Faculty Profiles - SHIOKAWA, Kazuo

Papers - SHIOKAWA, Kazuo

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Severe Magnetic Fluctuations in the Near-Earth Magnetotail: Spectral Analysis and Dependence on Solar Activity Reviewed

Heqiucen Xu, Kazuo Shiokawa

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 125 ( 7 ) 2020.7

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

Magnetic fluctuations in the near-Earth magnetotail are an important signature of substorm onset. In a previous statistical study, we reported their occurrence rates, spatial distributions, and relationship with plasma flows. In the present study, we investigated their spectral properties using 11 years of measurements from the Time History of Events and Macroscale Interactions during Substorms mission for 2008-2018. We found 10,848 severe magnetic fluctuation events with sigma(B)/(B) over bar) > 0.5, where sigma(B) and (B) over bar are the standard deviation and average, respectively, of the magnetic field intensity for the local proton gyroperiod. The occurrence rates of severe magnetic fluctuations show no clear dependence on the F-10.7 index in one solar cycle. We extracted 36 dipolarization events with severe magnetic fluctuations. In the power spectral density (PSD) of the magnetic fluctuations during dipolarizations, the steepness of the spectral slope increased with increasing frequency in almost all the events. The average PSDs are shown sorted by (a) distance to the neutral sheet and (b) ambient magnetic field intensity. In all groups, the slopes of the average PSDs increased abruptly from below similar to 10(-1.3) Hz (0.05 Hz) to above similar to 10(-1.3) Hz, which is close to the gyrofrequency of O+ ions. It is the first time that a change of slope near the proton gyrofrequency (frequency range: 0.05-1 Hz) was found in cases of larger ambient magnetic field intensity, implying that the magnetic fluctuations were relatively strong near the proton gyrofrequency. These results suggest that the magnetic fluctuations contribute to the nonmagnetohydrodynamic effect in the ion motion.

DOI： 10.1029/2020JA027834

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Modulation of Pc1 Wave Ducting by Equatorial Plasma Bubble Reviewed

Hyangpyo Kim, Kazuo Shiokawa, Jacheung Park, Yoshizumi Miyoshi, Junga Hwang, Akira Kadokura

GEOPHYSICAL RESEARCH LETTERS Vol. 47 ( 9 ) 2020.5

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

Ducting Pc1 waves are observed over a wide latitudinal range by the low-Earth orbit satellites as compressional mode. In this paper, we present the first observation of the modulation of ducting Pc1 waves by equatorial plasma bubbles (EPBs) based on the Swarm satellites. We show two ducting Pc1 events propagating across EPBs that occurred on 7 April 2016 and on 27 September 2017. We found that the EPBs modulate the Pc1 wave propagation by setting up reflection boundaries and leakage holes in the ionospheric waveguide. We also found that changes of Pc1 wave intensities generally follow the electron density variation and that the intensity is stronger at higher density region. From the comparison between Swarm-A and Swarm-C observations, we conclude that ionospheric plasma plays an important role for Pc1 waveguide even though their density is significantly low in EPBs.

DOI： 10.1029/2020GL088054

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Observations of equatorial plasma bubbles using a low-cost 630.0-nm all-sky imager in Ishigaki Island, Japan Reviewed

Keisuke Hosokawa, Kohei Takami, Susumu Saito, Yasunobu Ogawa, Yuichi Otsuka, Kazuo Shiokawa, Chia-Hung Chen, Chien-Hung Lin

EARTH PLANETS AND SPACE Vol. 72 ( 1 ) 2020.5

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

Here, we introduce a low-cost airglow imaging system developed for observing plasma bubble signatures in 630.0-nm airglow emission from the F region of the ionosphere. The system is composed of a small camera, optical filter, and fish-eye lens, and is operated using free software that automatically records video from the camera. A pilot system was deployed in Ishigaki Island in the southern part of Japan (Lat 24.4, Lon 124.4, Mlat 19.6) and was operated for similar to 1.5 years from 2014 to 2016 corresponding to the recent solar maximum period. The pilot observations demonstrated that it was difficult to identify the plasma bubble signature in the raw image captured every 4 s. However, the quality of the image could be improved by reducing the random noise of instrumental origin through an integration of 30 consecutive raw images obtained in 2 min and further by subtracting the 1-h averaged background image. We compared the deviation images to those from a co-existing airglow imager of OMTIs, which is equipped with a back-illuminated cooled CCD camera with a high quantum efficiency of similar to 90%. It was confirmed that the low-cost airglow imager is capable of imaging the spatial structure of plasma bubbles, including their bifurcating traces. The results of these pilot observations in Ishigaki Island will allow us to distribute the low-cost imager in a wide area and construct a network for monitoring plasma bubbles and their space weather impacts on satellite navigation systems.

DOI： 10.1186/s40623-020-01187-1

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Development of low-cost multi-wavelength imager system for studies of aurora and airglow Reviewed

Y. Ogawa, Y. Tanaka, A. Kadokura, K. Hosokawa, Y. Ebihara, T. Motoba, B. Gustavsson, U. Brändström, Y. Sato, S. Oyama, M. Ozaki, T. Raita, F. Sigernes, S. Nozawa, K. Shiokawa, M. Kosch, K. Kauristie, C. Hall, S. Suzuki, Y. Miyoshi, A. Gerrard, H. Miyaoka, R. Fujii

Polar Science Vol. 23 2020.3

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

© 2019 Elsevier B.V. and NIPR This paper introduces a new system that can monitor aurora and atmospheric airglow using a low-cost Watec monochromatic imager (WMI) equipped with a sensitive camera, a filter with high transmittance, and the non-telecentric optics. The WMI system with 486-nm, 558-nm, and 630-nm band-pass filters has observable luminosity of about ~200–4000 Rayleigh for 1.07-sec exposure time and about ~40–1200 Rayleigh for 4.27-sec exposure time, for example. It is demonstrated that the WMI system is capable of detecting 428-nm auroral intensities properly, through comparison with those measured with a collocated electron-multiplying charge-coupled device (EMCCD) imager system with narrower band-pass filter. The WMI system has two distinct advantages over the existing system: One makes it possible to reduce overall costs, and the other is that it enables the continuous observation even under twilight and moonlight conditions. Since 2013 a set of multi-wavelength WMIs has been operating in northern Scandinavia, Svalbard, and Antarctica to study meso- and large-scale aurora and airglow phenomena. Future development of the low-cost WMI system is expected to provide a great opportunity for constructing a global network for multi-wavelength aurora and airglow monitoring.

DOI： 10.1016/j.polar.2019.100501

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Multievent Analysis of Oscillatory Motion of Medium-Scale Traveling Ionospheric Disturbances Observed by a 630-nm Airglow Imager Over Tromso Reviewed

Yadav Sneha, Shiokawa K., Oyama S., Otsuka Y.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 125 ( 3 ) 2020.3

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Geophysical Research: Space Physics

We present a comprehensive investigation on the propagation characteristics of duskside medium-scale traveling ionospheric disturbances (MSTIDs) using 630.0-nm airglow emissions over Tromsø (69.6°N, 19.2°E; magnetic latitude: 66.7°N). The unique points of our observation are (1) duskside MSTIDs primarily exhibited eastward motion under quiet conditions but turned to the westward direction associated with geomagnetic disturbances, (2) the westward moving MSTIDs again turned to the eastward direction when the geomagnetic disturbance ceased, (3) the turning of MSTIDs to the westward direction was invariably associated with an increase of the northward component of the magnetic field observed by the local ground-based magnetometers and with the equatorward expansion of the auroral oval, and (4) the Super Dual Auroral Radar Network convection maps revealed that the location of Tromsø was inside (outside) the duskside convection cell during the time of appearance of westward (eastward) moving MSTIDs. The average eastward and westward velocities of MSTIDs were ~25–80 and ~40–140 m/s, respectively. The Doppler shift measurement of the 630-nm airglow by a Fabry-Perot interferometer at Tromsø showed that northeastward winds were predominant during the appearance of eastward moving MSTIDs. These experimental evidences suggest that the oscillatory motion of MSTIDs over high latitudes is driven by the convection electric field. The MSTIDs tend to move eastward under geomagnetically quiet conditions but show westward motion under the influence of convection electric field associated with auroral activities in the duskside of two-cell convection pattern.

DOI： 10.1029/2019JA027598

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Fine-Scale Visualization of Aurora in a Wide Area Using Color Digital Camera Images From the International Space Station Reviewed

Sota Nanjo, Yuta Hozumi, Keisuke Hosokawa, Ryuho Kataoka, Yoshizumi Miyoshi, Shin-ichiro Oyama, Mitsunori Ozaki, Kazuo Shiokawa, Satoshi Kurita

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 125 ( 3 ) 2020.3

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

The full-color photographs of aurora have been taken with digital single-lens reflex cameras mounted on the International Space Station (ISS). Since these photographs do not have accurate time and geographical information, in order to use them as scientific data, it is necessary to calibrate the imaging parameters (such as looking direction and angle of view of the camera) of the photographs. For this purpose, we calibrated the imaging parameters using a city light image taken from the Defense Meteorological Satellite Program satellite following the method of Hozumi et al. (2016, https://doi.org/10.1186/s40623-016-0532-z). We mapped the photographs onto the geographic coordinate system using the calibrated imaging parameters. To evaluate the accuracy of the mapping, we compared the aurora taken simultaneously from ISS and ground. Comparing the spatial structure of discrete aurora and the temporal variation of pulsating aurora, the accuracy of the data set is less than 0.3 s in time and less than 5 km in space in the direction perpendicular to the looking direction of the camera. The generated data set has a wide field of view (similar to 1,100 x 900 km), and their temporal resolution is less than 1 s. Not only that, the field of view can sweep a wide area (similar to 3,000km in longitude) in a short time (similar to 10 min). Thus, this new imaging capability will enable us to capture the evolution of fine-scale spatial structure of aurora in a wide area.

DOI： 10.1029/2019JA027729

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Equatorial Plasma Bubble Zonal Drift Velocity Variations in Response to Season, Local Time, and Solar Activity across Southeast Asia Reviewed

Sarudin I., Hamid N. S. A., Abdullah M., Buhari S. M., Shiokawa K., Otsuka Y., Yatini C. Y.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 125 ( 3 ) 2020.3

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Geophysical Research: Space Physics

This work investigated the zonal drift velocity, VE, of equatorial plasma bubbles (EPBs) which was calculated from the zonal keogram of the rate of total electron content index obtained using the Global Positioning System data from the Malaysia Real-Time Kinematics GNSS Network over Southeast Asia across the period 2008 until 2013 (498 nights). In general, a long-term study on the VE of EPBs and its dependence on the season, local time, and solar activity over Southeast Asia by using Global Positioning System multipoint receivers has never been conducted before. The case study showed that the VE estimated from Malaysia Real-Time Kinematics GNSS Network was basically within a similar range to that of the 630-nm airglow images obtained at Kototabang, Indonesia. Furthermore, the VE of EPBs was higher during high solar-activity period, which was in line with past studies. The time range of the EPB VE peak is more accurately determined compared to previous studies. In high solar activity conditions, a peak of VE was observed at 2130 LT (equinox), 2200 LT (June solstice), and after 2300 LT (December solstice). After this peak, the EPB VE steadily decreased with time during the equinox and was varied throughout the night during June solstice. Moreover, it was found to differ with time from the evening to the postmidnight period during December solstice.

DOI： 10.1029/2019JA027521

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ULF Modulation of Energetic Electron Precipitations Observed by VLF/LF Radio Propagation Invited Reviewed

Takuya Miyashita, Hiroyo Ohya, Fuminori Tsuchiya, Asuka Hirai, Mitsunori Ozaki, Kazuo Shiokawa, Yoshizumi Miyoshi, Nozomu Nishitani, Mariko Teramoto, Martin Connors, Simon G. Shepherd, Yoshiya Kasahara, Atsushi Kumamoto, Masafumi Shoji, Iku Shinohara, Hiroyuki Nakata, Toshiaki Takano

Radio Science Bulletin 2020.3

Wavenumber Spectra of Atmospheric Gravity Waves and Medium-Scale Traveling Ionospheric Disturbances Based on More Than 10-Year Airglow Images in Japan, Russia, and Canada Reviewed

Satoshi Tsuchiya, Kazuo Shiokawa, Yuichi Otsuka, Takuji Nakamura, Mamoru Yamamoto, Martin Connors, Ian Schofield, Boris Shevtsov, Igor Poddelsky

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 125 ( 3 ) 2020.3

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

We have studied atmospheric gravity waves (AGWs) and nighttime medium-scale traveling ionospheric disturbances (MSTIDs) by applying three-dimensional spectral analysis technique to 557.7- and 630.0-nm airglow images at Shigaraki (SGK) (35 degrees N, 136 degrees E, 1999-2017) and Rikubetsu (RIK) (44 degrees N, 144 degrees E, 1999-2017), Japan, Athabasca (ATH), Canada, (55 degrees N, 247 degrees E, 2005-2017), and Magadan (MGD), Russia (60 degrees N, 151 degrees E, 2008-2017), focusing on their horizontal wavenumber spectra. For the AGWs in 557.7-nm images, the power spectra in summer are stronger than in other seasons, probably due to stronger tropospheric convection. The highest energy content of the waves are mostly at wavelengths between 20 and 300 km at MGD, ATH, and RIK, while it is above 200 km at SGK. The largest power spectral density is obtained at RIK at wavelengths of 30-100 km and then ATH. The slopes of the horizontal wavenumber spectra varies from -2.77 to -3.22. From the MSTIDs in 630.0-nm images, the power spectra in summer at RIK and SGK are stronger than those in other seasons regardless of solar activity. The power spectra in solar quiet time are stronger than those in solar active time at all four stations. These features can be explained by the Perkins instability with coupling between sporadic E and F layers. The spectral slope decreases with increasing latitudes. Weak positive correlations were obtained between the daily wave power of AGWs in 557.7-nm images and MSTIDs in 630.0-nm images, suggesting that the MSTIDs in the thermosphere may be partially generated by the AGWs from the mesopause region.Plain Language Summary In this paper we study atmospheric gravity waves (AGWs) and nighttime medium-scale traveling ionospheric disturbances (MSTIDs) observed at four stations in Japan, Canada, and Russia, in 557.7- and 630.0-nm airglow images over more than 10 years. The 557.7-nm airglow has an emission layer at altitudes of 90-100 km (mesopause region). The waves seen in the 557.7-nm airglow images mainly indicates AGWs. The 630.0-nm airglow has an emission layer at altitudes of 200-300 km (bottomside ionosphere). The waves seen in the 630.0-nm airglow images mainly indicates MSTIDs in the ionosphere. The AGWs in the mesopause region are the main driver of global atmospheric circulation in the middle atmosphere. The MSTIDs in the bottomside ionosphere are one of the causes of the satellite positioning error. We show typical energy content, propagation direction, and wavelengths of these waves at these two altitudes and discuss possible reason of the observed characteristics. These results contribute to our understanding of generation and propagation of AGWs and MSTIDs in the upper atmosphere.

DOI： 10.1029/2019JA026807

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Multiple time-scale beats in aurora: precise orchestration via magnetospheric chorus waves Reviewed

Hosokawa K., Miyoshi Y., Ozaki M., Oyama S- I, Ogawa Y., Kurita S., Kasahara Y., Kasaba Y., Yagitani S., Matsuda S., Tsuchiya F., Kumamoto A., Kataoka R., Shiokawa K., Raita T., Turunen E., Takashima T., Shinohara I, Fujii R.

SCIENTIFIC REPORTS Vol. 10 ( 1 ) page： 3380 2020.2

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

The brightness of aurorae in Earth's polar region often beats with periods ranging from sub-second to a few tens of a second. Past observations showed that the beat of the aurora is composed of a superposition of two independent periodicities that co-exist hierarchically. However, the origin of such multiple time-scale beats in aurora remains poorly understood due to a lack of measurements with sufficiently high temporal resolution. By coordinating experiments using ultrafast auroral imagers deployed in the Arctic with the newly-launched magnetospheric satellite Arase, we succeeded in identifying an excellent agreement between the beats in aurorae and intensity modulations of natural electromagnetic waves in space called "chorus". In particular, sub-second scintillations of aurorae are precisely controlled by fine-scale chirping rhythms in chorus. The observation of this striking correlation demonstrates that resonant interaction between energetic electrons and chorus waves in magnetospheres orchestrates the complex behavior of aurora on Earth and other magnetized planets.

DOI： 10.1038/s41598-020-59642-8

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Probing afternoon detached aurora and high-latitude trough based on DMSP observations Reviewed

Su Zhou, Kazuo Shiokawa, Igor Poddelsky, Yuqing Chen, Jin Zhang

ADVANCES IN SPACE RESEARCH Vol. 65 ( 1 ) page： 214 - 220 2020.1

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCI LTD

The present work displays the observations of an afternoon detached aurora along with ionospheric high-latitude trough. The event was observed by DMSP F17 on 19 September 2014. The afternoon detached aurora was isolated from the auroral oval and was located between 12:00-18:00 magnetic local time (MLT) and 65-70 degrees geomagnetic latitude (MLAT). Particle observations indicate that the afternoon detached aurora was produced by energetic ring current ions with energies above similar to 10 keV where the main ion energy was likely to be above the upper limit of DMSP measurement (similar to 30 keV). Magnetometer observation from the ground implies that the energetic ions were likely scattered by EMIC waves. Both the detached aurora and the auroral oval are found to be well inside the high-latitude trough with MLAT between similar to 64 degrees and similar to 76 degrees (68-80 degrees GLAT). The auroral oval corresponds to a westward (sunward) plasma drift. It is expected that the westward drift transports the low-density plasma in the nightside toward the dayside, leading to the high-latitude trough formation. The afternoon detached aurora was well equatorward of the high-latitude trough, and the corresponding plasma drift was nearly zero. The plasma associated with the detached aurora is expected to be stagnant, and broaden the high-latitude trough equatorward. (C) 2019 COSPAR. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.asr.2019.10.003

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Conjugate Observations of Dayside and Nightside VLF Chorus and QP Emissions Between Arase (ERG) and Kannuslehto, Finland Reviewed

Claudia Martinez-Calderon, Yuto Katoh, Jyrki Manninen, Yoshiya Kasahara, Shoya Matsuda, Atsushi Kumamoto, Fuminori Tsuchiya, Ayako Matsuoka, Masafumi Shoji, Mariko Teramoto, Iku Shinohara, Kazuo Shiokawa, Yoshizumi Miyoshi

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 125 ( 1 ) 2020.1

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

We compare for the first time two conjugate events showing simultaneous very low frequency (VLF) wave observations between the same ground station and spacecraft, at different geomagnetic conditions and on opposite sides of the magnetosphere. Waves were observed at Kannuslehto (MLAT = 64.4 degrees N, L = 5.46), Finland, and on board Arase (Exploration of energization and Radiation in Geospace, ERG) in the inner magnetosphere. Case 1 on 28 March 2017 shows quasiperiodic (QP) emissions and chorus simultaneously observed on the postmidnight side during the recovery phase of a storm, with sustained high solar wind speed and AE index. Case 2 on 30 November 2017 shows clear one-to-one correspondence of QP elements on the noonside during geomagnetic quiet time (Dst > 10 nT and AE < 100 nT). We present the characteristics of both cases, focusing on coherence and spatial extent of the waves, electron density, and magnetic field variations. We report that the magnetic field gradient plays a role in the changes of spectral features of the waves.

DOI： 10.1029/2019JA026663

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Capability of Geomagnetic Storm Parameters to Identify Severe Space Weather Reviewed

Balan N., Zhang Qing-He, Xing Zanyang, Skoug R., Shiokawa K., Luehr H., Ram S. Tulasi, Otsuka Y., Zhao Lingxin

ASTROPHYSICAL JOURNAL Vol. 887 ( 1 ) 2019.12

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

The paper investigates the capability of geomagnetic storm parameters in the disturbance storm-time (Dst), Kp, and AE indices to distinguish between severe space weather (SvSW) that causes the reported electric power outages and/or telecommunication failures and normal space weather (NSW) that does not cause these severe effects in a 50 yr period (1958-2007). The parameters include the storm intensities DstMin (minimum Dst during the main phase, MP, of the storm), (dDst/dt)MPmax, Kpmax, and AEmax. In addition, the impulsive parameter is derived for the storms that are automatically identified in the Kyoto Dst and USGS Dst.∫TMP| DstMP| dt is the integral of the modulus of the Dst from onset of the MP (MPO) to the DstMin. T MP is the MP duration from MPO to DstMin. The corresponding mean values KpMP and AEMP are also calculated. Regardless of the significant differences in the storm parameters between the two Dst indices, the IpsDst in both indices seems to identify four of the five SvSW events (and the Carrington event) in more than 750 NSW events that have been reported to have occurred in 1958-2007, while all other parameters separate one or two SvSWs from the NSWs. Using the Kyoto IpsDst threshold of -250 nT, we demonstrate a 100% true SvSW identification rate with only one false NSW. Using the false NSW event (1972 August 4), we investigate whether using a higher resolution Dst might result in a more accurate identification of SvSWs. The mechanism of the impulsive action leading to large IpsDst and SvSW involves the coincidence that the fast interplanetary coronal mass ejection velocity V contains its shock (or front) velocity ΔV and large interplanetary magnetic field Bz southward covering ΔV.

DOI： 10.3847/1538-4357/ab5113

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A Neural Network-Based Ionospheric Model Over Africa From Constellation Observing System for Meteorology, Ionosphere, and Climate and Ground Global Positioning System Observations Reviewed

Daniel Okoh, Gopi Seemala, Babatunde Rabiu, John Bosco Habarulema, Shuanggen Jin, Kazuo Shiokawa, Yuichi Otsuka, Malini Aggarwal, Jean Uwamahoro, Patrick Mungufeni, Bolaji Segun, Aderonke Obafaye, Nada Ellahony, Chinelo Okonkwo, Mpho Tshisaphungo, Dadaso Shetti

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 124 ( 12 ) page： 10512 - 10532 2019.12

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

The first regional total electron content (TEC) model over the entire African region (known as AfriTEC model) using empirical observations is developed and presented. Artificial neural networks were used to train TEC observations obtained from Global Positioning System receivers, both on ground and onboard the Constellation Observing System for Meteorology, Ionosphere, and Climate satellites for the African region from years 2000 to 2017. The neural network training was implemented using inputs that enabled the networks to learn diurnal variations, seasonal variations, spatial variations, and variations that are connected with the level of solar activity, for quiet geomagnetic conditions (-20 nT <= Dst <= 20 nT). The effectiveness of three solar activity indices (sunspot number, solar radio flux at 10.7-cm wavelength [F10.7], and solar ultraviolet [UV] flux at 1 AU) for the neural network trainings was tested. The F10.7 and UV were more effective, and the F10.7 was used as it gave the least errors on the validation data set used. Equatorial anomaly simulations show a reduced occurrence during the June solstice season. The distance of separation between the anomaly crests is typically in the range from about 11.5 +/- 1.0 degrees to 16.0 +/- 1.0 degrees. The separation is observed to widen as solar activity levels increase. During the December solstice, the anomaly region shifts southwards of the equinox locations; in year 2012, the trough shifted by about 1.5 degrees and the southern crest shifted by over 2.5 degrees.

DOI： 10.1029/2019JA027065

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Three Different Episodes of Prompt Equatorial Electric Field Perturbations Under Steady Southward IMF Bz During St. Patrick's Day Storm Reviewed

S. Tulasi Ram, B. Nilam, N. Balan, Q. Zhang, K. Shiokawa, D. Chakrabarty, Z. Xing, K. Venkatesh, B. Veenadhari, A. Yoshikawa

Journal of Geophysical Research: Space Physics Vol. 124 ( 12 ) page： 10428 - 10443 2019.12

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

Three different episodes of prompt penetration electric field (PPEF) disturbances are observed during the main phase of the St. Patrick's Day storm on 17 March 2015 under steady southward interplanetary magnetic field (IMF) Bz conditions unlike the conventional PPEF associated with southward or northward turnings of IMF Bz. These PPEF events took place during the period when strong disturbance dynamo fields are prevailing in the background. The first event is triggered by a solar wind dynamic pressure pulse that caused a sharp eastward PPEF and strong enhancement of equatorial electrojet current in Brazilian dayside. The second event caused another short but strong westward PPEF on dayside due to the reversal of IMF By from duskward to dawnward under steady IMF Bz. The third event caused a longer eastward PPEF in association with a solar wind dynamic pressure pulse followed by the onset of a substorm, which has led to strong enhancement of equatorial electrojet, quick rejuvenation and symmetric redistribution of equatorial ionization anomaly in the Brazilian sector. The signatures of the PPEF with opposite polarity and smaller magnitudes are also observed in the Asian sector on the nightside. The possible mechanisms for the observed PPEF events under steady IMF Bz are discussed in terms of changes in the high-latitude field-aligned currents and reconfiguration of high-latitude convection fields using Active Magnetosphere and Planetary Electrodynamics Response Experiment and Super Dual Auroral Radar Network high-frequency radar observations.

DOI： 10.1029/2019JA027069

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Longitudinal Extent of Magnetospheric ELF/VLFWaves using Multipoint PWING Ground Stations at Subauroral Latitudes Reviewed

Yuhei Takeshita, Kazuo Shiokawa, Mitsunori Ozaki, Jyrki Manninen, Shin-Ichiro Oyama, Martin Connors, Dmitry Baishev, Vladimir Kurkin, Alexey Oinats

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 124 ( 12 ) page： 9881 - 9892 2019.12

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

Magnetospheric extremely low frequency/very low frequency (ELF/VLF) waves are plasma waves emitted from high-energy electrons in the magnetosphere. These waves have received much attention, as they contribute to the acceleration and loss of relativistic electrons in the radiation belts through wave-particle interactions. The longitudinal extent of ELF/VLF waves has not been well-understood, although the extent is important in quantitative evaluation of relativistic electron variations. In this study, we analyzed data from continuous ground-based simultaneous observations of ELF/VLF waves over a 2-month period in November and December of 2017, using six loop antennas located at roughly equal intervals around the north geomagnetic pole at similar to 60 degrees. magnetic latitudes. We estimated the longitudinal extent of magnetospheric ELF/VLF waves based on their occurrence rate. Our results showed that the ELF/VLF wave occurrence rate differed by twofold to threefold, depending on the longitudes of the observation sites. We explain this difference in terms of longitudinal differences in the ionosphere's magnetic field intensity, possibly due to the electron loss that occurs during the bounce motion at longitudes of small magnetic field intensity. Based on our statistical analysis, we estimated the typical longitudinal extent of ELF/VLF waves as similar to 76 degrees. Time series analysis results showed that the large longitudinal extent of the ELF/VLF waves occurs frequently during the main phase of geomagnetic storms and is also associated with substorms represented by the auroral electrojet index.

DOI： 10.1029/2019JA026810

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IpsDst of Dst Storms Applied to Ionosphere-Thermosphere Storms and Low-Latitude Aurora Reviewed

Balan N., Zhang Qing -He, Shiokawa K., Skoug R., Xing Zanyang, Ram S. Tulasi, Otsuka Y.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 124 ( 11 ) page： 9552 - 9565 2019.11

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Geophysical Research: Space Physics

Conventionally, the minimum value of Dst (DstMin) and maximum values of Kp and AE (Kpmax and AEmax) representing the geomagnetic storm intensities have been used for investigating space weather in Earth's environment. The present paper uses the derived parameters (IpsDst, IpsKp, and IpsAE) giving the mean values of Dst, Kp, and AE during the main phase (MP) of Dst storms for investigating ionosphere-thermosphere storms and low-latitude (630 nm) aurora. The derived parameters (IpsDst, IpsKp, and IpsAE) representing the impulsive strength of geomagnetic storms seem to have more systematic dependence among themselves than among the intensities (DstMin, Kpmax, and AEmax). The ionosphere-thermosphere storms observed by the CHAMP (Challenging Minisatellite Payload) satellite and low-latitude auroras observed by optical imagers are much more intense during high impulsive storms than high intensity storms. In a statistical sense, over 175 positive ionospheric storms (△NmF2) observed in 1985–2005 and the intensity of 20 red auroras observed in 1989–2004 at midlatitudes correlate better with the impulsive parameters than the intensity parameters, with the best correlation being with IpsDst. The mechanism of the impulsive action (high-energy input over a short duration) leading to large IpsDst arises from the impact of fast solar storms (interplanetary coronal mass ejections) with large IMF Bz southward at their front (or shock). The impulsive action results in bright low-latitude auroras and strong ionosphere-thermosphere storms.

DOI： 10.1029/2019JA027080

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Statistical Study of Auroral/Resonant-Scattering 427.8-nm Emission Observed at Subauroral Latitudes Over 14 Years Reviewed

Shiokawa K., Otsuka Y., Connors M.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS Vol. 124 ( 11 ) page： 9293 - 9301 2019.11

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Geophysical Research: Space Physics

Auroral emission at 427.8-nm from N2+ ions is caused by precipitation of energetic electrons, or by resonant scattering of sunlight by auroral N2+ ions. The latter often causes impressive purple aurora at high altitudes. However, statistical characteristics of auroral 427.8-nm emission have not been well studied. In this paper we report occurrence characteristics of high 427.8-nm emission intensities (more than 100 R) at subauroral latitudes, based on measurements by a filter-tilting photometer over 14 years (2005–2018) at Athabasca, Canada (magnetic latitude: ~62°). We divided the data set into solar elevation angles (θs) more than and less than −24° (shadow height of sunlight: 600 km) to separate the 427.8-nm emissions caused by resonant scattering of sunlight and those excited by auroral electrons, respectively. The occurrence rate of 427.8-nm emissions of more than 100 R is 10.6% and 7.65% for θs more than and less than −24°, respectively, confirming that resonant scattering of sunlight by N2+ ions is a cause of the strong 427.8-nm emissions of more than 100 R in the sunlit ionosphere. The occurrence rate is high in the postmidnight sector and increases with increasing geomagnetic activity, solar wind speed, and density. The occurrence rate is lowest in winter. A high occurrence rate was observed in 2015–2018, during the declining phase of the 11-year solar activity. Superposed epoch analysis indicates that the 427.8-nm emission exceeds 100 R when solar wind speed increases and solar wind density concurrently decreases, though the standard deviation of the data is rather large.

DOI： 10.1029/2019JA026704

Web of Science

Scopus

High-latitude thermospheric wind study using a Fabry-Perot interferometer at Tromso in Norway: averages and variations during quiet times Reviewed

Heqiucen Xu, Kazuo Shiokawa, Shin-ichiro Oyama, Satonori Nozawa

EARTH PLANETS AND SPACE Vol. 71 ( 1 ) 2019.10

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

The average winds in the thermosphere during geomagnetically quiet times are important because they provide a baseline wind in the upper atmosphere, but they remain insufficiently understood at high latitudes. This paper reports the first direct ground-based wind measurements of the quiet-time thermospheric wind pattern at Tromso in Norway using 2009-2015 data from a Fabry-Perot interferometer. We analyzed red-line wind measurements (630.0 nm; altitude: 200-300 km). On average, the zonal wind shows a decrease of eastward wind compared with diurnal tidal wind before midnight. A maximum speed of 100 m/s occurs at both the dusk and dawn sides. The meridional wind has a diurnal tide structure with a minimum value of - 130 m/s around midnight. We also found occasional large wind deviations (> 100 m/s) from the averages, even during geomagnetically quiet times. We suggest that these large wind deviations are caused by the plasma convection associated with weak substorm activities with auroral electrojet (AE) index values of less than 100 nT that occurred at local times different from that at Tromso.

DOI： 10.1186/s40623-019-1093-8

Web of Science

Scopus

Direct comparison between magnetospheric plasma waves and polar mesosphere winter echoes in both hemispheres Reviewed International journal

Tanaka, Y.-M, T. Nishiyama, A. Kadokura, M. Ozaki, K. Shiokawa, S. Oyama, M. Nosé, T. Nagatsuma, M. Tsutsumi, K. Nishimura, K. Sato, Y. Miyoshi, Y. Kasahara, A. Kumamoto, F. Tsuchiya, M. Hikishima, S. Matsuda, A. Matsuoka, M. Shinohara, A. Fujimoto, M. Teramoto, R. Nomura, R. Kataoka, A. Sessai Yukimatu

J. Geophys. Res. (SPA) Vol. 124 ( 11 ) page： 9626 - 9639 2019.10

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

We present the first and direct comparison between magnetospheric plasma waves and polar mesosphere winter echoes (PMWE) simultaneously observed by the conjugate observation with Arase satellite and high‐power atmospheric radars in both hemispheres, namely, the Program of the Antarctic Syowa Mesosphere, Stratosphere, and Troposphere/Incoherent Scatter Radar at Syowa Station (SYO; −69.00°S, 39.58°E), Antarctica, and the Middle Atmosphere Alomar Radar System at Andøya (AND; 69.30°N, 16.04°E), Norway. The PMWE were observed during 03–07 UT on 21 March 2017, just after the arrival of corotating interaction region in front of high‐speed solar wind stream. An isolated substorm occurred at 04 UT during this interval. Electromagnetic ion cyclotron (EMIC) waves and whistler mode chorus waves were simultaneously observed near the magnetic equator and showed similar temporal variations to that of the PMWE. These results indicate that chorus waves as well as EMIC waves are drivers of precipitation of energetic electrons, including relativistic electrons, which make PMWE detectable at 55‐ to 80‐km altitude. Cosmic noise absorption measured with a 38.2‐MHz imaging riometer and low‐altitude echoes at 55–70 km measured with an medium‐frequency radar at SYO also support the relativistic electron precipitation. We suggest a possible scenario in which the various phenomena observed in near‐Earth space, such as magnetospheric plasma waves (EMIC waves and chorus waves), pulsating auroras, cosmic noise absorption, and PMWE, can be explained by the interaction between the high‐speed solar wind containing corotating interaction regions and the magnetosphere.

DOI： 10.1029/2019JA026891

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