Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1038/s41598-023-36188-z

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PubMed

Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Astronomical Society  

Abstract

One key aspect of understanding the solar dynamo mechanism and the evolution of solar magnetism is to properly describe the emergence of solar active regions. In this Letter, we describe the Lagrangian photospheric flows dynamics during a simulated flux emergence that produces an active region formed by pores. We analyze the lower photospheric flow organization prior, during and following the rise of an active region, uncovering the repelling and attracting photospheric structures that act as sources and sinks for magnetic element transport. Our results show that around 10 hr before the simulated emergence, considerable global changes are taking place on mesogranular scales indicated by an increase of the number of regions acting as a source to the multiple and scattered emergences of small-scale magnetic flux. At the location of active region’s appearance, the converging flows become weaker and there is an arising of a diverging region 8 hr before the emergence time. Our study also indicates that the strong concentration of magnetic field affects the flow dynamics beyond the area of the actual simulated pores, leading to complex and strongly diverging flows in the neighboring regions. Our findings suggest that the Lagrangian analysis is a powerful tool to describe the changes in the photospheric flows due to magnetic flux emergence.

DOI： 10.3847/2041-8213/acd007

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Other Link： https://iopscience.iop.org/article/10.3847/2041-8213/acd007/pdf

Authorship：Lead author   Language：English   Publishing type：Part of collection (book)  

DOI： 10.1007/978-981-19-7765-7_10

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Oxford University Press (OUP)  

Abstarct

Strong solar flares occur in δ-spots characterized by the opposite-polarity magnetic fluxes in a single penumbra. Sunspot formation via flux emergence from the convection zone to the photosphere can be strongly affected by convective turbulent flows. It has not yet been shown how crucial convective flows are for the formation of δ-spots. The aim of this study is to reveal the impact of convective flows in the convection zone on the formation and evolution of sunspot magnetic fields. We simulated the emergence and transport of magnetic flux tubes in the convection zone using radiative magnetohydrodynamics code R2D2. We carried out 93 simulations by allocating the twisted flux tubes to different positions in the convection zone. As a result, both δ-type and β-type magnetic distributions were reproduced only by the differences in the convective flows surrounding the flux tubes. The δ-spots were formed by the collision of positive and negative magnetic fluxes on the photosphere. The unipolar and bipolar rotations of the δ-spots were driven by magnetic twist and writhe, transporting magnetic helicity from the convection zone to the corona. We detected a strong correlation between the distribution of the nonpotential magnetic field in the photosphere and the position of the downflow plume in the convection zone. The correlation could be detected 20–30 h before the flux emergence. The results suggest that high free energy regions in the photosphere can be predicted even before the magnetic flux appears in the photosphere by detecting the downflow profile in the convection zone.

DOI： 10.1093/mnras/stac2635

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

DOI： 10.1029/2021JD035658

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2021JD035658

Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Astronomical Society  

Abstract

We analyze the simulation result shown in Hotta & Kusano (2021) in which the solar-like differential rotation is reproduced. The Sun is rotating differentially with the fast equator and the slow pole. It is widely thought that the thermal convection maintains the differential rotation, but recent high-resolution simulations tend to fail to reproduce the fast equator. This fact is an aspect of one of the biggest problems in solar physics called the convective conundrum. Hotta & Kusano succeed in reproducing the solar-like differential rotation without using any manipulation with an unprecedentedly high-resolution simulation. In this study, we analyze the simulation data to understand the maintenance mechanism of the fast equator. Our analyses lead to conclusions that are summarized as follows. (1) The superequipatition magnetic field is generated by the compression, which can indirectly convert the massive internal energy to magnetic energy. (2) The efficient small-scale energy transport suppresses large-scale convection energy. (3) Non-Taylor–Proudman differential rotation is maintained by the entropy gradient caused by the anisotropic latitudinal energy transport enhanced by the magnetic field. (4) The fast equator is maintained by the meridional flow mainly caused by the Maxwell stress. The Maxwell stress itself also has a role in the angular momentum transport for the fast near-surface equator (we call it the Punching ball effect). The fast equator in the simulation is reproduced not due to the low Rossby number regime but due to the strong magnetic field. This study newly finds the role of the magnetic field in the maintenance of differential rotation.

DOI： 10.3847/1538-4357/ac7395

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Other Link： https://iopscience.iop.org/article/10.3847/1538-4357/ac7395/pdf

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1038/s41550-021-01459-0

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Other Link： https://www.nature.com/articles/s41550-021-01459-0

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1186/s40623-021-01530-0

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

Although solar activity may significantly impact the global environment and socioeconomic systems, the mechanisms for solar eruptions and the subsequent processes have not yet been fully understood. Thus, modern society supported by advanced information systems is at risk from severe space weather disturbances. Project for solar-terrestrial environment prediction (PSTEP) was launched to improve this situation through synergy between basic science research and operational forecast. The PSTEP is a nationwide research collaboration in Japan and was conducted from April 2015 to March 2020, supported by a Grant-in-Aid for Scientific Research on Innovative Areas from the Ministry of Education, Culture, Sports, Science and Technology of Japan. By this project, we sought to answer the fundamental questions concerning the solar-terrestrial environment and aimed to build a next-generation space weather forecast system to prepare for severe space weather disasters. The PSTEP consists of four research groups and proposal-based research units. It has made a significant progress in space weather research and operational forecasts, publishing over 500 refereed journal papers and organizing four international symposiums, various workshops and seminars, and summer school for graduate students at Rikubetsu in 2017. This paper is a summary report of the PSTEP and describes the major research achievements it produced.

DOI： 10.1186/s40623-021-01486-1

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.3847/1538-4357/abf3c1

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

We surveyed the relationship between the scale of space weather events and their occurrence rate in Japan, and we discussed the social impact of these phenomena during the Project for Solar-Terrestrial Environment Prediction (PSTEP) in 2015-2019. The information was compiled for domestic users of space weather forecasts for appropriate preparedness against space weather disasters. This paper gives a comprehensive summary of the survey, focusing on the fields of electricity, satellite operations, communication and broadcasting, satellite positioning usage, aviation, human space activity, and daily life on the Earth's surface, using the cutting-edge knowledge of space weather. Quantitative estimations of the economic impact of space weather events on electricity supply and aviation are also given. Some topics requiring future research, which were identified during the survey are also described.

DOI： 10.1186/s40623-021-01420-5

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

It is of great interest and importance to study the variabilities of solar EUV, UV and X-ray irradiance in heliophysics, in Earth's climate, and space weather applications. A careful study is required to identify, track, monitor and segment the different coronal features such as active regions (ARs), coronal holes (CHs), the background regions (BGs) and the X-ray bright points (XBPs) from spatially resolved full-disk images of the Sun. Variability of solar soft X-ray irradiance is studied for a period of 13 years (February 2007-March 2020, covers Solar Cycle 24), using the X-Ray Telescope on board the Hinode (Hinode/XRT) and GOES (1 - 8 angstrom). The full-disk X-ray images observed in Al_mesh filter from XRT are used, for the first time, to understand the solar X-ray irradiance variability measured, Sun as a star, by GOES instrument. An algorithm in Python has been developed and applied to identify and segment coronal X-ray features (ARs, CHs, BGs, and XBPs) from the full-disk soft X-ray observations of Hinode/XRT. The segmentation process has been carried out automatically based on the intensity level, morphology and sizes of the X-ray features. The total intensity, area, and contribution of ARs/CHs/BGs/XBPs features were estimated and compared with the full-disk integrated intensity (FDI) and GOES (1 - 8 angstrom) X-ray irradiance measurements. The XBPs have been identified and counted automatically over the full disk to investigate their relation to solar magnetic cycle. The total intensity of ARs/CHs/BGs/XBPs/FD regions are compared with the GOES (1 - 8 angstrom) X-ray irradiance variations. We present the results obtained from Hinode/XRT full-disk images (in Al_mesh filter) and compare the resulting integrated full-disk intensity (FDI) with GOES X-ray irradiance. The X-ray intensity measured over ARs/CHs/BGs/XBPs/FD is well correlated with GOES X-ray flux. The contributions of the segmented X-ray features to FDI and X-ray irradiance variations are determined. It is found that the background and active regions have a greater impact on the X-ray irradiance fluctuations. The mean contribution estimated for the whole observed period of the background regions (BGs) will be around 65 +/- 10.97%, whereas the ARs, XBPs and CHs are 30 +/- 11.82%, 4 +/- 1.18% and 1 +/- 0.52%, respectively, to total solar X-ray flux. We observed that the area and contribution of ARs and CHs varies with the phase of the solar cycle, whereas the BGs and XBPs show an anti-correlation. We find that the area of the coronal features is highly variable suggesting that their area has to be taken into account in irradiance models, in addition to their intensity variations. The time series results of XBPs suggest for an existence of anti-correlation between the number of XBPs and the sunspot numbers. It is also important to consider both the number variation and the contribution of XBPs in the reconstruction of total solar X-ray irradiance variability.

DOI： 10.1007/s11207-021-01785-6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

In our earlier study (Paper I) of this series, we examined the hemispheric sign preference (HSP) of magnetic helicity flux dH/dt across photospheric surfaces of 4802 samples of 1105 unique active regions (ARs) observed during solar cycle 24. Here, we investigate any association of the HSP, expressed as a degree of compliance, with flaring activity, analyzing the same set of dH/dt estimates as used in Paper I. The AR samples under investigation are assigned to heliographic regions (HRs) defined in the Carrington longitude-latitude plane with a grid spacing of 45 degrees in longitude and 15 degrees in latitude. For AR samples in each of the defined HRs, we calculate the degree of HSP compliance and the average soft X-ray flare index. The strongest flaring activity is found to be in one distinctive HR with an extremely low-HSP compliance of 41% as compared to the mean and standard deviation of 62% and 7%, respectively, over all HRs. This sole HR shows an anti-HSP (i.e., <50%) and includes the highly flare-productive AR NOAA 12673, however this AR is not uniquely responsible for the HR's low HSP. We also find that all HRs with the highest flaring activity are located in the southern hemisphere, and they tend to have lower degrees of HSP compliance. These findings point to the presence of localized regions of the convection zone with enhanced turbulence, imparting a greater magnetic complexity and a higher flaring rate to some rising magnetic flux tubes.

DOI： 10.3847/1538-4357/abea13

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.3847/1538-4357/abe414

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We report observational evidence of Lagrangian chaotic saddles in plasmas, given by the intersections of finite-time unstable and stable manifolds, using an approximate to 22h sequence of spacecraft images of the horizontal velocity field of solar photosphere. A set of 29 persistent objective vortices with lifetimes varying from 28.5 to 298.3 min are detected by computing the Lagrangian averaged vorticity deviation. The unstable manifold of the Lagrangian chaotic saddles computed for approximate to 11h exhibits twisted folding motions indicative of recurring vortices in a magnetic mixed-polarity region. We show that the persistent objective vortices are formed in the gap regions of Lagrangian chaotic saddles at supergranular junctions.

DOI： 10.1103/PhysRevE.102.060201

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PubMed

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

A hemispheric preference in the dominant sign of magnetic helicity has been observed in numerous features in the solar atmosphere, i.e., left-handed/right-handed helicity in the northern/southern hemisphere. The relative importance of different physical processes that may contribute to the observed hemispheric sign preference (HSP) of magnetic helicity is still under debate. Here, we estimate magnetic helicity flux (dH/dt) across the photospheric surface for 4802 samples of 1105 unique active regions (ARs) that appeared over an 8 yr period from 2010 to 2017 during solar cycle 24, using photospheric vector magnetic field observations by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). The estimates of dH/dt show that 63% and 65% of the investigated AR samples in the northern and southern hemispheres, respectively, follow the HSP. We also find a trend that the HSP of dH/dt increases from similar to 50%-60% up to similar to 70%-80% as ARs (1) appear at the earlier inclining phase of the solar cycle or higher latitudes and (2) have larger values of vertical bar dH/dt vertical bar, the total unsigned magnetic flux, and the average plasma-flow speed. These observational findings support the enhancement of the HSP mainly by the Coriolis force acting on a buoyantly rising and expanding flux tube through the turbulent convection zone. In addition, the differential rotation on the solar surface as well as the tachocline alpha-effect of a flux-transport dynamo may reinforce the HSP for ARs at higher latitudes.

DOI： 10.3847/1538-4357/abbb93

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The aim of this study is to generate maps of the hard X-ray emission produced by energetic electrons in a solar flare and compare them with observations. The ultimate goal is to test the viability of the combined MHD/test-particle approach for data-driven modeling of active events in the solar corona and their impact on the heliosphere. Based on an MHD model of X-class solar flare observed on 2017 September 8, we calculate trajectories of a large number of electrons and protons using the relativistic guiding-center approach. Using the obtained particle trajectories, we deduce the spatial and energy distributions of energetic electrons and protons, and calculate bremsstrahlung hard X-ray emission using the "thin-target" approximation. Our approach predicts some key characteristics of energetic particles in the considered flare, including the size and location of the acceleration region, energetic particle trajectories and energy spectra. Most importantly, the hard X-ray bremsstrahlung intensity maps predicted by the model are in good agreement with those observed by RHESSI. Furthermore, the locations of proton and electron precipitation appear to be close to the sources of helioseismic response detected in this flare. Therefore, the adopted approach can be used for observationally driven modeling of individual solar flares, including manifestations of energetic particles in the corona, as well as the inner heliosphere.

DOI： 10.3847/1538-4357/abb60e

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.3847/1538-4357/aba61a

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Language：English   Publishing type：Research paper (scientific journal)  

© 2020. The American Astronomical Society. All rights reserved. Solar flares and coronal mass ejections (CMEs) are eruptive phenomena caused by coronal magnetic fields. In particular, large eruptive events originate in active regions (AR) with strong surface magnetic fields. However, it is still unclear what determines the capability of an AR to specifically produce eruptive flares and CMEs, and this hinders our knowledge of the initiation mechanism for the eruptive component of these phenomena. In this study, we propose a new parameter r m to measure the possibility that a flare that occurs in an AR can be eruptive and produce a CME. The parameter r m is defined by the ratio of the magnetic flux of twist higher than a threshold T c to the surrounding-and specifically, the overlying-magnetic flux. The value of r m for each AR can be estimated using nonlinear force-free field extrapolation models of the coronal magnetic field. Based on the data obtained by the Solar Dynamics Observatory/Helioseismic and Magnetic Imager, we calculated the values of r m for 29 ARs at 51 times prior to flares larger than M5.0 class. We find that the footpoints of field lines with twist higher than 0.2 can represent the subsequent flare ribbons well, and field lines that overlie and "fence in" the highly twisted region will work to confine the eruption, generating confined flares. Discriminant function analysis is used to show that r m is moderately well able to distinguish ARs that have the capability of producing eruptive flares.

DOI： 10.3847/1538-4357/ab822c

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

A nonlinear force-free field (NLFFF) extrapolation is widely used to reconstruct the three-dimensional magnetic field in the solar corona from the observed photospheric magnetic field. However, the pressure gradient and gravitational forces are ignored in the NLFFF model, even though the photospheric and chromospheric magnetic fields are not in general force-free. Here we develop a magnetohydrodynamic (MHD) relaxation method that reconstructs the solar atmospheric (chromospheric and coronal) magnetic field as a non-force-free magnetic field (NFFF) in magnetohydrostatic equilibrium where the Lorentz, pressure gradient, and gravitational forces are balanced. The system of basic equations for the MHD relaxation method is derived, and mathematical properties of the system are investigated. A robust numerical solver for the system is constructed based on the modern high-order shock capturing scheme. Two-dimensional numerical experiments that include the pressure gradient and gravitational forces are also demonstrated.

DOI： 10.3847/1538-4365/ab64f2

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

A crucial challenge to successful flare prediction is forecasting periods that transition between "flare-quiet" and "flare-active." Building on earlier studies in this series in which we describe the methodology, details, and results of flare forecasting comparison efforts, we focus here on patterns of forecast outcomes (success and failure) over multiday periods. A novel analysis is developed to evaluate forecasting success in the context of catching the first event of flare-active periods and, conversely, correctly predicting declining flare activity. We demonstrate these evaluation methods graphically and quantitatively as they provide both quick comparative evaluations and options for detailed analysis. For the testing interval 2016-2017, we determine the relative frequency distribution of two-day dichotomous forecast outcomes for three different event histories (i.e., event/event, no-event/event, and event/no-event) and use it to highlight performance differences between forecasting methods. A trend is identified across all forecasting methods that a high/low forecast probability on day 1 remains high/low on day 2, even though flaring activity is transitioning. For M-class and larger flares, we find that explicitly including persistence or prior flare history in computing forecasts helps to improve overall forecast performance. It is also found that using magnetic/modern data leads to improvement in catching the first-event/first-no-event transitions. Finally, 15% of major (i.e., M-class or above) flare days over the testing interval were effectively missed due to a lack of observations from instruments away from the Earth-Sun line.

DOI： 10.3847/1538-4357/ab65f0

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Over a four hour period between 2014 June 12-13 a series of three flares were observed within AR 12087. This sequence of flares started with a non-eruptive M-class flare, followed by a non-eruptive C-class flare, and finally ended with a second C-class flare that had an associated filament eruption. In this paper we combine spectroscopic analysis of Interface Region Imaging Spectrometer observations of the Si IV line during the three flares along with a series of nonlinear force-free field (NLFFF) extrapolations in order to investigate the conditions that lead the final flare to be eruptive. From this analysis it is found to be unlikely that the eruption was triggered by either kink instability or by tether-cutting reconnection, allowing the flux rope to rise into a region where it would be susceptible to the torus instability. The NLFFF modeling does, however, suggest that the overlying magnetic field has a fan-spine topology, raising the possibility that breakout reconnection occurring during the first two flares weakened the overlying field, allowing the flux rope to erupt in the subsequent third flare.

DOI： 10.3847/1538-4357/ab6bc8

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1088/2514-3433/ab404ach2

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Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

© 2019. The American Astronomical Society. All rights reserved. We studied a flare onset process in terms of stability of a three-dimensional (3D) magnetic field in active region 12371 producing an eruptive M6.5 flare in 2015 June 22. In order to reveal the 3D magnetic structure, we first extrapolated the 3D coronal magnetic fields based on time series of the photospheric vector magnetic fields under a nonlinear force-free field (NLFFF) approximation. The NLFFFs nicely reproduced the observed sigmoidal structure which is widely considered to be preeruptive magnetic configuration. In particular, we found that the sigmoid is composed of two branches of sheared arcade loops. On the basis of the NLFFFs, we investigated the sheared arcade loops to explore the onset process of the eruptive flare using three representative magnetohydrodynamic instabilities: the kink, torus, and double arc instabilities (DAI). The DAI, recently proposed by Ishiguro & Kusano, is a double arc loop that can be more easily destabilized than a torus loop. Consequently, the NLFFFs are found to be quite stable against the kink and torus instabilities. However, the sheared arcade loops formed prior to the flare possibly become unstable against the DAI. As a possible scenario for the onset process of the M6.5 flare, we suggest a three-step process: (1) double arc loops are formed by the sheared arcade loops through the tether-cutting reconnection during an early phase of the flare, (2) the DAI contributes to the expansion of destabilized double arc loops, and (3) finally, the torus instability makes the full eruption.

DOI： 10.3847/1538-4357/ab5582

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Language：English   Publishing type：Research paper (scientific journal)  

© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society This is an erratum to the paper 'Supergranular turbulence in the quiet Sun: Lagrangian coherent structures' that was published in MNRAS, 488, 3076-3088 (2019). In the original version of stz1909, there were several instances where the author's corrections had not been implemented during the proofing stages. These have now been corrected as follows: (i) Page 3077: in the second paragraph, the reference 'G16 G17' has been corrected to 'Gošić et al. 2014, 2016'. (ii) Page 3079: in section 3.1, the reference 'G20' has been corrected to 'Haller (2015)'. (iii) Page 3087: the article ID 'eeau2307' has been added to reference 'Hotta H., Iijima H., Kusano K., 2019, Sci. Adv., 5'. The publisher apologises for these errors.

DOI： 10.1093/mnras/stz2296

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Language：English   Publishing type：Research paper (scientific journal)  

© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society The quiet Sun exhibits a wealth of magnetic activities that are fundamental for our understanding of solar magnetism. The magnetic fields in the quiet Sun are observed to evolve coherently, interacting with each other to form prominent structures as they are advected by photospheric flows. The aim of this paper is to study supergranular turbulence by detecting Lagrangian coherent structures (LCS) based on the horizontal velocity fields derived from Hinode intensity images at disc centre of the quiet Sun on 2010 November 2. LCS act as transport barriers and are responsible for attracting/repelling the fluid elements and swirling motions in a finite time. Repelling/attracting LCS are found by computing the forward/backward finite-time Lyapunov exponent (FTLE), and vortices are found by the Lagrangian-averaged vorticity deviation method. We show that the Lagrangian centres and boundaries of supergranular cells are given by the local maximum of the forward and backward FTLE, respectively. The attracting LCS expose the location of the sinks of photospheric flows at supergranular junctions, whereas the repelling LCS interconnect the Lagrangian centres of neighbouring supergranular cells. Lagrangian transport barriers are found within a supergranular cell and from one cell to other cells, which play a key role in the dynamics of internetwork and network magnetic elements. Such barriers favour the formation of vortices in supergranular junctions. In particular, we show that the magnetic field distribution in the quiet Sun is determined by the combined action of attracting/repelling LCS and vortices.

DOI： 10.1093/mnras/stz1909

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Language：English   Publishing type：Research paper (scientific journal)  

© 2019. The American Astronomical Society. All rights reserved. A workshop was recently held at Nagoya University (2017 October 31-November 2), sponsored by the Center for International Collaborative Research, at the Institute for Space-Earth Environmental Research, Nagoya University, Japan, to quantitatively compare the performance of today's operational solar flare forecasting facilities. Building upon Paper I of this series, in Paper II we described the participating methods for this latest comparison effort, the evaluation methodology, and presented quantitative comparisons. In this paper, we focus on the behavior and performance of the methods when evaluated in the context of broad implementation differences. Acknowledging the short testing interval available and the small number of methods available, we do find that forecast performance: (1) appears to improve by including persistence or prior flare activity, region evolution, and a human "forecaster in the loop"; (2) is hurt by restricting data to disk-center observations; (3) may benefit from long-term statistics but mostly when then combined with modern data sources and statistical approaches. These trends are arguably weak and must be viewed with numerous caveats, as discussed both here and in Paper II. Following this present work, in Paper IV (Park et al. 2019) we will present a novel analysis method to evaluate temporal patterns of forecasting errors of both types (i.e., misses and false alarms). Hence, most importantly, with this series of papers, we demonstrate the techniques for facilitating comparisons in the interest of establishing performance-positive methodologies.

DOI： 10.3847/1538-4357/ab2e11

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Language：English   Publishing type：Research paper (scientific journal)  

© 2019. The American Astronomical Society. All rights reserved.. Solar flares are extremely energetic phenomena in our solar system. Their impulsive and often drastic radiative increases, particularly at short wavelengths, bring immediate impacts that motivate solar physics and space weather research to understand solar flares to the point of being able to forecast them. As data and algorithms improve dramatically, questions must be asked concerning how well the forecasting performs; crucially, we must ask how to rigorously measure performance in order to critically gauge any improvements. Building upon earlier-developed methodology of Paper I (Barnes et al. 2016), international representatives of regional warning centers and research facilities assembled in 2017 at the Institute for Space-Earth Environmental Research, Nagoya University, Japan to, for the first time, directly compare the performance of operational solar flare forecasting methods. Multiple quantitative evaluation metrics are employed, with the focus and discussion on evaluation methodologies given the restrictions of operational forecasting. Numerous methods performed consistently above the "no-skill" level, although which method scored top marks is decisively a function of flare event definition and the metric used; there was no single winner. Following in this paper series, we ask why the performances differ by examining implementation details (Leka et al. 2019), and then we present a novel analysis method to evaluate temporal patterns of forecasting errors in Paper IV (Park et al. 2019). With these works, this team presents a well-defined and robust methodology for evaluating solar flare forecasting methods in both research and operational frameworks and today's performance benchmarks against which improvements and new methods may be compared.

DOI： 10.3847/1538-4365/ab2e12

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

The Sweet-Parker and Petschek models are well-established magnetohydrodynamics (MHD) models of steady magnetic reconnection. Recent findings on magnetic reconnection in high-Lundquist-number plasmas indicate that Sweet-Parker-type reconnection in marginally stable thin current sheets connecting plasmoids can produce fast reconnection. By contrast, it has proven difficult to achieve Petschek-type reconnection in plasmas with uniform resistivity because sustaining it requires localization of the diffusion region. However, Shibayama et al. [Phys. Plasmas 22, 100706 (2015)] recently noted that Petschek-type reconnection can he achieved spontaneously in a dynamical manner even under uniform resistivity through what they called dynamical Petschek reconnection. In this new type of reconnection, Petschek-type diffusion regions can be formed in connection with plasmoids. In this paper, we report the results of two-dimensional resistive MHD simulation with uniform resistivity, undertaken to determine the diffusion region localization mechanism under dynamical Petschek reconnection. Through this modeling, we found that the separation of the X-point from the flow stagnation point (S-point) plays a crucial role in the localization of the diffusion region because the motion of the X-point is restricted by the strong flow emanating from the flow stagnation point. This mechanism suggests that dynamical Petschek reconnection is possible even in large systems such as the solar corona. Published under license by AIP Publishing.

DOI： 10.1063/1.5084771

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：EDP Sciences  

<italic>Context.</italic> The reduced speed of sound technique (RSST) has been used for efficient simulation of low Mach number flows in solar and stellar convection zones. The basic RSST equations are hyperbolic and are suitable for parallel computation by domain decomposition. The application of RSST is limited to cases in which density perturbations are much smaller than the background density. In addition, nonconservative variables are required to be evolved using this method, which is not suitable in cases where discontinuities such as shock waves coexist in a single numerical domain.


<italic>Aims.</italic> In this study, we suggest a new semiconservative formulation of the RSST that can be applied to low Mach number flows with large density variations.


<italic>Methods.</italic> We derive the wave speed of the original and newly suggested methods to clarify that these methods can reduce the speed of sound without affecting the entropy wave. The equations are implemented using the finite volume method. Several numerical tests are carried out to verify the suggested methods.


<italic>Results.</italic> The analysis and numerical results show that the original RSST is not applicable when mass density variations are large. In contrast, the newly suggested methods are found to be efficient in such cases. We also suggest variants of the RSST that conserve momentum in the machine precision. The newly suggested variants are formulated as semiconservative equations, which reduce to the conservative form of the Euler equations when the speed of sound is not reduced. This property is advantageous when both high and low Mach number regions are included in the numerical domain.


<italic>Conclusions.</italic> The newly suggested forms of RSST can be applied to a wider range of low Mach number flows.

DOI： 10.1051/0004-6361/201834031

Language：English   Publishing type：Research paper (scientific journal)  

Copyright © 2019 The Authors. The solar convection zone is filled with turbulent convection in highly stratified plasma. Several theoretical and observational studies suggest that the numerical calculations overestimate the convection velocity. Since all deep convection zone calculations exclude the solar surface due to substantial temporal and spatial scale separations, the solar surface, which drives the thermal convection with efficient radiative cooling, has been thought to be the key to solve this discrepancy. Thanks to the recent development in massive supercomputers, we are successful in performing the comprehensive calculation covering the whole solar convection zone. We compare the results with and without the solar surface in the local domain and without the surface in the full sphere. The calculations do not include the rotation and the magnetic field. The surface region has an unexpectedly weak influence on the deep convection zone. We find that just including the solar surface cannot solve the problem.

DOI： 10.1126/sciadv.aau2307

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PubMed

Language：English   Publishing type：Research paper (scientific journal)  

© 2017 Elsevier Ltd Solar eruptions are the most energetic phenomena observed in the solar system observed as flares, coronal mass ejections (CMEs) and filament/prominence eruption. The helically twisted flux tube is widely thought to be the source and driver of solar eruptions and to carry the plasma into the interplanetary space. Those may eventually reach the magnetosphere and cause strong disturbances of the geomagnetic field. Therefore, the understanding of the onset of solar eruptions is important not only in the framework of solar physics but also for the space weather forecast. In this paper, we report on new insight into the onset mechanism of solar eruptions recently obtained from our new studies. We perform the studies in terms of the observational approach with state-of-the-art solar physics satellites and the numerical one with the latest super computer system. We specified two types of small magnetic perturbations of the photospheric magnetic field. These can enhance the magnetic reconnection in the pre-existing non-potential magnetic field, which produces a large flux tube and then drives the eruption. We further confirmed that this reconnection is a key process for the eruption in our latest data-constrained simulation. We report our latest results and our interpretation of the onset mechanism of solar eruptions.

DOI： 10.1016/j.jastp.2017.08.035

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Coronal magnetic fields are responsible for the onset of solar flares and solar eruptions. However, the type of magnetic field parameters that can be used to measure the critical condition for a solar eruption is still unclear. As an effort to understand the possible condition for a solar flare, we have examined the nondimensional parameter kappa introduced by Ishiguro & Kusano, which contains information about magnetic twist distribution and magnetic flux in an active region (AR). We introduce a new parameter kappa*, as a proxy for kappa, and we have analyzed the evolution of kappa* during the flaring period of an AR using the nonlinear force-free field extrapolated from the photospheric vector magnetic field data. Using data from the Solar Dynamics Observatory/Helioseismic and Magnetic Imager, we have calculated kappa* for the AR NOAA 11158 during its three-day flaring period. We found that kappa* increased to a certain level before two large flares and decreased significantly after their onset. The results suggest that kappa* may be used as an indicator of the necessary condition for the onset of a solar eruption in the AR. Based on this result, we propose a new method to assess the possibility of a large solar eruption from an AR by combining the parameter kappa* and information about the magnetic energy of the AR.

DOI： 10.3847/1538-4357/aad181

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The X1 flare and associated filament eruption occurring in NOAA Active Region 12017 on SOL2014-03-29 has been a source of intense study. In this work, we analyze the results of a series of nonlinear force-free field extrapolations of the flare's pre- and post-flare periods. In combination with observational data provided by the IRIS, Hinode, and Solar Dynamics Observatory missions, we have confirmed the existence of two flux ropes present within the active region prior to flaring. Of these two flux ropes, we find that intriguingly only one erupts during the X1 flare. We propose that the reason for this is due to tether cutting reconnection allowing one of the flux ropes to rise to a torus unstable region prior to flaring, thus allowing it to erupt during the subsequent flare.

DOI： 10.3847/1538-4357/aac5e1

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

© 2018 Elsevier Ltd Mercury is the closest orbiting planet around the sun and is therefore embedded in an intensive and highly varying solar wind. In-situ data from the MESSENGER spacecraft of the plasma environment near Mercury indicates that a coronal mass ejection (CME) passed the planet on 23 November 2011 over the span of the 12 h MESSENGER orbit. Slavin et al. (2014) derived the upstream parameters of the solar wind at the time of that orbit, and were able to explain the observed MESSENGER data in the cusp and magnetopause segments of MESSENGER's trajectory. These upstream parameters will be used for our first simulation run. We use the hybrid code A.I.K.E.F. which treats ions as individual particles and electrons as a mass-less fluid, to conduct hybrid simulations of Mercury's magnetospheric response to the impact of the CME on ion gyro time scales. Results from the simulation are in agreement with magnetic field measurements from the inner day-side magnetosphere and the bow-shock region. However, at the planet's nightside, Mercury's plasma environment seemed to be governed by different solar wind conditions, in conclusion, Mercury's interaction with the CME is not sufficiently describable by only one set of upstream parameters. Therefore, to simulate the magnetospheric response while MESSENGER was located in the tail region, we use parameters obtained from the MHD solar wind simulation code SUSANOO (Shiota et al. (2014)) for our second simulation run. The parameters of the SUSANOO model achieve a good agreement of the data concerning the plasma tail crossing and the night-side approach to Mercury. However, the polar and closest approach are hardly described by both upstream parameters, namely, neither upstream dataset is able to reproduce the MESSENGER crossing of Mercury's magnetospheric cusp. We conclude that the respective CME was too variable on the timescale of the MESSENGER orbit to be described by only two sets of upstream conditions. Our results suggest locally strong and highly variable dynamics of the CME on timescales of 15 min while MESSENGER was near closest approach.

DOI： 10.1016/j.pss.2017.12.016

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

© 2018. The American Astronomical Society. All rights reserved. The triggering mechanism(s) and critical condition(s) of solar flares are still not completely clarified, although various studies have attempted to elucidate them. We have also proposed a theoretical flare-trigger model based on MHD simulations in which two types of small-scale bipole fields, the so-called opposite polarity (OP) and reversed shear (RS), can trigger flares. In this study, we evaluated the applicability of our flare-trigger model to the observation of 32 flares that were observed by the Solar Dynamics Observatory, by focusing on geometrical structures. We classified the events into six types, including the OP and RS types, based on photospheric magnetic field configuration, presence of precursor brightenings, and shape of the initial flare ribbons. As a result, we found that approximately 30% of the flares were consistent with our flare-trigger model, and the number of RS-type triggered flares is larger than that of the OP type. We found that none of the sampled events contradict our flare model; though, we cannot clearly determine the trigger mechanism of 70% of the flares in this study. We carefully investigated the applicability of our flare-trigger model and the possibility that other models can explain the other 70% of the events. Consequently, we concluded that our flare-trigger model has certainly proposed important conditions for flare-triggering.

DOI： 10.3847/1538-4357/aaacd1

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

© The Author(s) 2018. Solar eruptions are well-known drivers of extreme space weather, which can greatly disturb the Earth's magnetosphere and ionosphere. The triggering process and initial dynamics of these eruptions are still an area of intense study. Here we perform a magnetohydrodynamic simulation taking into account the observed photospheric magnetic field to reveal the dynamics of a solar eruption in a real magnetic environment. In our simulation, we confirmed that tether-cutting reconnection occurring locally above the polarity inversion line creates a twisted flux tube, which is lifted into a toroidal unstable area where it loses equilibrium, destroying the force-free state, and driving the eruption. Consequently, a more highly twisted flux tube is built up during this initial phase, which can be further accelerated even when it returns to a stable area. We suggest that a nonlinear positive feedback process between the flux tube evolution and reconnection is the key to ensure this extra acceleration.

DOI： 10.1038/s41467-017-02616-8

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Scopus

PubMed

Language：English   Publishing type：Research paper (scientific journal)   Publisher：EDP SCIENCES S A  

Aims. We report the small temporal variation of the axial dipole moment near the solar minimum and its application to the solar-cycle prediction by the surface flux transport (SFT) model.
Methods. We measure the axial dipole moment using the photospheric synoptic magnetogram observed by theWilcox Solar Observatory (WSO), the ESA/NASA Solar and Heliospheric Observatory Michelson Doppler Imager (MDI), and the NASA Solar Dynamics Observatory Helioseismic and Magnetic Imager (HMI). We also use the SFT model for the interpretation and prediction of the observed axial dipole moment.
Results. We find that the observed axial dipole moment becomes approximately constant during the period of several years before each cycle minimum, which we call the axial dipole moment plateau. The cross-equatorial magnetic flux transport is found to be small during the period, although a significant number of sunspots are still emerging. The results indicate that the newly emerged magnetic flux does not contribute to the build up of the axial dipole moment near the end of each cycle. This is confirmed by showing that the time variation of the observed axial dipole moment agrees well with that predicted by the SFT model without introducing new emergence of magnetic flux. These results allow us to predict the axial dipole moment at the Cycle 24/25 minimum using the SFT model without introducing new flux emergence. The predicted axial dipole moment at the Cycle 24/25 minimum is 60-80 percent of Cycle 23/24 minimum, which suggests the amplitude of Cycle 25 is even weaker than the current Cycle 24.
Conclusions. The plateau of the solar axial dipole moment is an important feature for the longer-term prediction of the solar cycle based on the SFT model.

DOI： 10.1051/0004-6361/201731813

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

© 2017 Author(s). A well-known Taylor problem in the theory of forced magnetic reconnection is investigated in the framework of the Hall-Magnetohydrodynamics. In the first part of the paper, we deal with the linear theory of the Hall-mediated forced reconnection. Then, in the second part, these results are used for demonstrating how the secondary tearing (plasmoid) instability can develop in the course of this process.

DOI： 10.1063/1.4996982

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The stability of the magnetic field in the solar corona is important for understanding the causes of solar eruptions. Although various scenarios have been suggested to date, the tether-cutting reconnection scenario proposed by Moore et al. is one of the widely accepted models to explain the onset process of solar eruptions. Although the tether-cutting reconnection scenario proposes that the sigmoidal field formed by internal reconnection is the magnetic field in the pre-eruptive state, the stability of the sigmoidal field has not yet been investigated quantitatively. In this paper, in order to elucidate the stability problem of. the pre-eruptive state, we developed a simple numerical analysis in which the sigmoidal field is modeled by a double arc electric current loop and its stability is analyzed. As a result, we found that the double arc loop is more easily destabilized than the axisymmetric torus, and it becomes unstable even if the external field does not decay with altitude, which is in contrast to the axisymmetric torus instability. This suggests that tether-cutting reconnection may well work as the onset mechanism of solar eruptions, and if so, the critical condition for eruption under a certain geometry may be determined by a new type of instability rather than by the torus instability. Based on them, we propose a new type of instability called double arc instability (DAI). We discuss the critical conditions for DAI and derive a new parameter k, defined as the product of the magnetic twist and the normalized flux of the tether-cutting reconnection.

DOI： 10.3847/1538-4357/aa799b

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

In order to understand the flare trigger mechanism, we conduct three-dimensional magnetohydrodynamic simulations using a coronal magnetic field model derived from data observed by the Hinode satellite. Several types of magnetic bipoles are imposed into the photospheric boundary of the Nonlinear Force-free Field model of Active Region (AR) NOAA 10930 on 2006 December 13, to investigate what kind of magnetic disturbance may trigger the flare. As a result, we confirm that certain small bipole fields, which emerge into the highly sheared global magnetic field of an AR, can effectively trigger a flare. These bipole fields can be classified into two groups based on their orientation relative to the polarity inversion line: the so-called opposite polarity, and reversed shear structures, as suggested by Kusano et al. We also investigate the structure of the footpoints of reconnected field lines. By comparing the distribution of reconstructed field lines and observed flare ribbons, the trigger structure of the flare can be inferred. Our simulation suggests that the data-constrained simulation, taking into account both the large-scale magnetic structure and small-scale magnetic disturbance (such as emerging fluxes), is a good way to discover a flare-producing AR, which can be applied to space weather prediction.

DOI： 10.3847/1538-4357/aa750e

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The physical properties and their. contribution to the onset of a solar flare are still uncleare even. though chromospheric brightening is considered a precursor phenomenon of a flare. Many studies suggested that photospheric magnetic field changes cause destabilization of large-scale coronal structure. We aim to understand how a small photospheric change contributes to a flare and to reveal how the intermediary chromosphere behaves in the precursor phase. We analyzed the precursor brightening of the X1.6 flare on 2014 October 22 in the AR 12192 using the Interface Region Imaging Spectrograph (IRIS) and Hinode/EUV Imaging Spectrometer (EIS) data. We investigated a localized jet with the strong precursor brightening, and compared the intensity, Doppler velocity, and line width in C II, Mg II k, and. Si IV lines by IRIS and He II, Fe XII, and. Fe XV lines by Hinode/EIS. We also analyzed the photospheric magnetic field and chromospheric/coronal structures using the. Solar Dynamics Observatory (SDO)/Helioseismic and Magnetic Imager and Atmospheric Imaging Assembly. We found a significant blueshift (similar to 100 km s(-1)), which is related to the strong precursor brightening over a characteristic magnetic field structure, and the blueshift was observed at all of. the temperatures. This might indicate that the flow is accelerated by Lorentz force. Moreover, the large-scale coronal loop that connects the foot points of the flare ribbons was destabilized just after the precursor brightening with the blueshift. It suggests that magnetic reconnection locally occurred in the lower chromosphere and it triggered magnetic reconnection of the X1.6 flare in the corona.

DOI： 10.3847/1538-4357/aa6dfe

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1038/s41550-017-0085

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The solar magnetic field in a flare-producing active region (AR) is much more complicated than theoretical models, which assume a very simple magnetic field structure. The X1.0 flare, which occurred in AR 12192 on 2014 October 25, showed a complicated three-ribbon structure. To clarify the trigger process of the flare and to evaluate the applicability of a simple theoretical model, we analyzed the data from Hinode/Solar Optical Telescope and the Solar Dynamics Observatory/Helioseismic and Magnetic Imager, Atmospheric Imaging Assembly. We investigated the spatio-temporal correlation between the magnetic field structures, especially the non-potentiality of the horizontal field, and the bright structures in the solar atmosphere. As a result, we determined that the western side of the positive polarity, which is intruding on a negative polarity region, is the location where the flare was triggered. This is due to the fact that the sign of the. magnetic shear in that region was opposite that of. the major shear of the AR, and the significant brightenings were observed over the polarity inversion line (PIL) in that region before flare onset. These features are consistent with the recently proposed flare-trigger model that suggests that small reversed shear (RS) magnetic disturbances can trigger solar flares. Moreover, we found that the RS field was located slightly off the flaring PIL, contrary to the theoretical prediction. We discuss the possibility of an extension of the RS model based on an extra numerical simulation. Our result suggests that the RS field has a certain flexibility for displacement from a highly sheared PIL, and that the RS field triggers more flares than we expected.

DOI： 10.3847/1538-4357/aa6682

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1038/s41550-017-0085

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Observations of the Sun suggest that solar activities systematically create north-south hemispheric asymmetries. For instance, the hemisphere in which sunspot activity is more active tends to switch after the early half of each solar cycle. Svalgaard & Kamide recently pointed out that the time gaps of polar field reversal between the northern and southern hemispheres are simply consequences of the asymmetry of sunspot activity. However, the mechanism underlying the asymmetric feature in solar cycle activity is not yet well understood. In this paper, in order to explain the cause of the asymmetry from the theoretical point of view, we investigate the relationship between the dipole- and quadrupole-type components of the magnetic field in the solar cycle using the mean-field theory based on the flux transport dynamo model. As a result, we found that there are two different attractors of the solar cycle, in which either the north or the south polar field is first reversed, and that the flux transport dynamo model explains well the phase-asymmetry of sunspot activity and the polar field reversal without any ad hoc source of asymmetry.

DOI： 10.3847/1538-4357/835/1/84

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：THE JAPAN SOCIETY FOR AERONAUTICAL AND SPACE SCIENCES  

DOI： 10.14822/kjsass.65.1_22

CiNii Books

Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

This paper explores the characteristics of 42 solar X-class flares that were observed between February 2011 and November 2014, with data from the Solar Dynamics Observatory (SDO) and other sources. This flare list includes nine X-class flares that had no associated CMEs. In particular our aim was to determine whether a clear signature could be identified to differentiate powerful flares that have coronal mass ejections (CMEs) from those that do not. Part of the motivation for this study is the characterization of the solar paradigm for flare/CME occurrence as a possible guide to the stellar observations; hence we emphasize spectroscopic signatures. To do this we ask the following questions: Do all eruptive flares have long durations? Do CME-related flares stand out in terms of active-region size vs. flare duration? Do flare magnitudes correlate with sunspot areas, and, if so, are eruptive events distinguished? Is the occurrence of CMEs related to the fraction of the active-region area involved? Do X-class flares with no eruptions have weaker non-thermal signatures? Is the temperature dependence of evaporation different in eruptive and non-eruptive flares? Is EUV dimming only seen in eruptive flares? We find only one feature consistently associated with CME-related flares specifically: coronal dimming in lines characteristic of the quiet-Sun corona, i.e. 1 - 2 MK. We do not find a correlation between flare magnitude and sunspot areas. Although challenging, it will be of importance to model dimming for stellar cases and make suitable future plans for observations in the appropriate wavelength range in order to identify stellar CMEs consistently.

DOI： 10.1007/s11207-016-0923-0

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We analyze a three-dimensional (3D) magnetic structure and its stability in large solar active region (AR) 12192, using the 3D coronal magnetic field constructed under a nonlinear force-free field (NLFFF) approximation. In particular, we focus on the magnetic structure that produced an X3.1-class flare, which is one of the X-class flares observed in AR 12192. According to our analysis, the AR contains a multiple-flux-tube system, e.g., a large flux tube, with footpoints that are anchored to the large bipole field, under which other tubes exist close to a polarity inversion line (PIL). These various flux tubes of different sizes and shapes coexist there. In particular, the latter are embedded along the PIL, which produces a favorable shape for the tether-cutting reconnection and is related to the X-class solar flare. We further found that most of magnetic twists are not released even after the flare, which is consistent with the fact that no observational evidence for major eruptions was found. On the other hand, the upper part of the flux tube is beyond a critical decay index, essential for the excitation of torus instability before the flare, even though no coronal mass ejections were observed. We discuss the stability of the complicated flux tube system and suggest the reason for the existence of the stable flux tube. In addition, we further point out a possibility for tracing the shape of flare ribbons, on the basis of a detailed structural analysis of the NLFFF before a flare.

DOI： 10.3847/0004-637X/818/2/168

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Standard magnetohydrodynamic (MHD) theory predicts reconnection rate that is far too slow to account for a wide variety of reconnection events observed in space and laboratory plasmas. Therefore, it was commonly accepted that some non-MHD (kinetic) effects play a crucial role in fast reconnection. A recently renewed interest in simple MHD models is associated with the so-called plasmoid instability of reconnecting current sheets. Although it is now evident that this effect can significantly enhance the rate of reconnection, many details of the underlying multiple-plasmoid process still remain controversial. Here, we report results of a high-resolution computer simulation which demonstrate that fast albeit intermittent magnetic reconnection is sustained by numerous small-scale Petschek-type shocks spontaneously formed in the current sheet due to its plasmoid instability. (C) 2015 Author(s).

DOI： 10.1063/1.4934652

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

This letter presents a self-consistent description of nonlinear forced magnetic reconnection in Taylor's model of this process. If external boundary perturbation is strong enough, nonlinearity in the current sheet evolution becomes important before resistive effects come into play. This terminates the current sheet shrinking that takes place at the linear stage and brings about its nonlinear equilibrium with a finite thickness. Then, in theory, this equilibrium is destroyed by a finite plasma resistivity during the skin-time, and further reconnection proceeds in the Rutherford regime. However, realization of such a scenario is unlikely because of the plasmoid instability, which is fast enough to develop before the transition to the Rutherford phase occurs. The suggested analytical theory is entirely different from all previous studies and provides proper interpretation of the presently available numerical simulations of nonlinear forced magnetic reconnection. (C) 2015 AIP Publishing LLC.

DOI： 10.1063/1.4932079

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

DOI： 10.1002/2015SW001213

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.70.1.0_896

CiNii Books

Language：English   Publishing type：Research paper (scientific journal)   Publisher：OXFORD UNIV PRESS  

Understanding the mechanism that produces solar flares is important not only from the scientific point of view but also for improving space weather predictability. There are numerous observational and computational studies that have attempted to reveal the onset mechanism of solar flares. However, the underlying mechanism of flare onset remains elusive. To elucidate the flare trigger mechanism, we analyzed several flare events which were observed by Hinode/Solar Optical Telescope (SOT) in our previous study. Because of the limitation of the SOT field of view, however, only four events in the Hinode data sets have been usable. Therefore, increasing the number of events is required for evaluating the flare trigger models. We investigated the applicability of data obtained by the Solar Dynamics Observatory (SDO) to increase the data sample for a statistical analysis of the flare trigger process. SDO regularly observes the full disk of the sun and all flares, although its spatial resolution is lower than that of Hinode. We investigated the M6.6 flare which occurred on 2011 February 13, and compared the analyzed data of SDO with the results of our previous study using Hinode/SOT data. Filter and vector magnetograms obtained by the Helioseismic and Magnetic Imager and filtergrams from the Atmospheric Imaging Assembly (AIA) 1600 angstrom were employed. From the comparison of small-scale magnetic configurations and chromospheric emission prior to the flare onset, we confirmed that the trigger region is detectable with the SDO data. We also measured the magnetic shear angles of the active region and the azimuth and strength of the flare trigger field. The results were consistent with our previous study. We concluded that statistical studies of the flare trigger process are feasible with SDO as well as Hinode data. We also investigated the temporal evolution of the magnetic field before the flare onset with SDO.

DOI： 10.1093/pasj/psu091

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1093/pasj/psu092

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1093/pasj/psu092

Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

We present a comparison of the Solar Dynamics Observatory (SDO) analysis of NOAA Active Region (AR) 11158 and numerical simulations of flux-tube emergence, aiming to investigate the formation process of this flare-productive AR. First, we use SDO/Helioseismic and Magnetic Imager (HMI) magnetograms to investigate the photospheric evolution and Atmospheric Imaging Assembly (AIA) data to analyze the relevant coronal structures. Key features of this quadrupolar region are a long sheared polarity inversion line (PIL) in the central delta-sunspots and a coronal arcade above the PIL. We find that these features are responsible for the production of intense flares, including an X2.2-class event. Based on the observations, we then propose two possible models for the creation of AR 11158 and conduct flux-emergence simulations of the two cases to reproduce this AR. Case 1 is the emergence of a single flux tube, which is split into two in the convection zone and emerges at two locations, while Case 2 is the emergence of two isolated but neighboring tubes. We find that, in Case 1, a sheared PIL and a coronal arcade are created in the middle of the region, which agrees with the AR 11158 observation. However, Case 2 never builds a clear PIL, which deviates from the observation. Therefore, we conclude that the flare-productive AR 11158 is, between the two cases, more likely to be created from a single split emerging flux than from two independent flux bundles.

DOI： 10.1007/s11207-014-0502-1

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We develop a nonlinear force-free field (NLFFF) extrapolation code based on the magnetohydrodynamic (MHD) relaxation method. We extend the classical MHD relaxation method in two important ways. First, we introduce an algorithm initially proposed by Dedner et al. to effectively clean the numerical errors associated with del . B. Second, the multigrid type method is implemented in our NLFFF to perform direct analysis of the high-resolution magnetogram data. As a result of these two implementations, we successfully extrapolated the high resolution force-free field introduced by Low & Lou with better accuracy in a drastically shorter time. We also applied our extrapolation method to the MHD solution obtained from the flux-emergence simulation by Magara. We found that NLFFF extrapolation may be less effective for reproducing areas higher than a half-domain, where some magnetic loops are found in a state of continuous upward expansion. However, an inverse S-shaped structure consisting of the sheared and twisted loops formed in the lower region can be captured well through our NLFFF extrapolation method. We further discuss how well these sheared and twisted fields are reconstructed by estimating the magnetic topology and twist quantitatively.

DOI： 10.1088/0004-637X/780/1/101

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Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We present a 2.5 dimensional magnetohydrodynamic simulation of a magnetic flux rope (FR) propagating in the heliosphere and investigate the cause of the observed sharp plasma beta transition. Specifically, we consider a strong internal magnetic field and an explosive fast start, such that the plasma beta is significantly lower in the FR than in the sheath region that is formed ahead. This leads to an unusual FR morphology in the first stage of propagation, while the more traditional view (e. g., from space weather simulations like Enlil) of a pancake-shaped FR is observed as it approaches 1 AU. We investigate how an equipartition line, defined by a magnetic Weber number, surrounding a core region of a propagating FR, can demarcate a boundary layer where there is a sharp transition in the plasma beta. The substructure affects the distribution of toroidal flux, with the majority of the flux remaining in a small core region that maintains a quasi-cylindrical structure. We quantitatively investigate a locus of points where the kinetic energy density of the relative inflow field is equal to the energy density of the transverse magnetic field (i.e., effective tension force). The simulation provides compelling evidence that at all heliocentric distances the distribution of toroidal magnetic flux away from the FR axis is not linear, with 80% of the toroidal flux occurring within 40% of the distance from the FR axis. Thus, our simulation displays evidence that the competing ideas of a pancaking structure observed remotely can coexist with a quasi-cylindrical magnetic structure seen in situ.

DOI： 10.1088/0004-637X/779/2/142

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Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

To better understand a preferred magnetic field configuration and its evolution during coronal mass ejection (CME) events, we investigated the spatial and temporal evolution of photospheric magnetic fields in the active region NOAA 9236 that produced eight flare-associated CMEs during the time period of 2000 November 23-26. The time variations of the total magnetic helicity injection rate and the total unsigned magnetic flux are determined and examined not only in the entire active region but also in some local regions such as the main sunspots and the CME-associated flaring regions using SOHO/MDI magnetogram data. As a result, we found that (1) in the sunspots, a large amount of positive (right-handed) magnetic helicity was injected during most of the examined time period, (2) in the flare region, there was a continuous injection of negative (left-handed) magnetic helicity during the entire period, accompanied by a large increase of the unsigned magnetic flux, and (3) the flaring regions were mainly composed of emerging bipoles of magnetic fragments in which magnetic field lines have substantially favorable conditions for making reconnection with large-scale, overlying, and oppositely directed magnetic field lines connecting the main sunspots. These observational findings can also be well explained by some MHD numerical simulations for CME initiation (e. g., reconnection-favored emerging flux models). We therefore conclude that reconnection-favored magnetic fields in the flaring emerging flux regions play a crucial role in producing the multiple flare-associated CMEs in NOAA 9236.

DOI： 10.1088/0004-637X/778/1/13

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We investigated four major solar flare events that occurred in active regions NOAA 10930 (2006 December 13 and 14) and NOAA 11158 (2011 February 13 and 15) by using data observed by the Solar Optical Telescope on board the Hinode satellite. To reveal the trigger mechanism of solar flares, we analyzed the spatio-temporal correlation between the detailed magnetic field structure and the emission image of the Ca ii H line at the central part of flaring regions for several hours prior to the onset of the flares. In all the flare events, we observed that the magnetic shear angle in the flaring regions exceeded 70 degrees, as well as that characteristic magnetic disturbances developed at the centers of flaring regions in the pre-flare phase. These magnetic disturbances can be classified into two groups depending on the structure of their magnetic polarity inversion lines; the so-called opposite-polarity and reversed-shear magnetic field recently proposed by our group, although the magnetic disturbance in one event of the four samples is too subtle to clearly recognize the detailed structure. The result suggests that some major solar flares are triggered by rather small magnetic disturbances. We also show that the critical size of the flare-trigger field varies among flare events and briefly discuss how the flare-trigger process depends on the evolution of active regions.

DOI： 10.1088/0004-637X/778/1/48

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Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：OXFORD UNIV PRESS  

We consider transition to Hall-magnetohydrodynamics regimes of the resistive tearing instability in weakly ionized plasmas. A particular emphasis is put on a possible role of the resulting fast magnetic reconnection in such astrophysical objects as solar chromosphere, interstellar molecular clouds and protostellar discs. It is shown that Hall-mediated reconnection can be relevant to explosive phenomena in the latter, though it is unlikely to take place in the former two environments.

DOI： 10.1093/mnras/stt1155

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Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We report a detailed event analysis of the M6.6 class flare in the active region (AR) NOAA 11158 on 2011 February 13. AR 11158, which consisted of two major emerging bipoles, showed prominent activity including one X- and several M-class flares. In order to investigate the magnetic structures related to the M6.6 event, particularly the formation process of a flare-triggering magnetic region, we analyzed multiple spacecraft observations and numerical results of a flare simulation. We observed that, in the center of this quadrupolar AR, a highly sheared polarity inversion line (PIL) was formed through proper motions of the major magnetic elements, which built a sheared coronal arcade lying over the PIL. The observations lend support to the interpretation that the target flare was triggered by a localized magnetic region that had an intrusive structure, namely, a positive polarity penetrating into a negative counterpart. The geometrical relationship between the sheared coronal arcade and the triggering region is consistent with the theoretical flare model based on the previous numerical study. We found that the formation of the trigger region was due to the continuous accumulation of small-scale magnetic patches. A few hours before the flare occurred, the series of emerged/advected patches reconnected with a pre-existing field. Finally, the abrupt flare eruption of the M6.6 event started around 17:30 UT. Our analysis suggests that in the process of triggering flare activity, all magnetic systems on multiple scales are included, not only the entire AR evolution but also the fine magnetic elements.

DOI： 10.1088/0004-637X/773/2/128

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

DOI： 10.1002/swe.20045

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Ahedo and Ramos [Phys. Plasmas 19, 072519 (2012)] revisited what they called the supersonic regime of the Hall-mediated resistive tearing instability and arrived to results that disagree with the previously known ones (Fruchtman and Strauss [Phys. Fluids B 5, 1408 (1993)], Bian and Vekstein [Phys. Plasmas 14, 072107 (2007)]). The present Comment aims to clarify the origin of this disagreement and to confirm in this way the validity of the earlier findings. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4789449]

DOI： 10.1063/1.4789449

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1253/circj.CJ-66-0054

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PubMed

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Solar flares and coronal mass ejections, the most catastrophic eruptions in our solar system, have been known to affect terrestrial environments and infrastructure. However, because their triggering mechanism is still not sufficiently understood, our capacity to predict the occurrence of solar eruptions and to forecast space weather is substantially hindered. Even though various models have been proposed to determine the onset of solar eruptions, the types of magnetic structures capable of triggering these eruptions are still unclear. In this study, we solved this problem by systematically surveying the nonlinear dynamics caused by a wide variety of magnetic structures in terms of three-dimensional magnetohydrodynamic simulations. As a result, we determined that two different types of small magnetic structures favor the onset of solar eruptions. These structures, which should appear near the magnetic polarity inversion line (PIL), include magnetic fluxes reversed to the potential component or the nonpotential component of major field on the PIL. In addition, we analyzed two large flares, the X-class flare on 2006 December 13 and the M-class flare on 2011 February 13, using imaging data provided by the Hinode satellite, and we demonstrated that they conform to the simulation predictions. These results suggest that forecasting of solar eruptions is possible with sophisticated observation of a solar magnetic field, although the lead time must be limited by the timescale of changes in the small magnetic structures.

DOI： 10.1088/0004-637X/760/1/31

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We perform four numerical magnetohydrodynamic simulations in 2.5 dimensions ( 2.5D) of fast coronal mass ejections (CMEs) and their associated shock fronts between 10 Rs and 300 Rs. We investigate the relative change in the shock standoff distance, Delta, as a fraction of the CME radial half-width, D-OB (i.e., Delta/D-OB). Previous hydrodynamic studies have related the shock standoff distance for Earth's magnetosphere to the density compression ratio (DR; rho(u)/rho(d)) measured across the bow shock. The DR coefficient, k(dr), which is the proportionality constant between the relative standoff distance (Delta/D-OB) and the compression ratio, was semi-empirically estimated as 1.1. For CMEs, we show that this value varies linearly as a function of heliocentric distance and changes significantly for different radii of curvature of the CME's leading edge. We find that a value of 0.8 +/- 0.1 is more appropriate for small heliocentric distances (&lt;30 Rs) which corresponds to the spherical geometry of a magnetosphere presented by Seiff. As the CME propagates its cross section becomes more oblate and the k(dr) value increases linearly with heliocentric distance, such that k(dr) = 1.1 is most appropriate at a heliocentric distance of about 80 Rs. For terrestrial distances (215 Rs) we estimate k(dr) = 1.8 +/- 0.3, which also indicates that the CME cross-sectional structure is generally more oblate than that of Earth's magnetosphere. These alterations to the proportionality coefficients may serve to improve investigations into the estimates of the magnetic field in the corona upstream of a CME as well as the aspect ratio of CMEs as measured in situ.

DOI： 10.1088/0004-637X/759/2/103

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

The Solar TErrestrial RElations Observatory (STEREO) provides high cadence and high resolution images of the structure and morphology of coronal mass ejections (CMEs) in the inner heliosphere. CME directions and propagation speeds have often been estimated through the use of time-elongation maps obtained from the STEREO Heliospheric Imager (HI) data. Many of these CMEs have been identified by citizen scientists working within the SolarStormWatch project (www.solarstormwatch.com) as they work towards providing robust real-time identification of Earth-directed CMEs. The wide field of view of HI allows scientists to directly observe the two-dimensional (2D) structures, while the relative simplicity of time-elongation analysis means that it can be easily applied to many such events, thereby enabling a much deeper understanding of how CMEs evolve between the Sun and the Earth. For events with certain orientations, both the rear and front edges of the CME can be monitored at varying heliocentric distances (R) between the Sun and 1 AU. Here we take four example events with measurable position angle widths and identified by the citizen scientists. These events were chosen for the clarity of their structure within the HI cameras and their long track lengths in the time-elongation maps. We show a linear dependency with R for the growth of the radial width (W) and the 2D aspect ratio (.) of these CMEs, which are measured out to approximate to 0.7 AU. We estimated the radial width from a linear best fit for the average of the four CMEs. We obtained the relationships W = 0.14R + 0.04 for the width and chi = 2.5R + 0.86 for the aspect ratio (W and R in units of AU).

DOI： 10.1007/s11207-012-0041-6

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Nonlinear force-free field (NLFFF) extrapolation is a powerful tool for the modeling of the magnetic field in the solar corona. However, since the photospheric magnetic field does not in general satisfy the force-free condition, some kind of processing is required to assimilate data into the model. In this paper, we report the results of new preprocessing for the NLFFF extrapolation. Through this preprocessing, we expect to obtain magnetic field data similar to those in the chromosphere. In our preprocessing, we add a new term concerning chromospheric longitudinal fields into the optimization function proposed by Wiegelmann et al. We perform a parameter survey of six free parameters to find minimum force-and torque-freeness with the simulated-annealing method. Analyzed data are a photospheric vector magnetogram of AR 10953 observed with the Hinode spectropolarimeter and a chromospheric longitudinal magnetogram observed with SOLIS spectropolarimeter. It is found that some preprocessed fields show the smallest force-and torque-freeness and are very similar to the chromospheric longitudinal fields. On the other hand, other preprocessed fields show noisy maps, although the force-and torque-freeness are of the same order. By analyzing preprocessed noisy maps in the wave number space, we found that small and large wave number components balance out on the force-free index. We also discuss our iteration limit of the simulated-annealing method and magnetic structure broadening in the chromosphere.

DOI： 10.1088/0004-637X/752/2/126

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We use autocorrelation to investigate evolution in flow fields inferred by applying Fourier local correlation tracking (FLCT) to a sequence of high-resolution (0 ''.3), high-cadence (similar or equal to 2 minute) line-of-sight magnetograms of NOAA active region (AR) 10930 recorded by the narrowband filter imager of the Solar Optical Telescope aboard the Hinode satellite over 2006 December 12 and 13. To baseline the timescales of flow evolution, we also autocorrelated the magnetograms, at several spatial binnings, to characterize the lifetimes of active region magnetic structures versus spatial scale. Autocorrelation of flow maps can be used to optimize tracking parameters, to understand tracking algorithms' susceptibility to noise, and to estimate flow lifetimes. Tracking parameters varied include: time interval Delta t between magnetogram pairs tracked, spatial binning applied to the magnetograms, and windowing parameter sigma used in FLCT. Flow structures vary over a range of spatial and temporal scales (including unresolved scales), so tracked flows represent a local average of the flow over a particular range of space and time. We define flow lifetime to be the flow decorrelation time, tau. For Delta t &gt; tau, tracking results represent the average velocity over one or more flow lifetimes. We analyze lifetimes of flow components, divergences, and curls as functions of magnetic field strength and spatial scale. We find a significant trend of increasing lifetimes of flow components, divergences, and curls with field strength, consistent with Lorentz forces partially governing flows in the active photosphere, as well as strong trends of increasing flow lifetime and decreasing magnitudes with increases in both spatial scale and Delta t.

DOI： 10.1088/0004-637X/747/2/130

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ASTRONOMICAL SOC PACIFIC  

In order to discuss the propagation of Alfven wave in the real solar atmosphere, we calculate three dimensional magnetic field using magnetic field observation obtained with Hinode and a high resolution potential field model. The modeled field reproduces fun out (canopy) structure rooted to patchy concentrations of magnetic flux (kG-patches) in the polar region. Combined with a atmosphere model, the model shows that most of the Alfven wave propagating along the magnetic field lines can hardly pass through the transition region due to reflection there. The model suggests that strong concentration of the magnetic field may make retardation of Alfven wave phase between neighboring field lines rooted on the edge of the canopy. The situation may be potential situation for Alfven wave dissipation due to phase mixing process.

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ASTRONOMICAL SOC PACIFIC  

In this paper we report shear motions (3-5 km s(-1)) of magnetic elements observed at a preflare of an X3.4 flare (around 02:00 UT on 2006 December 13) in active region 10930. This preflare is observed by X-ray Telescope onboard Hinode. Velocity fields are obtained from filter magnetograms of the Solar Optical Telescope. We also analyzed velocity fields divided into divergent and rotation components. From these velocity components, two local twisting motions are found. One twisting motion continues for 0.5 hours, and another continues at least for 2.5 hours.

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Twist and connectivity of magnetic field lines in the flare-productive active region NOAA 10930 are investigated in terms of the vector magnetograms observed by the Solar Optical Telescope on board the Hinode satellite and the nonlinear force-free field (NLFFF) extrapolation. First, we show that the footpoints of magnetic field lines reconstructed by the NLFFF correspond well to the conjugate pair of highly sheared flare ribbons on the Ca II images, which were observed by Hinode as an X3.4 class flare on 2006 December 13. This demonstrates that the NLFFF extrapolation may be used to analyze the magnetic field connectivity. Second, we find that the twist of magnetic field lines anchored on the flare ribbons increased as the ribbons moved away from the magnetic polarity inversion line in the early phase of the flare. This suggests that magnetic reconnection might commence from a region located below the most strongly twisted field. Third, we reveal that the magnetic flux twisted more than a half turn and gradually increased during the last one day prior to the onset of the flare, and that it quickly decreased for two hours after the flare. This is consistent with the store-and-release scenario of magnetic helicity. However, within this active region, only a small fraction of the flux was twisted by more than one full turn and the field lines that reconnected first were twisted less than one turn. These results imply that the kink mode instability could hardly occur, at least before the onset of flare. Based on our results, we discuss the trigger process of solar flares.

DOI： 10.1088/0004-637X/738/2/161

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST AERONAUT ASTRONAUT  

DOI： 10.2514/1.B34141

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Using a model of particle acceleration and transport in solar flares, we investigate the height distribution of coronal electrons by focusing on the energy-dependent pitch-angle scattering. When pitch-angle scattering is not included, the peak heights of loop-top electrons are constant, regardless of their energy, owing to the continuous acceleration and compression of the electrons via shrinkage of magnetic loops. On the other hand, under pitch-angle scattering, the electron heights are energy-dependent: intermediate-energy electrons are at a higher altitude, whereas lower and higher energy electrons are at lower altitudes. This implies that the intermediate-energy electrons are inhibited from following the shrinking field lines to lower altitudes because pitch-angle scattering causes efficient precipitation of these electrons into the footpoint and their subsequent loss from the loop. This result is qualitatively consistent with the position of the above-the-loop-top hard X-ray (HXR) source that is located above coronal HXR loops emitted by lower energy electrons and microwaves emitted by higher energy electrons. Quantitative agreement with observations might be achieved by considering primary acceleration before the onset of loop shrinkage and additional pitch-angle scattering via wave-particle interactions.

DOI： 10.1088/0004-637X/732/2/111

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Interplanetary coronal mass ejections (ICMEs) are often observed to travel much faster than the ambient solar wind. If the relative speed between the two exceeds the fast magnetosonic velocity, then a shock wave will form. The Mach number and the shock standoff distance ahead of the ICME leading edge is measured to infer the vertical size of an ICME in a direction that is perpendicular to the solar wind flow. We analyze the shock standoff distance for 45 events varying between 0.5AU and 5.5AU in order to infer their physical dimensions. We find that the average ratio of the inferred vertical size to measured radial width, referred to as the aspect ratio, of an ICME is 2.8 +/- 0.5. We also compare these results to the geometrical predictions from Paper I that forecast an aspect ratio between 3 and 6. The geometrical solution varies with heliocentric distance and appears to provide a theoretical maximum for the aspect ratio of ICMEs. The minimum aspect ratio appears to remain constant at 1 (i.e., a circular cross section) for all distances. These results suggest that possible distortions to the leading edge of ICMEs are frequent. But, these results may also indicate that the constants calculated in the empirical relationship correlating the different shock front need to be modified; or perhaps both distortions and a change in the empirical formulae are required.

DOI： 10.1088/0004-637X/732/2/117

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

At launch, coronal mass ejections (CMEs) are often approximated as locally cylindrical objects with circular cross sections. However, CMEs have long been known to propagate almost radially away from the Sun along with the bulk solar wind. This has important consequences for the structure of CMEs; an initially circular cross section will be severely flattened by this radial motion. Yet calculations of total flux and helicity transport by CMEs based on in situ observations still use the assumption of a locally cylindrical object. In this paper, we investigate the morphology of an interplanetary CME based upon geometric arguments. By radially propagating an initial cylindrical object that maintains a constant ratio between its expansion speed and bulk flow, A, we show that the flattening, or "pancaking," of the two-dimensional cross section effectively ceases; the aspect ratios of these CMEs converge to a fixed value as they propagate further into the heliosphere. Thereafter the CME morphology is scale invariant. We predict aspect ratios of 5 +/- 1 at terrestrial distances. By correlating a planetary shock with an interplanetary shock linked to a CME, these aspect ratios are estimated using in situ measurements in Paper II. These estimates are made at various heliocentric distances.

DOI： 10.1088/0004-637X/731/2/109

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Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

Two mission concepts (plan A: out-of-ecliptic mission and plan B: high resolution spectroscopic mission) have been studied for the next Japanese-led solar mission Solar-C, which will follow the scientific success of the Hinode mission. The both mission concepts are concluded as equally important and attractive for the promotion of space solar physics. In the meantime we also had to make efforts for prioritizing the two options, in order to proceed to next stage of requesting the launch of Solar-C mission at the earliest opportunity. This paper briefly describes the two mission concepts and the current status on our efforts for prioritizing the two options. More details are also described for the plan B option as the first-priority Solar-C mission. The latest report from the Solar-C mission concept studies was documented as "Interim Report on the Solar-C Mission Concept."

DOI： 10.1117/12.893228

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Language：English   Publishing type：Research paper (scientific journal)  

Several divergence-cleaning techniques for multi-dimensional Godunov-type magnetohydrodynamic schemes are comparatively investigated. We also propose a new divergence-cleaning technique that is improved from an earlier projection method to improve the robustness. © 2011 The Japan Society of Plasma Science and Nuclear Fusion Research.

DOI： 10.1585/pfr.6.2406124

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

A magnetohydrodynamic (MHD) algorithm for global simulations of planetary magnetospheres is developed based on an approximate nonlinear Riemann solver, the so-called Harten-Lax-van Leer-Discontinuities (HLLD) approximate Riemann solver. An approximate nonlinear solution of the MHD Riemann problem, in which the contributions of the background potential magnetic field are subtracted and multispecies plasmas as well as general equation of state are included, can be algebraically obtained under the assumptions that the normal velocity and the background potential magnetic field in the Riemann fan are constant. The theoretical aspects of the HLLD approximate Riemann solver are focused on, in particular.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

A magnetohydrodynamic (MHD) algorithm for global simulations of planetary magnetospheres is developed based on an approximate nonlinear Riemann solver, the so-called Harten-Lax-van Leer-Discontinuities (HLLD) approximate Riemann solver. An approximate nonlinear solution of the MHD Riemann problem, in which the contributions of the background potential magnetic field are subtracted and multispecies plasmas as well as general equation of state are included, can be algebraically obtained under the assumptions that the normal velocity and the background potential magnetic field in the Riemann fan are constant. The theoretical aspects of the HLLD approximate Riemann solver are focused on, in particular.

DOI： 10.1109/TPS.2010.2057451

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We performed a magnetohydrodynamic simulation of a formation process of coronal mass ejections (CMEs), focusing on the interaction (reconnection) between an ejecting flux rope and its ambient field. We examined three cases with different ambient fields: one had no ambient field, while the other two had dipole fields with opposite directions, parallel and anti-parallel to that of the flux rope surface. We found that while the flux rope disappears in the anti-parallel case, in the other cases the flux ropes can evolve to CMEs and show different amounts of flux rope rotation. The results imply that the interaction between an ejecting flux rope and its ambient field is an important process for determining CME formation and CME orientation, and also show that the amount and direction of the magnetic flux within the flux rope and the ambient field are key parameters for CME formation. The interaction (reconnection) plays a significant role in the rotation of the flux rope especially with a process similar to "tilting instability" in a spheromak-type experiment of laboratory plasma.

DOI： 10.1088/0004-637X/718/2/1305

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

We performed a magnetohydrodynamic simulation of a formation process of coronal mass ejections (CMEs), focusing on the interaction (reconnection) between an ejecting flux rope and its ambient field. We examined three cases with different ambient fields: one had no ambient field, while the other two had dipole fields with opposite directions, parallel and anti-parallel to that of the flux rope surface. We found that while the flux rope disappears in the anti-parallel case, in the other cases the flux ropes can evolve to CMEs and show different amounts of flux rope rotation. The results imply that the interaction between an ejecting flux rope and its ambient field is an important process for determining CME formation and CME orientation, and also show that the amount and direction of the magnetic flux within the flux rope and the ambient field are key parameters for CME formation. The interaction (reconnection) plays a significant role in the rotation of the flux rope especially with a process similar to "tilting instability" in a spheromak-type experiment of laboratory plasma.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

A 3-D magnetohydrodynamic (MHD) simulation is performed to reconstruct the interplanetary propagation of a coronal mass ejection (CME) that occurred on 13 December 2006. A spheromak-type magnetic field is superposed on a realistic ambient solar wind to reproduce the large-scale interplanetary magnetic field (IMF) associated with the CME. Here we show that a westward and southward directed spheromak CME with reasonable geometric, dynamic, and magnetic parameters reproduces the magnetic cloud, interplanetary shock, and sheath profiles as observed by in situ spacecraft. We suggest that the simple solar wind model developed in this study is topologically complex enough to be consistent with in situ observations, such as southward IMF associated with CMEs.

DOI： 10.1029/2009JA014167

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

A 3-D magnetohydrodynamic (MHD) simulation is performed to reconstruct the interplanetary propagation of a coronal mass ejection (CME) that occurred on 13 December 2006. A spheromak-type magnetic field is superposed on a realistic ambient solar wind to reproduce the large-scale interplanetary magnetic field (IMF) associated with the CME. Here we show that a westward and southward directed spheromak CME with reasonable geometric, dynamic, and magnetic parameters reproduces the magnetic cloud, interplanetary shock, and sheath profiles as observed by in situ spacecraft. We suggest that the simple solar wind model developed in this study is topologically complex enough to be consistent with in situ observations, such as southward IMF associated with CMEs.

Language：English   Publishing type：Research paper (scientific journal)  

A novel, particle-based, probabilistic approach for the Simulation of cloud microphysics is proposed, which is named the super-droplet method (SDM). This method enables the accurate simulation of cloud microphysics with a less demanding cost in Computation. SDM is applied to a warm-cloud system, which incorporates sedimentation, condensation/evaporation and stochastic coalescence. The methodology to couple super-droplets and a non-hydrostatic model is also developed. It is confirmed that the result of our Monte Carlo scheme for the stochastic coalescence of super-droplets agrees fairly well with the solutions of the stochastic coalescence equation. The behaviour of the model is evaluated using a simple test problem, that of a shallow maritime cumulus formation initiated by a warm bubble. Possible extensions of SDM are briefly discussed. A theoretical analysis suggests that the Computational cost of SDM becomes lower than the spectral (bin) method when the number of attributes - the variables that identify the state of each super-droplet - becomes larger than some critical value, which we estimate to be in the range 2 similar to 4. Copyright (C) 2009 Royal Meteorological Society

Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-BLACKWELL  

A novel, particle-based, probabilistic approach for the Simulation of cloud microphysics is proposed, which is named the super-droplet method (SDM). This method enables the accurate simulation of cloud microphysics with a less demanding cost in Computation. SDM is applied to a warm-cloud system, which incorporates sedimentation, condensation/evaporation and stochastic coalescence. The methodology to couple super-droplets and a non-hydrostatic model is also developed. It is confirmed that the result of our Monte Carlo scheme for the stochastic coalescence of super-droplets agrees fairly well with the solutions of the stochastic coalescence equation. The behaviour of the model is evaluated using a simple test problem, that of a shallow maritime cumulus formation initiated by a warm bubble. Possible extensions of SDM are briefly discussed. A theoretical analysis suggests that the Computational cost of SDM becomes lower than the spectral (bin) method when the number of attributes - the variables that identify the state of each super-droplet - becomes larger than some critical value, which we estimate to be in the range 2 similar to 4. Copyright (C) 2009 Royal Meteorological Society

DOI： 10.1002/qj.441

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPRINGER-VERLAG BERLIN  

A multiscale simulation method for gas detonations is proposed. This method is performed by the interlocking of a continuum fluid model and a particle based molecular model. The simulation system is spatially divided into the two kinds of domains, in one of which the near-thermal equilibrium is satisfied but not in the other. The dynamics of the near-thermal equilibrium domains is modeled by the continuum fluid dynamic equations. In the nonthermal equilibrium domain, which corresponds to the denotation front, the dynamics is modeled by the Boltzmann equation. This method has been tested on a model detonation system.

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Agency for Marine-Earth Science and Technology  

The operation of the Earth Simulator started in March, 2002, and 7 years have passed. For these years, the Earth Simulator has been used by not only researchers of JAMSTEC but also many researchers of the domestic and international institutions, and many remarkable results have been made. The Earth Simulator Center was established at the same time as the birth of the Earth Simulator. The researchers of the Earth Simulator Center have also developed new simulation technologies.<br>The simulation is the only one method by which we can predict the future scientifically. Therefore, it has the important role in the science as well as the technology which can contribute to the life of the people, and has the important meaning in modeling the future of the human beings.

DOI： 10.5918/jamstecr.9.1_75

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

The reversal mechanism of a dipole magnetic field generated by dynamo action in a rotating spherical shell is investigated by a three-dimensional nonlinear magnetohydrodynamic simulation as well as a linear stability analysis. The emphasis of the study is on understanding the relationship between dipole reversal and the symmetry properties of the dynamo solution. As a result, first, it is found that there is a threshold of the magnetic Prandtl number, below which the dipole field is never reversed, and above which the reversal occurs at irregular intervals like the paleomagnetic evolution of the geodynamo. Second, it is shown that the dynamo process responsible for the generation of a dipole field (called "a-dynamo" in this paper) consists only of the antimirror symmetric magnetic field and the mirror symmetric velocity field with respect to the equatorial plane. Third, it is found that the components of the opposite symmetry to the a-dynamo grow only during the polarity reversal events and quickly decay afterwards. This indicates that the dipole field reversal and the loss of equatorial symmetry are tightly connected. In fact, it is clearly demonstrated by numerical analyses that the a-dynamo process is linearly unstable for the perturbation of opposite symmetry when the magnetic Prandtl number exceeds the threshold for dipole reversal. Mode coupling between the longitudinal Fourier components plays a crucial role in creating the instability. Based on the above results, it is proposed that symmetry-breaking instability could be the mechanism for dipole field reversal in the geodynamo process. The energy conversion between components of different symmetry is also analyzed in the quasistable polarity phase and in the polarity reversal phase, respectively. (C) 2008 American Institute of Physics.

DOI： 10.1063/1.2959120

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：UNIV CHICAGO PRESS  

DOI： 10.1086/588021

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

Solar flares and coronal mass ejections are associated with rapid changes in field connectivity and are powered by the partial dissipation of electrical currents in the solar atmosphere. A critical unanswered question is whether the currents involved are induced by the motion of preexisting atmospheric magnetic flux subject to surface plasma flows or whether these currents are associated with the emergence of flux from within the solar convective zone. We address this problem by applying state-of-the-art nonlinear force-free field (NLFFF) modeling to the highest resolution and quality vector-magnetographic data observed by the recently launched Hinode satellite on NOAA AR 10930 around the time of a powerful X3.4 flare. We compute 14 NLFFF models with four different codes and a variety of boundary conditions. We find that the model fields differ markedly in geometry, energy content, and force-freeness. We discuss the relative merits of these models in a general critique of present abilities to model the coronal magnetic field based on surface vector field measurements. For our application in particular, we find a fair agreement of the best-fit model field with the observed coronal configuration, and argue (1) that strong electrical currents emerge together with magnetic flux preceding the flare, (2) that these currents are carried in an ensemble of thin strands, ( 3) that the global pattern of these currents and of field lines are compatible with a large-scale twisted flux rope topology, and (4) that the similar to 10(32) erg change in energy associated with the coronal electrical currents suffices to power the flare and its associated coronal mass ejection.

Language：English   Publishing type：Research paper (scientific journal)  

The quantitative relationship between the magnetohydrodynamic ( MHD) activity of solar coronal arcade and the magnetic helicity injection, which is caused by shearing motion, has been investigated, using azimuthally symmetric model of MHD simulation. We have calculated several cases in which the width of the shearing region is varied and examined the relationship between the magnetic arcade dynamics and magnetic helicity evolution. As a result, it is found that as the shearing motion is imposed on narrower regions along each side of the magnetic inversion line, the magnetic arcade can be easily destabilized by the resistive tearing mode. However, in this case, even though reconnection driven by the tearing mode produces plasmoids, the plasmoid elevation is almost in proportion to the total amount of magnetic helicity contained in the arcade, and it is too slow to explain the trigger process of coronal mass ejections ( CMEs). On the other hand, in the case where the shearing motion is imposed on the entire region, much larger magnetic helicity injection is required to injected arcade in order to destabilize the system, compared to practical helicity injection measured in the solar corona. The results suggest that it may be difficult to trigger a CME just by the axisymmetric shearing motion and that some other mechanisms should be involved in the triggering process of a CME. The results also imply that the relation between the magnetic helicity and the overlying magnetic flux can be a key parameter for the CME occurrence.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

The quantitative relationship between the magnetohydrodynamic ( MHD) activity of solar coronal arcade and the magnetic helicity injection, which is caused by shearing motion, has been investigated, using azimuthally symmetric model of MHD simulation. We have calculated several cases in which the width of the shearing region is varied and examined the relationship between the magnetic arcade dynamics and magnetic helicity evolution. As a result, it is found that as the shearing motion is imposed on narrower regions along each side of the magnetic inversion line, the magnetic arcade can be easily destabilized by the resistive tearing mode. However, in this case, even though reconnection driven by the tearing mode produces plasmoids, the plasmoid elevation is almost in proportion to the total amount of magnetic helicity contained in the arcade, and it is too slow to explain the trigger process of coronal mass ejections ( CMEs). On the other hand, in the case where the shearing motion is imposed on the entire region, much larger magnetic helicity injection is required to injected arcade in order to destabilize the system, compared to practical helicity injection measured in the solar corona. The results suggest that it may be difficult to trigger a CME just by the axisymmetric shearing motion and that some other mechanisms should be involved in the triggering process of a CME. The results also imply that the relation between the magnetic helicity and the overlying magnetic flux can be a key parameter for the CME occurrence.

DOI： 10.1029/2007JA012516

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Solar flares and coronal mass ejections are associated with rapid changes in field connectivity and are powered by the partial dissipation of electrical currents in the solar atmosphere. A critical unanswered question is whether the currents involved are induced by the motion of preexisting atmospheric magnetic flux subject to surface plasma flows or whether these currents are associated with the emergence of flux from within the solar convective zone. We address this problem by applying state-of-the-art nonlinear force-free field (NLFFF) modeling to the highest resolution and quality vector-magnetographic data observed by the recently launched Hinode satellite on NOAA AR 10930 around the time of a powerful X3.4 flare. We compute 14 NLFFF models with four different codes and a variety of boundary conditions. We find that the model fields differ markedly in geometry, energy content, and force-freeness. We discuss the relative merits of these models in a general critique of present abilities to model the coronal magnetic field based on surface vector field measurements. For our application in particular, we find a fair agreement of the best-fit model field with the observed coronal configuration, and argue (1) that strong electrical currents emerge together with magnetic flux preceding the flare, (2) that these currents are carried in an ensemble of thin strands, ( 3) that the global pattern of these currents and of field lines are compatible with a large-scale twisted flux rope topology, and (4) that the similar to 10(32) erg change in energy associated with the coronal electrical currents suffices to power the flare and its associated coronal mass ejection.

DOI： 10.1086/527413

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ASTRONOMICAL SOC PACIFIC  

We have developed a new interlocked simulation model to connect an Ion-Particle Hybrid simulation model (Hybrid) and an Energetic-Particle Hybrid simulation (EP-Hyb) model. In the conventional Ion-Particle Hybrid model, all ions are kinetically treated as particles. In the EP-Hyb model, non-thermal energetic ions are kinetically treated, and the thermal component is calculated as a fluid. The interlocked model is applied to an one-dimensional collision-less shock problem. The data of each hybrid model are exchanged through the boundary at every time-step. Resultantly, the present simulation can handle the full ion kinetics to investigate the injection problem at the shock transition region by the Hybrid model, and also the wave-particle interactions even in the far upstream region to investigate the diffusive acceleration process by the EP-Hyb model. Since the calculation cost of the EP-Hyb model is much smaller than that of the Hybrid model, with the interlocked simulation we can considerably reduce the computational demand.

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ASTRONOMICAL SOC PACIFIC  

Robust and efficient approximate Riemann solvers for magnetohydrodynamics (MHD) are constructed. Particularly, a family of positively conservative Harten-Lax-van Leer (HLL)-type Riemann solvers, the so-called HLLD ('D' denotes Discontinuities), HLLR ('R' denotes Rotational), HLLC ('C' denotes Contact), and HLL solvers, is systematically considered.

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ASTRONOMICAL SOC PACIFIC  

Active region NOAA 10930 produced the first X-class flare observed by the Hinode satellite. Fortunately, the data covers the time span before and after the X-class flare arising on December 13, 2006 in this active region. During this event, the Hinode satellite clearly observed the typical features of a flare: two ribbon and post flare loop structures seen by SOT, sigmoidal structure before the onset, and subsequent cusp loop structure seen by XRT. We analyzed the three-dimensional magnetic structure of the region before and after the flare on Dec.13, 2006 using the Non-Linear Force-Free (NLFF) extrapolation method based on extended magnetofrictional method.
As a result of the NLFF extrapolation, we found that, before the flare onset, strong sheared structures were formed on the neutral line. Furthermore, we revealed that a sigmoidal structure was not formed of a single loop, but was composed of strong multiple sheared field. On other the hand, after the flare, elongated magnetic flux is partially formed and a part of magnetic shear was released. The result indicates that the part; of NLFF relaxes toward the potential like field in this event.

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

Recently, several methods that measure the velocity of magnetized plasma from time series of photospheric vector magnetograms have been developed. Velocity fields derived using such techniques can be used both to determine the fluxes of magnetic energy and helicity into the corona, which have important consequences for understanding solar flares, coronal mass ejections, and the solar dynamo, and to drive time-dependent numerical models of coronal magnetic fields. To date, these methods have not been rigorously tested against realistic, simulated data sets, in which the magnetic field evolution and velocities are known. Here we present the results of such tests using several velocity-inversion techniques applied to synthetic magnetogram data sets, generated from anelastic MHD simulations of the upper convection zone with the ANMHD code, in which the velocity field is fully known. Broadly speaking, the MEF, DAVE, FLCT, IM, and ILCT algorithms performed comparably in many categories. While DAVE estimated the magnitude and direction of velocities slightly more accurately than the other methods, MEF's estimates of the fluxes of magnetic energy and helicity were far more accurate than any other method's. Overall, therefore, the MEF algorithm performed best in tests using the ANMHD data set. We note that ANMHD data simulate fully relaxed convection in a high-beta plasma, and therefore do not realistically model photospheric evolution.

Language：English   Publishing type：Research paper (scientific journal)  

Many kinds of simulation models have been developed to understand the complex plasma systems. However, these simulation models have been separately performed because the fundamental assumption of each model is different and restricts the physical processes in each spatial and temporal scales. On the other hand, it is well known that the interactions among the multiple scales may play crucial roles in the plasma phenomena (e.g. magnetic reconnection. collisionless shock), where the kinetic processes in the micro-scale may interact with the global structure in the fluid dynamics. To take self-consistently into account such multi-scale phenomena, we have developed a new simulation model by directly interlocking the fluid simulation of the magnetohyrdodynamics (MHD) model and the kinetic simulation of the particle-in-cell (PIC) model. The PIC domain is embedded in a small part of MHD domain. The both simulations are performed simultaneously in each domain and the bounded data are frequently exchanged each other to keep the consistency between the models. We have applied our new interlocked simulation to Alfven wave propagation problem as a benchmark test and confirmed that the waves can propagate smoothly through the boundaries of each domain. (C) 2007 Elsevier Inc. All rights reserved.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACADEMIC PRESS INC ELSEVIER SCIENCE  

Many kinds of simulation models have been developed to understand the complex plasma systems. However, these simulation models have been separately performed because the fundamental assumption of each model is different and restricts the physical processes in each spatial and temporal scales. On the other hand, it is well known that the interactions among the multiple scales may play crucial roles in the plasma phenomena (e.g. magnetic reconnection. collisionless shock), where the kinetic processes in the micro-scale may interact with the global structure in the fluid dynamics. To take self-consistently into account such multi-scale phenomena, we have developed a new simulation model by directly interlocking the fluid simulation of the magnetohyrdodynamics (MHD) model and the kinetic simulation of the particle-in-cell (PIC) model. The PIC domain is embedded in a small part of MHD domain. The both simulations are performed simultaneously in each domain and the bounded data are frequently exchanged each other to keep the consistency between the models. We have applied our new interlocked simulation to Alfven wave propagation problem as a benchmark test and confirmed that the waves can propagate smoothly through the boundaries of each domain. (C) 2007 Elsevier Inc. All rights reserved.

DOI： 10.1016/j.jcp.2007.09.011

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Recently, several methods that measure the velocity of magnetized plasma from time series of photospheric vector magnetograms have been developed. Velocity fields derived using such techniques can be used both to determine the fluxes of magnetic energy and helicity into the corona, which have important consequences for understanding solar flares, coronal mass ejections, and the solar dynamo, and to drive time-dependent numerical models of coronal magnetic fields. To date, these methods have not been rigorously tested against realistic, simulated data sets, in which the magnetic field evolution and velocities are known. Here we present the results of such tests using several velocity-inversion techniques applied to synthetic magnetogram data sets, generated from anelastic MHD simulations of the upper convection zone with the ANMHD code, in which the velocity field is fully known. Broadly speaking, the MEF, DAVE, FLCT, IM, and ILCT algorithms performed comparably in many categories. While DAVE estimated the magnitude and direction of velocities slightly more accurately than the other methods, MEF's estimates of the fluxes of magnetic energy and helicity were far more accurate than any other method's. Overall, therefore, the MEF algorithm performed best in tests using the ANMHD data set. We note that ANMHD data simulate fully relaxed convection in a high-beta plasma, and therefore do not realistically model photospheric evolution.

DOI： 10.1086/522422

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ASTRONOMICAL SOC PACIFIC  

In this study, we investigated magnetic helicity injection rates among active regions of various sizes. We analyzed 78 active regions (more than 600 magnetograms), using the vector magnetograms obtained with the Solar Flare Telescope of NAOJ and SoHO/MDI magnetograms. We used a method proposed by Kusano et al. (2002, 2004) to evaluate the helicity injection. Magnetic fluxes of analyzed regions are from 2 x 10(12) Wb to 4 x 10(14) Wb. Unsigned magnetic helicity injection rates are from 10(19) Wb(2) s(-1) to 10(23) Wb(2) S-1. From a scatter plot of the magnetic flux and the unsigned magnetic helicity injection rate, we found that the magnetic helicity injection rate has an upper limit for a given value of the magnetic flux, and this upper limit is inversely proportional to the magnetic flux. We discuss these results with a model of the helicity injection due to helical turbulence (E-Effect, Longcope, Fisher, & Pevtsov 1998).

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPRINGER-VERLAG BERLIN  

A new methodology for the, simulation of multiscale processes, called Macro-Micro Interlocked (MMI) Simulation, is introduced. The MMI simulation is carried out by the two-way connection of different numerical models, which may handle macroscopic and microscopic dynamics, respectively. The MMI simulation are applied to several multiscale phenomena, for instance, cloud formation, gas detonation, and plasma dynamics. The results indicate that the MMI simulation provide us an effective and prospective framework for multiscale simulation.

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ASTRONOMICAL SOC PACIFIC  

We numerically investigated the three-dimensional (3D) stability and the nonlinear dynamics of flux tube embedded in magnetic arcade. As a results, we found that the flux tube is unstable to the kink mode instability, as the system approach to the loss-of-equilibrium state. The 3D simulation shows that when the flux tube is long enough, it can escape from arcade with almost constant speed after the accelerated launching due to the kink instability. In the case of the short flux tube, however, the flux tube ascending is failed at some height. The strong current sheet is formed in the lower part of the long flux tube at late phase as a consequence of the instability, whereas the current sheet can not be maintained in the case of short flux tube. Therefore, the formation of the current sheet at late phase may have some relation to the ejective eruption of the flux tube.

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ASTRONOMICAL SOC PACIFIC  

Some important observations have shown that the strong correlation between emerging flux and eruptions of quiescent filaments (Feynman & Martin, 1995; Wang & Sheeley, 1999). Chen & Shibata (2000) performed two-dimensional simulations including a flux rope in the corona, and their results suggest that the eruption process is triggered by the emerging flux through the reconnection. Our purpose in this paper is to investigate that '' how a filament is produced and how an eruption process can be initiated by the emerging flux and what effects of three-dimensionality appear in the process of eruption ''. For that purpose, we performed three-dimensional numerical simulations of the emerging flux model. nom our results, a filamentary structure is produced from the coronal arcade field by the reconnection process, and when the reconnection process proceeds effectively, the produced structure is ejected by the magnetic force. These processes can thought to be a new mechanism of the eruption which is different from, for example, the one in Fan & Gibson (2004).

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ASTRONOMICAL SOC PACIFIC  

The formation mechanism of sigmoidal structure in the solar corona and the transition process from long-lived sigmoid to flare eruption are studied on the basis of the Taylor's minimum energy principle and the three-dimensional magnetohydrodynamic numerical simulations. The simulation results indicate that magnetic reconnection driven by the resistive tearing mode instability growing on current sheet, where magnetic shear is steeply reversed, may cause the formation of sigmoidal structure. The mechanism of that can be explained as the spontaneous manifestation of the Taylor's minimum energy state in magnetic arcade geometry. Furthermore, it is also numerically demonstrated that the formation of sigmoid can be followed by the eruption of magnetic flux. In the eruption, however, the sigmoidal flux itself is not ejected upward, but it is collapsed by the reconnection with overlaying magnetic field. The results are consistent with the reversed shear flare model recently proposed by Kusano et al., which predicts that the coronal eruption is triggered by the collapsing of magnetic arcade due to the annihilation of positive and negative magnetic helicity.

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)  

The compute power required in the simulation of mutually cooperating systems in the physical sciences is huge. Therefore, it is common to simulate one system and treat others as parameters or environmental conditions to the simulated system. Macro-Micro Interlocked (MMI) simulation framework has been proposed to simulate such cooperating systems. In this paper, we study the feasibility of applying this framework on the simulation of macro-micro economic model. This simulation allows us to observe the system's behaviour at the macro and micro levels concurrently. Policy changes at the macroeconomic level may affect the behaviour of the entities at the microeconomic level. Similarly, the behaviour changes at the microeconomic level may effect the macroeconomic system. We implement an MMI simulation library suitable for execution on a cluster of PCs. At this early stage, the experiment shows a promising result which provides us with a foundation to experiment with a larger model. © 2007 IEEE.

DOI： 10.1109/PADS.2007.22

Scopus

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

Three-dimensional magnetohydrodynamics of the Solar coronal plasma is investigated by numerical simulation, aiming to understand the mechanisms of the Solar flare onset. It is demonstrated by the simulations that the resistive tearing mode instability growing on the magnetic shear inversion layer can drive the large-scale eruption through the mutual excitation of double reconnections. It is also revealed that the instability is able to cause the magnetohydrodynamic energy relaxation, in which the typical sigmoidal structure is self-organized prior to the onset of eruption. The Simulation results predict that both the formation of sigmoids and the onset of flares should occur around the electric current sheet where the magnetic shear is steeply reversed. It is consistent with the reversed-shear flare model and the vector magnetograph observations.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：CAMBRIDGE UNIV PRESS  

A new simulation model is developed, in which the interaction between the macroscopic and microscopic plasma processes is able to be taken into account self-consistently. Magnetohydrodynamic (MHD) simulation and particle-in-cell (PIC) simulation are directly connected and performed simultaneously. This MHD-PIC connection model is first applied to the study of the quiet auroral are formation process. It is an example of our challenge to create the holistic simulation framework, that is, the heterogeneous schemes that can be unified by the state-of-the-art numerical technique. In this paper, we will explain the algorithm of our new model and show the results which have been calculated on the Earth Simulator.

DOI： 10.1017/S0022377806005356

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：CAMBRIDGE UNIV PRESS  

Three-dimensional magnetohydrodynamics of the Solar coronal plasma is investigated by numerical simulation, aiming to understand the mechanisms of the Solar flare onset. It is demonstrated by the simulations that the resistive tearing mode instability growing on the magnetic shear inversion layer can drive the large-scale eruption through the mutual excitation of double reconnections. It is also revealed that the instability is able to cause the magnetohydrodynamic energy relaxation, in which the typical sigmoidal structure is self-organized prior to the onset of eruption. The Simulation results predict that both the formation of sigmoids and the onset of flares should occur around the electric current sheet where the magnetic shear is steeply reversed. It is consistent with the reversed-shear flare model and the vector magnetograph observations.

DOI： 10.1017/S0022377806005423

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：CAMBRIDGE UNIV PRESS  

The CAVE-type virtual reality (VR) system was introduced for scientific visualization of large-scale data in the plasma simulation community about a decade ago. Since then, we have been developing a VR visualization software. VFIVE, for general CAVE systems. Recently, we have integrated an open-source visualization library, the Visualization Toolkit (VTK), into VFIVE. Various visualization methods of TTK can be incorporated and used interactively in VFIVE.

DOI： 10.1017/S0022377806005253

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The three-dimensional stability and the nonlinear dynamics of a flux rope embedded in a magnetic arcade are investigated using magnetohydrodynamic (MHD) simulations, with the goal of understanding the mechanism of filament eruption in the solar corona. The flux rope equilibrium proposed by Forbes in 1990 is adopted as the initial state of the three-dimensional simulation, and we find that the equilibrium is linearly unstable to the kink mode instability, when the system approaches the loss-of-equilibrium state. The three-dimensional simulation demonstrates that when the flux rope is long enough, it can escape from the arcade at an almost constant speed after the accelerated launching phase due to the kink instability. The continuous ascending of the flux rope is driven by the nonlinear growth of several kink modes. In the case of a short flux rope, however, the flux rope ascension fails at some height. This suggests that the flux rope eruption must proceed through the multiple stages, which are driven by the loss of stability in the launching phase and by the loss of equilibrium in the late ascending phase. Since the short rope could not come up to the critical height for the transition from the first to the second stage, the ascending must stop. The role of magnetic reconnection in the late ascending phase and the formation mechanism of density cavity, which corresponds to dimming region observed in CMEs, is also discussed using the simulation results.

DOI： 10.1086/503153

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Two-dimensional, nonlinear magnetohydrodynamic simulations in a compressible plasma to investigate magnetic energy release during the process of forced magnetic reconnection are carried out. This is in order to study the heating of the Sun's corona where it is believed that reconnection is induced by the photospheric motions. A sheared force-free field is perturbed by a transitory slow disturbance (pulse) at the boundary. This disturbance triggers the formation of a current sheet that subsequently releases stored magnetic energy through magnetic reconnection. Previously, it has been shown that for small boundary perturbations, the simulation results are consistent with the previous analytic theory based on a linear approach. For larger amplitude perturbations, or close to the threshold for tearing instability, the evolution shows nonlinear behavior. Solar coronal heating may arise due to a series of reconnection events, and a primary aim of this work is to study the interaction of such heating events. Thus, the perturbations are applied at the boundary by successive pulses. It is found that following the second driving pulse, the current sheet expands along the separatrix before relaxing to a reconnective equilibrium with magnetic islands and releasing even more magnetic energy for the same amplitude perturbation. Here, the previous work is extended to study long wavelength perturbations for which the system exhibits even stronger nonlinear aspects. (c) 2006 American Institute of Physics.

DOI： 10.1063/1.2200630

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The formation mechanism of sigmoidal structure in the solar coronal magnetic field is studied using the three-dimensional magnetohydrodynamic numerical simulations, based on the so-called reversed-shear flare model recently proposed by Kusano et al. The simulation results clearly indicate that magnetic reconnection driven by the resistive tearing mode instability growing on the magnetic shear inversion layer can cause the spontaneous formation of sigmoidal structure. Furthermore, it is also numerically demonstrated that the formation of the sigmoids can be followed by the explosive energy liberation, if the sigmoids contain sufficient magnetic flux. This implies that the reversed-shear flare model can provide a self-consistent explanation for the formation of sigmoids as well as for the onset of eruption, which is driven by magnetic reconnection above sigmoids. The geometric relationship between the sigmoidal structure and the minimum energy state predicted by J. B. Taylor in 1974 is examined. The result suggests that the sigmoidal formation could be understood as a manifestation of the minimum energy state, which has the excess magnetic helicity compared to the bifurcation criterion of the linear force-free field. The consistency with the observations of magnetic helicity is also discussed.

DOI： 10.1086/432570

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACADEMIC PRESS INC ELSEVIER SCIENCE  

A new multi-state Harten-Lax-van Leer (HLL) approximate Riemann solver for the ideal magnetohydrodynamic (MHD) equations is developed based on the assumption that the normal velocity is constant over the Riemann fan. This assumption is same as that used in the HLLC ("C" denotes Contact) approximate Riemann solver for the Euler equations. From the assumption, it is naturally derived that the Riemann fan should consist of four intermediate states for Bx ≠ 0, whereas the number of the intermediate states is reduced to two when Bx = 0. Since the intermediate states satisfied with all jump conditions in this approximate Riemann system are analytically obtained, the multi-state HLL Riemann solver can be constructed straightforwardly. It is shown that this solver can exactly resolve isolated discontinuities formed in the MHD system, and hence named as HLLD Riemann solver. (Here, "D" stands for Discontinuities.) It is also analytically proved that the HLLD Riemann solver is positively conservative like the HLLC Riemann solver. Indeed, the HLLD Riemann solver corresponds to the HLLC Riemann solver when the magnetic field vanishes. Numerical tests demonstrate that the HLLD Riemann solver is more robust and efficient than the linearized Riemann solver, and its resolution is equally good. It indicates that the HLLD solver must be useful in practical applications for the ideal MHD equations. © 2005 Elsevier Inc. All rights reserved.

DOI： 10.1016/j.jcp.2005.02.017

Scopus

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

The formation mechanism of sigmoidal structure in the solar coronal magnetic field is studied using the three-dimensional magnetohydrodynamic numerical simulations, based on the so-called reversed-shear flare model recently proposed by Kusano et al. The simulation results clearly indicate that magnetic reconnection driven by the resistive tearing mode instability growing on the magnetic shear inversion layer can cause the spontaneous formation of sigmoidal structure. Furthermore, it is also numerically demonstrated that the formation of the sigmoids can be followed by the explosive energy liberation, if the sigmoids contain sufficient magnetic flux. This implies that the reversed-shear flare model can provide a self-consistent explanation for the formation of sigmoids as well as for the onset of eruption, which is driven by magnetic reconnection above sigmoids. The geometric relationship between the sigmoidal structure and the minimum energy state predicted by J. B. Taylor in 1974 is examined. The result suggests that the sigmoidal formation could be understood as a manifestation of the minimum energy state, which has the excess magnetic helicity compared to the bifurcation criterion of the linear force-free field. The consistency with the observations of magnetic helicity is also discussed.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We studied the relationship between magnetic helicity injection and the formation of sigmoidal loops. We analyzed seven active regions: three regions showed coronal loops similar to the potential field, and four regions showed the sigmoidal loops. The magnetic helicity injection rate was evaluated using the method proposed by Kusano et al. In order to compare the helicity of regions of various sizes, we defined the normalized helicity injection rate as the magnetic helicity injection rate divided by the magnetic flux squared. We found that the sigmoidal regions and nonsigmoidal regions have comparable normalized helicity injection rates. Next, we calculated the magnetic helicity content of the sigmoidal loops by using the magnetic flux tube model (Longcope & Welsch) and compared it with the magnetic helicity injected from around the footpoints of three sigmoidal loops. For two sigmoidal loops, it is found that these values are comparable. Another loop showed significant disagreement between helicity injection rate and its magnetic helicity content. Excluding this region on the basis of its complexity ( perhaps multiple loops forming a sigmoidal loop), we can conclude that geometric twist of the sigmoidal loops is consistent with the magnetic helicity injected from around the footpoints of the sigmoidal loops.

DOI： 10.1086/429363

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The correlation between magnetic helicity injection across the photosphere and soft X-ray activity in the solar corona is statistically investigated for seven active regions appearing in the years 1997 - 2000. The magnetic helicity flux into the solar corona is analyzed by the induction equation method, using magnetograms observed by the Michelson Doppler Imager on SOHO and by the vector magnetograph at the National Astronomical Observatory of Japan. Soft X-ray activity is evaluated from the data observed by the Yohkoh soft X-ray telescope (SXT). Soft X-ray activity of active regions in nonflare phases is found to correlate better with unsigned magnetic helicity flux than with the simple integration of the magnetic helicity flow. In addition, several magnetic variables, e. g., magnetic flux and electric current flux, are investigated, and it is confirmed that any fluxes given by the area integration of magnetic variables are well correlated with soft X-ray activity. However, for themagnetic helicity flow, not only the whole area flux but also the local intensity correlates well with the soft X-ray intensity. The relation between the spatial structure of the magnetic shear and soft X-ray activity is also investigated, and it is revealed that structural complexity in the magnetic shear tends to increase the efficiency of energy liberation in the solar corona. These results indicate that the magnetic helicity injected from the photosphere is relevant to the heating process in the solar corona, although several magnetic variables, not only magnetic helicity, could be related to that.

DOI： 10.1086/426806

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Magnetic reconnection is a strong candidate for a coronal heating mechanism, and heating by forced magnetic reconnection is investigated here. Two dimensional, nonlinear magnetohydrodynamic simulations are used to investigate forced magnetic reconnection in a compressible plasma. The reconnection occurs when a sheared force-free field is perturbed by a slow disturbance (pulse) at the boundary which is representative of the solar corona where the reconnection is induced by the photospheric motions. The case of driving by successive pulses, which generate a series of heating events which may interact with each other, is considered. This is in order to model the heating of the corona by a series of nanoflare events. For small perturbations, the simulation results are consistent with the previous analytic theory based on linear approach where a current sheet is formed initially at the resonant surface followed by reconnection and then release of magnetic energy. For large amplitude perturbations, or close to the threshold for tearing instability, the system exhibits strong nonlinear aspects. Following the second driving pulse, the current sheet expands along the separatrix before relaxing to a reconnective equilibrium and releasing even more magnetic energy for the same amplitude perturbation. (C) 2005 American Institute of Physics.

DOI： 10.1063/1.1831278

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ASTRONOMICAL SOC PACIFIC  

Generation and annihilation processes of magnetic helicity in solar coronal active regions are investigated based on the observations and the simulations. We first examined the reliability of the numerical techniques, which enable to measure the magnetic helicity flux through the photosphere based on the magnetogram data. Secondly, in terms of the new technique, we found that magnetic helicities of the both signs are simultaneously injected into active regions. Motivated by this result, finally, we investigated the nonlinear process of the magnetic helicity annihilation, using the three-dimensional numerical simulations. The simulations clearly indicated that the helicity reversal can cause the eruption of large-scale plasmoid through the nonlinear process of the resistive instability growing on the helicity inversion layer. From these studies, we point out that the annihilation of magnetic helicity is a key process for the onset mechanism of solar flares.

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：CAMBRIDGE UNIV PRESS  

DOI： 10.1017/S1743921305000669

Web of Science

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

We have investigated the possibility that magnetic reconnection between oppositely sheared magnetic loops works as a trigger mechanism of solar flares, based on three-dimensional numerical simulations. The simulations were carried out by applying a slow footpoint motion, which reverses a preloaded magnetic shear, in the vicinity of the magnetic neutral line. The simulation results clearly indicated that the reversal of magnetic shear can cause a large-scale eruption of the magnetic arcade through a series of two different kinds of magnetic reconnections. The first reconnection is initiated by the resistive-tearing mode instability growing on the magnetic shear inversion layer and annihilates the sheared magnetic fluxes, which are oppositely directed along the magnetic neutral line. As a result of this, the magnetic arcade collapses into the reconnection point, and a new current sheet is generated above and below the shear inversion layer. The generation of new current sheets is followed by another magnetic reconnection, which drives the eruption of the sheared magnetic arcade. Mutual excitation of the two reconnections may explain the explosive property of the flare onset.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：UNIV CHICAGO PRESS  

We have investigated the possibility that magnetic reconnection between oppositely sheared magnetic loops works as a trigger mechanism of solar flares, based on three-dimensional numerical simulations. The simulations were carried out by applying a slow footpoint motion, which reverses a preloaded magnetic shear, in the vicinity of the magnetic neutral line. The simulation results clearly indicated that the reversal of magnetic shear can cause a large-scale eruption of the magnetic arcade through a series of two different kinds of magnetic reconnections. The first reconnection is initiated by the resistive-tearing mode instability growing on the magnetic shear inversion layer and annihilates the sheared magnetic fluxes, which are oppositely directed along the magnetic neutral line. As a result of this, the magnetic arcade collapses into the reconnection point, and a new current sheet is generated above and below the shear inversion layer. The generation of new current sheets is followed by another magnetic reconnection, which drives the eruption of the sheared magnetic arcade. Mutual excitation of the two reconnections may explain the explosive property of the flare onset.

DOI： 10.1086/421547

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

Three dimensional local simulation was performed to investigate the energy conversion and the dynamo action in the magnetohydrodynamic (MHD) turbulence due to the magnetorotational instability (MRI) in a differentially rotating disk, using Arbitrary Lagrangian Eulerian method. As a result, it was found that the turbulence plays a role to convert the magnetic energy to the kinetic energy, if the shear parameter q defined by -dln Ω/dln R is less than 2.3, where Ω and R are the angular velocity and the disk radius, respectively. It means that amplification of the magnetic energy in the MRI is mainly due to the twisting effect of differential rotation rather than a result of the turbulent dynamo action. When q ≥ 2.5, the energy conversion rate from the kinetic energy to the magnetic energy is switched to positive, because low modes, which are destabilized by the Rayleigh instability, can work for the turbulent dynamo. On the other hand, it is also revealed that, the α-dynamo effect exists at least for q ≥ 0.75, although the contribution to the amplification of the magnetic energy is very small. From these results, it can be concluded that, even though the MRI is able to amplify the magnetic energy, the turbulence due to the MRI is less effective as a driver of the conventional dynamo process, unless the Rayleigh instability arises. ©2004 The Physical Society of Japan.

DOI： 10.1143/JPSJ.73.94

Scopus

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ASTRONOMICAL SOC PACIFIC  

We analyzed the magnetic helicity injection in NOAA Active Region 8011 with the method proposed by Kusano et al. (2002). It was found that the helical geometry of a sigmoidal loop in this region is consistent with the magnetic helicity injected from the foot points of the loop. We also made a statistical study of the correlation between X-ray intensity and magnetic helicity injection. We found that the sum of the absolute values of the magnetic helicity injection shows good correlation with the X-ray luminosity. The estimated flux of magnetic free energy injection is not highly correlated with the X-ray flux, because the former quantity is overestimated in our simple method.

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ASTRONOMICAL SOC PACIFIC  

Several new methodologies to detect the magnetic helicity injection activity across the photosphere into the solar corona have been developed recently. We briefly review the basic principle of the typical two methods, the so-called local correlation tracking (LCT) method and the induction equation (IE) method. The reliability of them is examined using a three-dimensional model field. The benchmark examination indicates that, if the vector magnetic field data are available, the IE method can greatly improve the accuracy of the helicity flux measurement. The physical implication of the recent results of the magnetic helicity measurement is also discussed. In particular, we emphasize the possibility that not only the intensity but also the complexity of the magnetic helicity injection from the photosphere could activate the energy liberation process in the solar corona.

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1585/jspf.79.496

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Physics Publishing  

On the basis of the three-dimensional numerical simulation, we study the effect of density stratification on the thermal convection in a rotating spherical shell. Three different density-stratified equilibria, which have different polytropic indices, are adopted as the initial state of the simulation. The density ratio between the bottom and the top of the domain is in the range from 2.0 to 10.7, and the Taylor number is fixed to 104. It is found that when the Rayleigh number is slightly above the instability threshold, the convection structure is hardly affected by the difference of stratification, whereas when the Rayleigh number is much larger than the instability threshold, the nonlinear solution in the strongly stratified equilibria bifurcates into the sectorial convection and the quasi-axisymmetric zonal convection. Through the bifurcation from the sectorial solution to the quasi-axisymmetric solution, the sign of the averaged kinetic helicity is changed in each hemisphere (negative to positive in the northern hemisphere). The results suggest that the solar and the stellar convections must be sensitive to the density stratification.

DOI： 10.1086/344138

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Physics Publishing  

We develop a new methodology that can determine magnetic helicity flux as well as Poynting flux across the photosphere based on magnetograph observation. By applying this method, we study the injection mechanism of magnetic helicity and magnetic free energy into the solar corona. In order to derive the helicity and energy fluxes, first the velocity tangential to the solar surface is constructed by applying a correlation tracking technique on the magnetic observation, and second, the velocity component normal to the photosphere is derived from the condition that the magnetic evolution must be consistent with the induction equation. Through this procedure, we can determine the helicity and energy fluxes separately for the shear motion effect and for the flux emergence effect. Based on this new method, NOAA Active Region 8100 was analyzed from 1997 November 1 to 5 using data observed by the Solar and Heliospheric Observatory Michelson Doppler Interferometer and the vector magnetograph at the National Astronomical Observatory of Japan (NAOJ) in Tokyo. The results indicate that the photospheric shear motion and the flux emergence process have equally contributed to the helicity injection and have supplied magnetic helicity of opposite signs into this active region.

DOI： 10.1086/342171

Scopus

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

We develop a new methodology that can determine magnetic helicity flux as well as Poynting flux across the photosphere based on magnetograph observation. By applying this method, we study the injection mechanism of magnetic helicity and magnetic free energy into the solar corona. In order to derive the helicity and energy fluxes, first the velocity tangential to the solar surface is constructed by applying a correlation tracking technique on the magnetic observation, and second, the velocity component normal to the photosphere is derived from the condition that the magnetic evolution must be consistent with the induction equation. Through this procedure, we can determine the helicity and energy fluxes separately for the shear motion effect and for the flux emergence effect. Based on this new method, NOAA Active Region 8100 was analyzed from 1997 November 1 to 5 using data observed by the Solar and Heliospheric Observatory Michelson Doppler Interferometer and the vector magnetograph at the National Astronomical Observatory of Japan (NAOJ) in Tokyo. The results indicate that the photospheric shear motion and the flux emergence process have equally contributed to the helicity injection and have supplied magnetic helicity of opposite signs into this active region.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Physics Publishing  

Three-dimensional magnetohydrodynamic (MHD) instability in the solar coronal magnetic arcades, which are subject to the constant shearing motion at the feet, was investigated based on the numerical simulations. It was found that the magnetic arcade instability for three-dimensional modes undulating the magnetic arcade can grow if the aspect ratio (defined as the ratio of the arcade length to the width) is larger than unity. In particular, when the aspect ratio is much larger than unity, the nonlinear coupling among several modes dominates the dynamics. As a result, in a very long magnetic arcade, three-dimensional reconnection, which takes place in a limited region in the magnetic arcade, may proceed continuously.

DOI： 10.1086/339910

Scopus

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

We developed a new methodology to measure magnetic helicity as well as magnetic energy injection into the solar corona from magnetograph observations, and we studied the relationship between helicity injection and X-ray activity in the solar corona. In order to calculate the gauge-invariant helicity flux and the Poynting flux across the photosphere, first the velocity tangential to the photospheric surface is constructed by applying the correlation tracking technique on SOHO/MDI observations, and second the normal velocity is calculated by solving the induction equation as an inverse-problem using data from SOHO/MDI and the Solar Flare Telescope at NAOJ in Tokyo. The helicity injection as well as the free energy build-up is analyzed for active region NOAA 8100 from November 1 to 4, 1997. The results indicate that the emerging flux and the shear flow inject magnetic helicity of opposite sign (positive and negative, respectively) into the active region prior to a series of flares. Furthermore it is found that during the helicity injection process magnetic free energy in the amount of 5 x 1032 erg was supplied, which is much more than the X-ray flux emitted by the flares.

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

Two-dimensional numerical simulations are used to investigate nonlinear aspects of forced magnetic reconnection in a zero β highly conducting plasma. This is representative of the solar corona, where reconnection may be induced by external perturbations, e.g., at the photospheric boundary of the corona. The aim is to investigate the energy dissipation by the reconnection, which may provide a mechanism for heating the coronal plasma. The field is taken to he initially a sheared force-free equilibrium in a slab, and the effects of applying a slow deformation to the boundaries are investigated. Previous analytical studies assuming small departures from the initial equilibrium have found that a current sheet forms during an initial ideal phase of evolution, which subsequently relaxes to a reconnected equilibrium, releasing some magnetic energy. The linear theory predicts that the energy release has a singularity when the field is marginally stable to the tearing mode. The nonlinear evolution of the field is calculated numerically, focusing on the energy release. In particular, the strongly nonlinear behavior is studied in the parameter regime in which the linear theory breaks down. It is found that nonlinearities become strong close to the marginal stability point, and for such highly sheared fields, the energy released by reconnection is large even for weak boundary deformations. © 2001 American Institute of Physics.

DOI： 10.1063/1.1329151

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ASTRONOMICAL SOC PACIFIC  

The density stratification effects on the thermal convection in a rotating spherical shell, which is the representative of the solar convection zone, are investigated by three dimensional numerical simulations. It is found that, the convection structure in the strongly stratified system is switched from parallel cells aligned to the rotation axis to zonal rolles dominated by the longitudinally averaged mode, as the Rayleigh number increases much larger than the stability threshold. Corresponding to this structural transition, the averaged kinetic helicity reverses the sign in each hemisphere (from negative to positive in the northern hemisphere). The results indicate that the density stratification is much important for the nonlinear convection process in the rotating spherical shell.

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：TERRA SCIENTIFIC PUBL CO  

A three-dimensional reconnection process, which is caused by the magnetic arcade instability, is investigated using numerical simulations. It is found that the aspect ratio (defined as the ratio of the length to the width of the magnetic arcade) is a crucial parameter to control the nonlinear dynamics. When the aspect ratio is rather small, the dynamics is governed by a symmetric mode, and more explosive reconnection happens. In a longer arcade, on the other hand, an undulating three-dimensional activity arises. It suggests that the variety in solar coronal activity may be caused by the differece in arcade shape.

DOI： 10.1186/BF03353260

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPRINGER  

Web of Science

Language：English   Publishing type：Part of collection (book)   Publisher：Blackwell Publishing Ltd  

Solar coronal magnetic field plays a role as a channel through which the magnetic helicity is transported from the sun into the interplanetary space. If the coronal field forms the arcades, and if the magnetic Reynolds number is sufficiently large, the helicity transportation proceeds intermittently, even in the case that the helicity is constantly supplied from the sun. The intermittent transportation is a cyclic process which consists of the helicity storage, magnetic reconnection, and the plasmoid ejection. It is a result of the competition between the destabilization by the photospheric activity and the spontaneous stabilization caused by the magnetic arcade instability. The total helicity is well conserved even in the energy relaxation process, so that Taylor’s hypothesis on the selective dissipation is applicable also to the solar corona. However, the numerical simulations indicate that the energy relaxation is even partial, and that the free energy cannot be exhausted, because the photospheric boundary gives a stronger constraint than Taylor’s theory predicted. The coronal field could evolve around the marginally stable limit, which locates in the intermediate region between the stable and the unstable linear force free solutions. The results suggest that, even if there is no external trigger, the coronal plasma is able to generate intermittent activities like flares.

DOI： 10.1029/GM111p0149

Scopus

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPRINGER  

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

The propagation of electro-static waves in a-fluid dusty plasma system is investigated. We study the dynamics of the dust particles and ions in the a-fluid dusty plasma system. We find that the electro-static waves in dusty plasma are developed as the new modes. When we assume that the charge-up process in the equilibrium can be eliminated, these modes approach to Ion-Acoustic mode (Q(d0) = 0) Or Dust-ion-Acoustic mode (P-d0 = 0).

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Institute of Physics Inc.  

Nonlinear dynamics of magnetic buoyancy instabilities for the Parker-type and interchange-type modes are numerically investigated with the aim of understanding the emerging mechanism of solar coronal magnetic loops. Two-dimensional nonlinear magnetohydrodynamic (MHD) simulations of the magnetic buoyancy instabilities are performed in a highly stratified domain consisting of two temperature regions corresponding to the corona and the chromosphere, respectively. First, it is revealed that the angle between the wave vector and the magnetic field at the top interface of the magnetic layer is a crucial parameter controlling the basic properties of the instability. Second, it is found that the magnetic buoyancy instability in the sheared field grows nonlinearly larger than in the shearless field, although the magnetic shear reduces the linear growth rate. This is due to the Parker-type mode being destabilized by the nonlinear effect of the interchange-type instabilities. Third, it is shown that the magnetic buoyancy instability can drive magnetic reconnections which create isolated flux regions. © 1998 American Institute of Physics.

DOI： 10.1063/1.872944

Scopus

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：JAPAN SOCIETY PLASMA SCIENCE &NUCLEAR FUSION RESEARCH  

Three-dimensional global magnetohydrodynamic (MHD) simulations are performed in order to investigate the global structure and the dynamics of the Jovian magnetosphere. Particularly, we investigate the influence of the planetary rotation onto the magnetospheric structure. The effects of the interplanetary magnetic field (IMF) in the solar wind are also considered. The simulation results clearly indicate that the dawn-dusk asymmetries are generated due to the effect of the planetary rotation. The plasmoid can be also observed in the tail region. The neutral lines of the magnetic field, however, appear obliquely from the dawn-dusk line, so that the location of the plasmoid is quite different from that in the Earth's magnetosphere.

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：JAPAN SOCIETY PLASMA SCIENCE &NUCLEAR FUSION RESEARCH  

The theoretical proposition, that solar hares are self-organization processes to realize the Taylor's minimum energy state [K.Kusano, ct al., Astrophys. J., 441 942 (1995)], is examined by the numerical simulations. The simulation shows that the self-organization process indeed takes place, and that a rapid energy relaxation process is achieved through magnetic reconnection on thin current sheets, as expected from the theory. However, it is also clarified that, since the line-tied condition on the photosphere severely restricts the energy relaxation in addition to Taylor's hypothesis on total helicity conservation. Furthermore, we consider the storage mechanism of the free energy to drive solar flares, and find that a substantial free energy can be stored in the coronal magnetic field through a competitive process between the helicity injection due to the photospheric motion and the instability in the coronal arcade system.

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：JAPAN SOCIETY PLASMA SCIENCE &NUCLEAR FUSION RESEARCH  

Magnetic reconnection, which arises from the nonlinearity of the magnetohydrodynamic instability in the solar coronal loops, is analyzed in detail by the numerical simulations. It is revealed that the time-scale of the current sheet formation does not depend on the linear growth rate of the initial instability, and it is scaled by the Alfven transit times. It is also found that! when the magnetic Reynolds number is in the range from 10(3) to 10(4), the length of the current layer in the reconnection site is geometrically limited to the scale of the loop feet interval and is hardly affected by the change in the resistivity. Consequently, the width of the current layer is proportional to the square root of the resistivity.

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：JAPAN SOCIETY PLASMA SCIENCE &NUCLEAR FUSION RESEARCH  

We perform the tyro-dimensional magnetohydrodynamic (MHD) simulation which can calculate either the generation and the evolution of the solar coronal magnetic loops with a magnetic shear. The linear stability analysis reveals that, if the magnetic field has a stronger shear, higher-wavenumber perturbations are destabilized because of the excitation of the interchange mode, so that the unstable region in the wavenumber space is enlarged. In the nonlinear simulation, it is observed that as the result of the initial perturbation growing, the magnetic flux emerges into the corona and the coronal loop is generated, It is also observed that when the perturbation of the short-wavelength is imposed to the strongly sheared magnetic held. the isolated flux tube is generated through the magnetic reconnection process.

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：JAPAN SOCIETY PLASMA SCIENCE &NUCLEAR FUSION RESEARCH  

We study the relaxation phenomena of the reversed field pinch (RFP) plasma with a radio-frequency (rf) current drive. We develop the magnetohydrodynamic code, in which the effect of the rf current drive is included in the induction equation as an external electric field obtained from the ray tracing of the fast magnetosonic wave. As the results of simulations, we observe that the rf current drive can effectively reduce the disturbance due to the kink mode instabilities and it may sustain the reversed held without the MHD relaxation dynamics, These results suggest that the rf current drive is a prospective method to enable a quiet sustainment of the RFP configuration.

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：JAPAN SOCIETY PLASMA SCIENCE &NUCLEAR FUSION RESEARCH  

The magnetohydrodynamic (MHD) relaxation process in the solar coronal magnetic field is investigated in terms of three-dimensional numerical simulations. We consider the dynamics of magnetic loops, which is caused by the photospheric twisting motion. It is found that, as a result of the rising of the magnetic loops, isolated flux tubes (plasmoids) are generated on top of the loops through the magnetic reconnection process. It is revealed that the plasmoid formation can be explained as a transition between two different solutions of the Woltjer-Taylor minimum energy state. (Kusano, Suzuki, & Nishikawa 1995). Furthermore, the three-dimensional effect in the transition dynamics is investigated in detail by the Fourier mode analysis.

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：JAPAN SOCIETY PLASMA SCIENCE &NUCLEAR FUSION RESEARCH  

The three-dimensional (3D) compressible magnetoconvection in a rectangular domain with a horizontal magnetic field is investigated using the numerical simulation. The 3D calculations are carried out adopting the result from the two-dimensional convection simulations as the initial condition. It is observed that the stable roll convection in the 2D system is unstable in the 3D system, and a complex spatial-temporal structure is generated.

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Institute of Physics Inc.  

Effects of electron-positron pair creation on the stationary one-dimensional propagation of relativistically coupled electromagnetic and electrostatic waves are studied based on the self-consistent model presented in the previous paper for an electron-ion plasma [L. N. Tsintsadze and K. Nishikawa, Phys. Plasmas 3, 511 (1996)]. The pair-created particles are treated as a cold electron-positron plasma at their creation point. In addition to the results similar to those obtained for the electron-ion plasma, a new type of instability of the finite amplitude plane wave is found which is purely growing in the wave frame near threshold. A novel solution describing an envelope shock which represents a wake-field excitation by a solitary electromagnetic pulse is obtained by taking into account the trapping of the pair-created particles in the upstream region. © 1997 American Institute of Physics.

DOI： 10.1063/1.872207

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union  

The nonlinear interaction of a rapidly rotating magnetosphere with the external supersonic plasma flow is investigated by using the three-dimensional global magneto-hydrodynamic (MHD) simulation. The results of the simulations clearly indicate that various dawn-dusk asymmetries are generated due to the effect of the rotation. Particularly, the location of the plasmoid, which is generated in the tail region of the rotating magnetosphere, is quite different from the case without rotation. It is also found that distinctive magnetospheric structures such as the plasma disc are largely modified in response to the dynamic pressure in the external plasma flow. Copyright 1997 by the American Geophysical Union.

DOI： 10.1029/97GL52739

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Yukawa Institute for Theoretical Physics  

INSAM (Institute for Numerical Simulations and Applied Mathematics) in Hiroshima University is a group consists of researchers whose major are Numerical Simulations and/or Applied Mathematics. It covers wide range of science: mathematics, physics, chemistry, biology and earth &amp
planetary system. In this report the current activities of INSAM are presented. The performance of Intel Paragon, which is a main computer resource of INSAM, and its implementation for our applications will be also discussed.

DOI： 10.1143/PTPS.122.51

Scopus

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：KLUWER ACADEMIC PUBL  

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：KLUWER ACADEMIC PUBL  

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Physics Publishing  

The general solution of the Woltjer-Taylor minimum energy state is found in the solar coronal geometry, which is modeled as a rectangular domain. It is shown that the solution bifurcates into two different states when the magnetic helicity integral or the geometrical factor defined as the ratio of the height to the width of the domain is satisfactorily increased. Based on this analysis, it is theoretically proposed that a solar flare is a transition process between two different Woltjer-Taylor states, which are generated as a result of the bifurcation. The theory predicts that a solar flare is triggered when the vertical size of the coronal magnetic loop becomes longer than the horizontal size. The topological structure in the coronal magnetic field must be dramatically changed in the transition process. The result implies that in general, magnetic reconnection must be accompanied by a solar flare. The numerical simulation shows that the transition process is dynamically realized as a result of a twisting motion on the photosphere. Based on this theory, disagreements among recent simulation works on the plasmoid formation problem can be consistently explained.

DOI： 10.1086/175413

Scopus

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

The general solution of the Woltjer-Taylor minimum energy state is found in the solar coronal geometry, which is modeled as a rectangular domain. It is shown that the solution bifurcates into two different states when the magnetic helicity integral or the geometrical factor defined as the ratio of the height to the width of the domain is satisfactorily increased. Based on this analysis, it is theoretically proposed that a solar flare is a transition process between two different Woltjer-Taylor states, which are generated as a result of the bifurcation. The theory predicts that a solar flare is triggered when the vertical size of the coronal magnetic loop becomes longer than the horizontal size. The topological structure in the coronal magnetic field must be dramatically changed in the transition process. The result implies that in general, magnetic reconnection must be accompanied by a solar flare. The numerical simulation shows that the transition process is dynamically realized as a result of a twisting motion on the photosphere. Based on this theory, disagreements among recent simulation works on the plasmoid formation problem can be consistently explained.

Language：English   Publishing type：Research paper (scientific journal)  

The energy conversion process as well as the plasma heating mechanism in the starting-up phase of driven reconnection is investigated in detail using two-dimensional magnetohydrodynamic (MHD) simulation. It is revealed that the compressional heating effect is predominant in this phase rather than the Ohmic and the viscous heating. The predominance of the compressional heating means that the plasma flow energy is considerably converted to the thermal energy. The strong compression takes place in the process during which the fast shock is generated in the downstream region of the reconnection point. The plasma heating due to nonsteady reconnection is briefly discussed as a promising mechanism for the anomalous ion heating, which is observed in reversed-field pinch (RFP) experiments. © 1993 American Institute of Physics.

DOI： 10.1063/1.860521

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

Whether it is induced by an ideal (current driven) instability or by an external force, plasma flow causes a change in the magnetic field configuration and often gives rise to a current intensification locally, thereby a fast driven reconnection being driven there. Many dramatic phenomena in magnetically confined plasmas such as magnetospheric substorms, solar flares, magnetohydrodynamic (MHD) self-organization, and tokamak sawtooth crash, may be attributed to this fast driven reconnection. Using a fourth-order MHD simulation code it is confirmed that compressibility of the plasma plays a crucial role in leading to a fast (MHD time scale) driven reconnection. This indicates that the incompressible representation is not always applicable to the study of a global dynamical behavior of a magnetically confined plasma. © 1992 American Institute of Physics.

DOI： 10.1063/1.860295

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Plasma Science and Nuclear Fusion Research  

The computer simulation has played an important role for the research of reversed-field pinch (REP) physics. Especially, in order to elucidate essential nonlinear mechanisms, the simulation study is a much useful methodology. It is described how the computer simulation has been applied to solve the self-reversal and the self-sustainment processes.

DOI： 10.1585/jspf1958.65.528

CiNii Books

Other Link： https://jlc.jst.go.jp/DN/JALC/00009529311?from=CiNii

Language：English   Publishing type：Research paper (scientific journal)  

The toroidal phase locking process of kink modes in a reversed field pinch (RFP) plasma is investigated in detail by means of magnetohydrodynamic (MHD) simulation. The physical mechanism of phase locking is clarified. The two most dominant linearly unstable kink modes govern the evolution of other kink modes, whereby phase locking takes place. It is confirmed that the phase locking process is not a special phenomenon of plasmas with a resistive boundary, but a common feature of the MHD relaxation process in RFPs. The relation between the phase locking process and the MHD relaxation process is briefly discussed. It is found that phase locking is reproduced cyclically in the sustainment process. © 1991, IOP publishing Ltd.

DOI： 10.1088/0029-5515/31/10/010

Scopus

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Progress of Theoretical Physics  

Recent progress in super-computers has opened a new tool for the study of nonlinear dynamics in plasmas. This paper reviews the recent computer simulation studies for the self-organization and self-relaxation processes in MHD plasmas carried out by the Hiroshima group. Taylor's conjecture for the selective dissipation of magnetic energy keeping the total magnetic helicity canstant is confirmed and the mechanisms for formation and maintenance of the reversed field pinch configuration are explained in terms of the self-relaxation of the plasma due to a driven magnetic reconnection triggered by ideal MHD helical kink unstable modes and their nonlinear couplings.

DOI： 10.1143/PTPS.99.206

CiNii Books

Other Link： http://dl.ndl.go.jp/info:ndljp/pid/10883515

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1039/c39890001208

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

Based on theoretical and computational considerations, it is confirmed that magnetic reconnection driven nonlinearly by a kink instability plays a key role in the magnetohydrodynamic relaxation. An important finding is that the relaxation takes the form of either global relaxation (e.g., the reversed field pinch relaxation) or local relaxation (e.g., the tokamak sawtooth relaxation), depending on whether the driven reconnection is total (antiparallel) or partial (partially antiparallel). © 1988 American Institute of Physics.

DOI： 10.1063/1.859100

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

The sixth order wave equation which results from a finite temperature expansion of the Vlasov equation is solved globally in the ion cyclotron range of frequencies. A perpendicularly stratified, onedimensional slab plasma is assumed. The diamagnetic drift and the associated anisotropy are included in the unperturbed distribution function to ensure a self-adjoint system. All x-dependence in the plasma pressure and magnetic field is retained along with the electric field parallel to B. Thus, Landau damping of the ion Bernstein wave is included self-consistently. Because of the global nature of the solution, the evanescent short wavelength Bernstein waves do not grow exponentially as in shooting methods. Strong variations occur in the absorption and in the structure of the wave fields as resonance topology is varied. Solutions to the complete sixth order differential equation are compared to those from an approximate second order equation based on local dispersion theory. © 1988 IOP Publishing Ltd.

DOI： 10.1088/0029-5515/28/1/007

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

DOI： 10.1063/1.866085

Web of Science

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

The characteristics of a toroidal discharge with qa ≈ 1/2 is studied on TORIUT-6. Ultra low q equilibria that are stable against the global kink mode are realized through MHD relaxation and are shown to be characterized by dq/dr &lt
0 and a hollow current profile. The stable state lasts for 600 μs, which is of the order of the classical diffusion time. These experimental observations are consistent with non-linear MHD simulation results. © IOP Publishing Ltd.

DOI： 10.1088/0029-5515/27/7/010

Scopus

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

We propose a theory that a new type of magnetic reconnection, nonlinear driven reconnection, is triggered during the nonlinear development of an m=1 helical kink-mode instability. This reconnection process can well explain nonlinear reconnection observed in the previous simulation and can be a candidate for the self-reversal mechanism in the reversed-field pinch. © 1985 The American Physical Society.

DOI： 10.1103/PhysRevLett.54.808

Scopus

Language：Japanese

Language：Japanese

Language：Japanese

Language：English

Language：Japanese

Total pages：178p   Language：Japanese

CiNii Books

Language：Japanese

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CiNii Books

Other Link： http://dl.ndl.go.jp/info:ndljp/pid/10459106

Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

Language：Japanese  

CiNii Books

Other Link： http://hdl.handle.net/2115/55063

Language：Japanese  

CiNii Books

Language：Japanese  

CiNii Books

Language：English   Publisher：Earth Simulator Center, Japan Agency for Marine-Earth Science and Technology  

CiNii Books

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

J-GLOBAL

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

Language：Japanese   Publisher：岩波書店  

CiNii Books

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CiNii Books

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CiNii Books

Other Link： https://projects.repo.nii.ac.jp/?action=repository_uri&item_id=376745

Language：Japanese  

CiNii Books

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

Language：Japanese   Publisher：一般社団法人日本物理学会  

CiNii Books

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

J-GLOBAL

Language：Japanese  

CiNii Books

Language：Japanese  

CiNii Books

Language：Japanese  

CiNii Books

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

Language：Japanese  

Toward peta-scale computing era, a next-generation MHD solver that is accurate and robust enough for any parameter regimes from almost incompressible turbulences to hypersonic flows should be developed. One of the key techniques for the next-generation solver must be a divergence cleaning method for the magnetic filed, which primarily affects the robustness of the multidimensional MHD solver particularly in high-resolution simulations. In this paper, several divergence cleaning methods including new methods are comparatively studied based on the HLLD Riemann solver.

CiNii Books

Language：Japanese   Publishing type：Research paper, summary (national, other academic conference)   Publisher：National Committee for IUTAM  

The physics of quiet auroral arcs formation has been studied with Macro-Micro Interlocked (MMI) simulations. In the macro part of our simulation code, the macroscopic instability between the magnetosphere and ionosphere is described by the magneto-hydro-dynamics (MHD) model. On the other hand, in the micro part, the microscopic instability is calculated by the particle-in-cell (PIC) model. In the MHD simulation, it is shown that longitudinally striated structures of the field-aligned current are formed by an ionospheric feedback instability[1]. Further, the emission of aurora arc is obtained from the energy spectrum of accelerated electrons with the PIC simulation.
[1] T. Sato, J. Geophys. Res., 83, 1042, 1978.

DOI： 10.11345/japannctam.57.0.214.0

Language：Japanese   Publisher：National Committee for IUTAM  

Solar activity may impact our environment on the Earth. For instance, solar flares and coronal mass ejections (CME), which are the biggest explosion in our solar system, are believed to give a severe influence to the electromagnetic condition around the earth, so that the prediction of such space weather phenomena is required also from the view point to protect satellite systems. On the other hand, since it has been suggested even from long ago that the solar spot activity correlates with the global temperature of the earth, it is important to reveal the mechanism how solar and cosmic dynamics are related to our environment. In this study, we have tried to develop a new type of simulation model, in which several numerical models are interlocked with each other to capture complex physical processes involved in the space weather and the space climate phenomena. The current status and the future prospect of the interlocked simulation study will be reported.

DOI： 10.11345/japannctam.57.0.216.0

Language：Japanese  

We have recently developed a new interlocked MHD model of the Sun-Earth system, in order to understand the relationship between solar and geomagnetic storms, and to predict space weather phenomena. In this paper, we introduce the entire design of the new model, and show the preliminary results of numerical experiments for early phase of a solar storm.

CiNii Books

Language：Japanese  

We have recently developed a new interlocked MHD modeling of the Sun-Earth system to understand the relationship between solar and geomagnetic storms, and to predict the associated space weather phenomena. In this study, we report our modeling results of the global solar wind structures including the propagation of a coronal mass ejection.

CiNii Books

Language：Japanese   Publisher：The Geological Society of Japan  

CiNii Books

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

J-GLOBAL

Language：Japanese   Publisher：日本気象学会  

CiNii Books

Language：Japanese  

A carbuncle-free multi-state Harten-Lax-van Leer (HLL)-type approximate Riemann solver for HD and MHD is newly proposed. From phenomenological consideration for numerical shock instabilities like a carbuncle phenomenon, we guess that the instabilities may appear when a numerical viscosity is insufficient for a shear wave rather than for a contact wave. In this study, the numerical viscosity for the tangential momentum is selectively added assuming the single-state tangential velocity in the Riemann fan. The new solver can be effectively embedded into the higher HLLC and HLLD Riemann solver using the usual shock-fix algorithm.

CiNii Books

Language：Japanese   Publisher：The Visualization Society of Japan  

Cloud formations and precipitation are highly-complicated phenomena, and the accurate modeling of them is a crucial subject for meteorological simulations. Recently, we developed a new algorithm, which enables the first-principle simulation of cloud formation process, by introducing a new numerical concept called "super-droplet".<BR>In this paper, we report about the visualization of the cloud formation-precipitation simulation using the super-droplet method. The technique to visualize the distribution of all droplets included in a cumulus cloud will be explained, and some application results of it will be also presented for the analysis of droplet size spectrum, cloud motion, and the optical property of cloud, respectively.

DOI： 10.3154/jvs.26.Supplement2_27

J-GLOBAL

Language：Japanese   Publisher：可視化情報学会  

DOI： 10.3154/jvs.26.Supplement1_87

CiNii Books

J-GLOBAL

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

Language：Japanese  

We develop a novel cloud microphysics model, called Super Droplet Model (SDM), which enables accurate calculation for the motion, growth by condensation and evaporation, and coalescence of all the droplets in a cloud with reasonable cost in computation. We couple SDM and a non-hydrostatic cloud dynamics model and demonstrate a regional simulation of cloud formation and precipitation.

CiNii Books

Language：Japanese   Publisher：日本気象学会  

CiNii Books

Language：Japanese  

We developed a new three-dimensional Magnetohydrodynamic (MHD) code for global simulation of coronal mass ejections in the solar corona. In this code, ideal MHD equations with gravity are solved by finite volume method with the monotonic upstream schemes for conservation laws (MUSCL) which achieves 3rd order spatial resolution, the HLLD nonlinear approximate Riemann solver, and the 2nd order Runge-Kutta time integration. An initial condition is built up with magneto-hydrostatic equilibrium of source surface potential field.

CiNii Books

Language：Japanese  

A fluid description of plasmas is useful to investigate the global structure in space plasma, such as the magnetosphere and heliosphere. It can reproduce the time evolution of macroscopic variations of density, bulk velocity and pressure. On the other hand, the microscopic dynamics of plasmas should be described by spatial and velocity distribution in the phase space. Wave-particle interaction process is the typical example for the application of the microscopic description. For these micro- and macroscopic description, different kinds of numerical approach may work well, respectively, e.g. MHD simulation for the macroscopic phenomena and PIC simulation for microscopic process. However, it is widely accepted that the cross-scale interaction between micro and macro processes plays a crucial role, for instance, in the diffusion region in the magnetic reconnection process. Therefore, a new model which can treat MHD-scale dynamics including particle kinetic effects is necessary. We have developed the new simulation method called "Interlocked simulation", in which MHD and PIC simulations are simultaneously performed. Here, we show its algorithm and some examples.

CiNii Books

Language：Japanese  

CiNii Books

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

Language：Japanese  

We performed three-dimensional numerical simulations of the emergence of magnetic flux from the convectiove zone into the corona and its reconnection with a preexisting ambient coronal arcade field. Initially the flux tube in the convective zone is perturbed and becomes unstable to the Parker instability. The instability causes the rise and the expansion of the flux tube into the corona and the field comes into contact with the arcade field. When the upcoming field and the coronal arcade field generate current sheets between them, the reconnection occurs, causing profound changes in their connectivity. The local heating and the high-speed outflows caused by the reconnection can be seen in our simulations. We investigate how the changes in the angle of the arcade relative to the magnetic neutral line, lead to the different physical processes.

CiNii Books

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

Language：English   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

The analysis of the magnetic helicity injection into active region NOAA 8100 is shown. This region is known as a source of many coronal activities such as flares. It is found that most of the flare events occurred about half a day after the helicity injection rate changed its sign. And the positions of Ha emission in flares well correspond to the helicity inversion lines in space. These results suggest that the sign-reversal of the helicity injection rate is a key signature of flare activities. (C) 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/S0273-1177(03)00773-7

Web of Science

Language：Japanese   Publisher：The Japan Society of Plasma Science and Nuclear Fusion Research  

Although magnetic reconnection is believed to be a key process in solar flares, the flares' triggering mechanism remains a long-standing problem. In order to resolve this issue, the physical linkage between the magnetohydrodynamic (MHD) process in the macroscopic scale and the plasma kinetics in the microscopic scale has to be understood. Multiple-scale simulation, which can precisely describe the nonlinear process of the MHD equations as well as the complexity in the solar corona, will play a crucial role in this field of study.

DOI： 10.1585/jspf.79.496

CiNii Books

Other Link： https://jlc.jst.go.jp/DN/JALC/00229591198?from=CiNii

Language：English   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

We developed a new methodology which can determine magnetic helicity flux across the photosphere based on the magnetograph observation. In order to derive the helicity flux, first the velocity tangential to the solar surface is constructed by applying a correlation tracking technique on the magnetic observation, and secondly the velocity component across the photosphere is derived from the condition that the magnetic evolution must be consistent with the induction equation. Through this procedure, we can determine the helicity flow across the photosphere as a function of time and space. Based on this new method, we analyzed magnetic helicity of the active regions NOAA 906 and 9077, using the data taken by SOHO/MDI and the vector magnetograph at NAOJ/Tokyo. As a result, it was revealed that positive and negative helicity is supplied in each region simultaneously. In particular, temporary activation of the helicity injection of the both signs was observed prior to X-class flare events in the GOES classification. The investigation on the helicity distribution suggests some relationship between flare onset and the increase of complexity in the helicity structure. (C) 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/S0273-1177(03)00772-5

Web of Science

Language：English   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Although magnetic reconnection is believed to be a key process for solar flares, the onset mechanism of flares remains as a long-standing problem. In this paper, we propose a new model for solar flares, in which magnetic reconnection converts oppositely-sheared field into the shear-free field. This process can be understood as the annihilation of magnetic helicities with, different signs, and it is consistent with the recent results of the helicity observations. Numerical simulations were carried, which show that, if the helicity is sharply, reversed within a magnetic arcade, reconnection quickly grows in the helicity inversion layer, driving explosive dynamics. The explosive process appears as a result of nonlinear instability, which grows faster than exponentially with time. Based on the results, we predict that the coexistence of the positive and negative helicity is crucial for the onset of flares. (C) 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/S0273-1177(03)00765-8

Web of Science

Language：Japanese  

Magnetohydrodynamics (MHD) provide a very important model for our understanding of the structure and evolution of space and astronomical plasmas, such as the Sun, solar corona, and planetary magnetosphere. This paper outlines two key concepts, reconnection and dynamo, those play an essential role in the energetic activity of a variety of astronomical and space plasmas.

CiNii Books

Other Link： http://dl.ndl.go.jp/info:ndljp/pid/10454465

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/butsuri1946.53.656

CiNii Books

Other Link： https://jlc.jst.go.jp/DN/JALC/00056151794?from=CiNii

Language：English   Publisher：KOREAN PHYSICAL SOC  

The nonlinear magnetohydrodynamics (MHD) in solar flares is considered using two dimensional numerical simulations. The results are analyzed based on the bifurcation theory of the linear force-free field [K. Kusano, et al., Astrophys. J. 441 942 (1995)]. The simulation results show that a transition from a loop type configuration into an island type configuration indeed takes place as the bifurcation theory predicted. The dependence of the transition dynamics on the resistivity is considered in detail.

Web of Science

Language：English   Publisher：KOREAN PHYSICAL SOC  

The nonlinear magnetohydrodynamics (MHD) in solar flares is considered using two dimensional numerical simulations. The results are analyzed based on the bifurcation theory of the linear force-free field [K. Kusano, et al., Astrophys. J. 441 942 (1995)]. The simulation results show that a transition from a loop type configuration into an island type configuration indeed takes place as the bifurcation theory predicted. The dependence of the transition dynamics on the resistivity is considered in detail.

Web of Science

Language：Japanese  

CiNii Books

Language：Japanese  

CiNii Books

Language：English   Publisher：UNIV CHICAGO PRESS  

Three-dimensional dynamics of solar coronal magnetic loops, which is caused by the photospheric twisting motion, is investigated in detail by using magnetohydrodynamic numerical simulations. It is found that, as a result of the rising of the magnetic loops, isolated flux tubes (plasmoids) are generated on top of the loops through magnetic reconnection. During the reconnection process, the magnetic energy of the mode coupled with the potential held is partially converted into the mode decoupled from it. We also analyze the linear stability of the coupled mode and reveal that it is destabilized when the loop height and the magnetic helicity exceed the critical values predicted from the bifurcation theory. Furthermore, we reveal that three-dimensional mode couplings have the effect of concentrating the helicity into an unstable mode, as well as the effect of reducing the growth rate of the instability.

Web of Science

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

Language：English   Publisher：UNIV CHICAGO PRESS  

The bifurcation structure as well as the stability property of the solar coronal magnetic arcades in a linear force-free field (LFFF) is carefully investigated. It is clarified that two different modes can cause the bifurcation of the minimum energy arcade profile: the symmetric mode (SM), which has a zero wavevector, and the undulating mode (UM), which has a nonzero wavevector parallel to the magnetic neutral lines. Depending on the geometry and the magnetic helicity contained in the arcades, the minimum energy state is switched among three different solutions of the LFFF: the coupled solution (CS), which consists only of the components coupled with the potential held; the mixed solution (MS), which is composed of the CS and the SM; and the MS of the CS and the UM. It is found that, once the LFFF bifurcates into the CS and the MS, the CS is always unstable against the SM or the UM. Based on the discussion about the relationship between the excess energy and the growth rate, it is proposed that the preflare state is a nonlinearly growing stage of these instabilities rather than a metastable state.

Web of Science

Language：English   Publisher：PHYSICAL SOC JAPAN  

One-dimensional transport equations for pulsed-laser induced space-charge distributions in GaAs crystals have been computed using a finite difference procedure by two models; (a) Model A for i-GaAs with an applied dc electric field under a uniform illumination over the crystal and (b) Model B for n- and i-GaAs without the electric field under a half illumination to simulate the ''transient thermoelectric effect'' (TTE). With Model A, the temporal and spatial evolutions of restored space-charge density of photoexcited electrons and holes are obtained, from which an induced electric field is found to oscillate with a single frequency of plasma oscillations (0.3-2THz), in satisfactory agreement with reported experiments. Model B calculations have revealed that near the boundary between the illuminated and non-illuminated regions, the appreciable enhancement of space-charge densities and induced electric fields are observed, which diffuse or drift toward a positive x direction, showing characteristic damping oscillations with two frequency components f(0) due to the doped donor electrons and f(1) due to the doped and photoexcited electrons of plasma oscillations. Furthermore, the induced TTE voltages measured across both ends of the crystal without dc electric field are simulated to show similar coherent plasma oscillations.

DOI： 10.1143/JPSJ.65.803

Web of Science

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

Language：English   Publishing type：Rapid communication, short report, research note, etc. (scientific journal)   Publisher：INT ATOMIC ENERGY AGENCY  

Double tearing modes in ultra-low-q equilibria have been simulated by three-dimensional MHD calculations. Spatially localized MHD relaxation is shown to be driven by double tearing modes, which bring the field configuration in the unstable region to a Taylor-like relaxed state. When the pitch minimum of the initial state is located near the centre of the plasma, the central value of q decreases rapidly and the outer magnetic island replaces the original magnetic axis. When the pitch minimum is located near the wall, the central core region remains stable and reconnection occurs in a limited intermediate region.

DOI： 10.1088/0029-5515/31/1/017

Web of Science

Language：English   Publisher：INT ATOMIC ENERGY AGENCY  

DOI： 10.1088/0029-5515/30/10/009

Web of Science

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

Language：English   Publisher：National Institute for Fusion Science  

CiNii Books

Language：Japanese   Publisher：一般社団法人日本物理学会  

CiNii Books

Language：Japanese   Publisher：一般社団法人日本物理学会  

CiNii Books

Language：English   Publisher：PHYSICAL SOCIETY JAPAN  

DOI： 10.1143/JPSJ.56.963

Web of Science

DOI： 10.1063/1.866085

DOI： 10.1063/1.866085

Language：English  

By means of 3D magnetohydrodynamic simulations, the relaxation process in the finite β reversed-field pinch (RFP) is investigated. It is confirmed that the nonlinear reconnection process is the dominant mechanism of the RFP relaxation even in the system of finite β. The nonlinear reconnection rate is almost halved compared with that in the zero β case. The relaxed state of finite β is dynamically achieved. This state has a flat pressure profile and is well approximated by the Bessel-function model in the central region. The simulation result agrees with the partially relaxed state model. The β-limit is estimated based on the Suydam criterion, and consequently, it is found that the stable area on the F-θ diagram is narrowed as the heat loss decreases. © 1987, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

DOI： 10.1143/JPSJ.56.963

Scopus

Language：English   Publishing type：Book review, literature introduction, etc.  

The relaxation process in the reversed field pinch has been studied extensively with three-dimensional magnetohydrodynamic simulations. It is found that the non-linear coupling between multiple helicity modes plays a leading role in the relaxation process. Specifically, the (m
n)=(0
1) island is excited as a result of non-linear coupling between the linearly unstable m=l modes, with a toroidal mode number difference of one. The m=0 island is then subject to axisymmetric non-linear reconnection whereby reversed flux is effectively generated in the outer region. It should be noted that although the axisymmetric non-linear reconnection of m=0 island is the dominant relaxation process, helical non-linear reconnection of linearly unstable m=l modes also plays some role in the relaxation of the whole system. It is also found that through this multiple helicity relaxation process, Taylor's minimum energy state is realized. The simulation results are generally consistent with experimental observations in the sustainment phase. This indicates that the multiple helicity relaxation process is of fundamental importance in the maintenance of the reversed field during the sustainment phase as well as in the self-reversal during the set-up phase. The relation between relaxation and aspect ratio has also been examined and it is found that the relaxation process is not strongly affected by the aspect ratio. © IOP Publishing Ltd.

DOI： 10.1088/0029-5515/27/5/011

Scopus

Language：English   Publisher：INT ATOMIC ENERGY AGENCY  

DOI： 10.1088/0029-5515/26/8/005

Web of Science

Language：Japanese   Publisher：一般社団法人日本物理学会  

CiNii Books

Language：Japanese   Publisher：一般社団法人日本物理学会  

CiNii Books