研究者詳細 - 塩川　和夫

論文 - 塩川　和夫

分割表示 483 件中 61 - 80 件目／全件表示 >>

Investigation of Small-Scale Electron Density Irregularities Observed by the Arase and Van Allen Probes Satellites Inside and Outside the Plasmasphere 査読有り

Thomas Neethal, Shiokawa Kazuo, Miyoshi Yoshizumi, Kasahara Yoshiya, Shinohara Iku, Kumamoto Atsushi, Tsuchiya Fuminori, Matsuoka Ayako, Kasahara Satoshi, Yokota Shoichiro, Keika Kunihiro, Hori Tomoaki, Asamura Kazushi, Wang Shiang-Yu, Kazama Yoichi, Tam Sunny Wing-Yee, Chang Tzu-Fang, Wang Bo-Jhou, Wygant John, Breneman Aaron, Reeves Geoffrey

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS 126 巻 ( 3 ) 2021年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Geophysical Research: Space Physics

In situ electron density profiles obtained from Arase in the night magnetic local time (MLT) sector and from RBSP-B covering all MLTs are used to study the small-scale density irregularities present in the plasmasphere and near the plasmapause. Electron density perturbations with amplitudes >10% from background density and with time-scales less than 30-min are investigated here as the small-scale density irregularities. The statistical survey of the density irregularities is carried out using nearly 2 years of density data obtained from RBSP-B and 4 months of data from Arase satellites. The results show that density irregularities are present globally at all MLT sectors and L-shells both inside and outside the plasmapause, with a higher occurrence at L > 4. The occurrence of density irregularities is found to be higher during disturbed geomagnetic and interplanetary conditions. The case studies presented here revealed: (1) The plasmaspheric density irregularities observed during both quiet and disturbed conditions are found to coexist with the hot plasma sheet population. (2) During quiet periods, the plasma waves in the whistler-mode frequency range are found to be modulated by the small-scale density irregularities, with density depletions coinciding well with the decrease in whistler intensity. Our observations suggest that different source mechanisms are responsible for the generation of density structures at different MLTs and geomagnetic conditions.

DOI： 10.1029/2020JA027917

その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2020JA027917

Statistical Analysis of Pc1 Wave Ducting Deduced From Swarm Satellites 査読有り

Kim Hyangpyo, Shiokawa Kazuo, Park Jaeheung, Miyoshi Yoshizumi, Stolle Claudia, Buchert Stephan

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS 126 巻 ( 3 ) 2021年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Geophysical Research: Space Physics

Transverse Pc1 waves propagating from magnetospheric source regions undergo mode conversion to the compressional mode in the ionosphere due to the induced Hall current. Mode converted Pc1 waves propagate across the magnetic field through the ionospheric waveguide. This process is called Pc1 wave ducting (PWD). PWDs have been observed by magnetometers on both ground and low Earth orbit satellites over a wide latitudinal and longitudinal range. In this work, we present the statistical analysis results of PWD exploiting Swarm satellites from 2015 to 2019. Spatial distributions show that the PWDs are mainly observed over the South Atlantic Anomaly longitudes, possibly due to the high Hall conductivity and F-region density, and at subauroral/auroral latitudes ((Formula presented.) MLAT). The occurrence rate of PWD increases with increasing AE and (Formula presented.) SYM-H (Formula presented.) indices. Seasonal dependence shows that PWD exhibits a high occurrence rate during equinox and local summer while local winter hosts only a low occurrence. The asymmetry between summer and winter can be explained by the ionospheric plasma density. The high occurrence rate in equinox may result from intense geomagnetic activity during the equinox, probably due to the Russell-McPherron effect. From our statistical analysis, we conclude that the occurrence of PWD is controlled by both ionospheric plasma conditions and geomagnetic activity, and that the mode conversion and PWD occur more efficiently as plasma density increases.

DOI： 10.1029/2020JA029016

Formation of an additional density peak in the bottom side of the sodium layer associated with the passage of multiple mesospheric frontal systems 査読有り

Narayanan Viswanathan Lakshmi, Nozawa Satonori, Oyama Shin-Ichiro, Mann Ingrid, Shiokawa Kazuo, Otsuka Yuichi, Saito Norihito, Wada Satoshi, Kawahara Takuya D., Takahashi Toru

ATMOSPHERIC CHEMISTRY AND PHYSICS 21 巻 ( 4 ) 頁： 2343 - 2361 2021年2月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Atmospheric Chemistry and Physics

We present a detailed investigation of the formation of an additional sodium density peak at altitudes of 79-85 km below the main peak of the sodium layer based on sodium lidar and airglow imager measurements made at Ramfjordmoen near Tromsø, Norway, on the night of 19 December 2014. The airglow imager observations of OH emissions revealed four passing frontal systems that resembled mesospheric bores, which typically occur in ducting regions of the upper mesosphere. For about 1.5 h, the lower-altitude sodium peak had densities similar to that of the main peak of the layer around 90 km. The lower-altitude sodium peak weakened and disappeared soon after the fourth front had passed. The fourth front had weakened in intensity by the time it approached the region of lidar beams and disappeared soon afterwards. The column-integrated sodium densities increased gradually during the formation of the lower-altitude sodium peak. Temperatures measured with the lidar indicate that there was a strong thermal duct structure between 87 and 93 km. Furthermore, the temperature was enhanced below 85 km. Horizontal wind magnitudes estimated from the lidar showed strong wind shears above 93 km. We conclude that the combination of an enhanced stability region due to the temperature profile and intense wind shears have provided ideal conditions for evolution of multiple mesospheric bores revealed as frontal systems in the OH images. The downward motion associated with the fronts appeared to have brought air rich in H and O from higher altitudes into the region below 85 km, wherein the temperature was also higher. Both factors would have liberated sodium atoms from the reservoir species and suppressed the reconversion of atomic sodium into reservoir species so that the lower-altitude sodium peak could form and the column abundance could increase. The presented observations also reveal the importance of mesospheric frontal systems in bringing about significant variation of minor species over shorter temporal intervals.

DOI： 10.5194/acp-21-2343-2021

Multi-Wavelength Imaging Observations of STEVE at Athabasca, Canada 査読有り

Yadav Sneha, Shiokawa Kazuo, Otsuka Yuichi, Connors Martin, St Maurice J-P

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS 126 巻 ( 2 ) 2021年2月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Geophysical Research: Space Physics

We present the first multi-wavelength imaging observations of strong thermal emission velocity enhancement (STEVE) using an all-sky imager at Athabasca (magnetic latitude = 61.5°N), Canada. This study is based on three STEVE events which were accompanied by picket fence structures in the green-line. Although the STEVE arc was dominant in 630 and 557.7-nm, weak emissions were also found in other wavelengths including OI at 844.6, Hβ, Na, and the nominal background filter at 572.5-nm. As observed at 630 and 557.7-nm, the STEVE arc started as a faint arc close to the auroral oval and moved equatorward. The 557.7-nm arc exhibited picket fence structure at later times after it moved equatorward. The picket fence was sometimes found to persist even after the 630-nm arc had disappeared. During a particular event, the STEVE arcs in both the 630 and 557.7-nm were found to carry a ribbon-like motion moving along the arc. We have found that STEVE arcs are embedded in a region of weak diffuse auroral emissions. The STEVE arcs have sharp boundaries and these boundaries are different in red- and green-line. The sharp decrease in the intensity at the immediate poleward edge of the STEVE arc appears as a “dark-band” in the green-line images. Based on the horizontal component of the geomagnetic field at Fort Smith (magnetic latitude 67.28°N), we find that the STEVE arc detachment, its equatorward motion, and its brightness coincided with changes in the magnetic activity during the recovery phase of a substorm.

DOI： 10.1029/2020JA028622

Multievent Study of Characteristics and Propagation of Naturally Occurring ELF/VLF Waves Using High-Latitude Ground Observations and Conjunctions With the Arase Satellite 査読有り

Martinez-Calderon C., Katoh Y., Manninen J., Santolik O., Kasahara Y., Matsuda S., Kumamoto A., Tsuchiya F., Matsuoka A., Shoji M., Teramoto M., Shinohara I, Shiokawa K., Miyoshi Y.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS 126 巻 ( 2 ) 2021年2月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Geophysical Research: Space Physics

We report the properties of the ionospheric exit point and characteristics of several types of very low frequency waves, including chorus and quasiperiodic emissions, based on a comprehensive dataset of simultaneous observations between ground and space. Whistler-mode waves were observed at Kannuslehto (L = 5.5, KAN), Finland, and in the inner magnetosphere by the Japanese Arase satellite. During the 2017–2018 winter campaign, we found 13 cases showing one-to-one correspondence of wave spectra between KAN and Arase. This is the first time that such a large number of conjugated events have been reported at once. The duration of the events ranged from a few minutes up to 3 h, with 90% of events detected in the afternoon sector. While the occurrence rate is higher during daytime, this can also be related to a majority of the detected waves being quasiperiodic emissions, a known dayside phenomenon. Arase was usually located within 30° of the equator, at L ∼4–5, and detected mostly waves propagating at oblique angles (≥20°). Frequently, the ionospheric magnetic footprint of Arase was located equatorwards (south) from KAN, often in the same geographical area. We investigated the probable location of the ionospheric exit point of the waves from the location of the footprint of Arase and the angle of arrival of waves detected at KAN. Using density measurements at Arase we discuss magnetospheric wave propagation; we find that, in most cases, waves were unducted in their propagation from the satellite to the ground.

DOI： 10.1029/2020JA028682

その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2020JA028682

Study of Spatiotemporal Development of Global Distribution of Magnetospheric ELF/VLF Waves Using Ground-Based and Satellite Observations, and RAM-SCB Simulations, for the March and November 2017 Storms 査読有り

Takeshita Yuhei, Shiokawa Kazuo, Miyoshi Yoshizumi, Ozaki Mitsunori, Kasahara Yoshiya, Oyama Shin-ichiro, Connors Martin, Manninen Jyrki, Jordanova Vania K., Baishev Dmitry, Oinats Alexey, Kurkin Vladimir

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS 126 巻 ( 2 ) 2021年2月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Geophysical Research: Space Physics

Magnetospheric Extremely Low-Frequency/Very Low-Frequency (ELF/VLF) waves have an important role in the acceleration and loss of energetic electrons in the magnetosphere through wave-particle interaction. It is necessary to understand the spatiotemporal development of magnetospheric ELF/VLF waves to quantitatively estimate this effect of wave-particle interaction, a global process not yet well understood. We investigated spatiotemporal development of magnetospheric ELF/VLF waves using 6 PWING ground-based stations at subauroral latitudes, Exploration of energization and Radiation in Geospace and RBSP satellites, POES/MetOp satellites, and the RAM-SCB model, focusing on the March and November 2017 storms driven by corotating interaction regions in the solar wind. Our results show that the ELF/VLF waves are enhanced over a longitudinal extent from midnight to morning and dayside associated with substorm electron injections. In the main to early storm recovery phase, we observe continuous ELF/VLF waves from ∼0 to ∼12 MLT in the dawn sector. This wide extent seems to be caused by frequent occurrence of substorms. The wave region expands eastward in association with the drift of source electrons injected by substorms from the nightside. We also observed dayside ELF/VLF wave enhancement, possibly driven by magnetospheric compression by solar wind, over an MLT extent of at least 5 h. Ground observations tend not to observe ELF/VLF waves in the post-midnight sector, although other methods clearly show the existence of waves. This is possibly due to Landau damping of the waves, the absence of the plasma density duct structure, and/or enhanced auroral ionization of the ionosphere in the post-midnight sector.

DOI： 10.1029/2020JA028216

その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2020JA028216

Active auroral arc powered by accelerated electrons from very high altitudes 査読有り

Imajo Shun, Miyoshi Yoshizumi, Kazama Yoichi, Asamura Kazushi, Shinohara Iku, Shiokawa Kazuo, Kasahara Yoshiya, Kasaba Yasumasa, Matsuoka Ayako, Wang Shiang-Yu, Tam Sunny W. Y., Chang Tzu-Fang, Wang Bo-Jhou, Angelopoulos Vassilis, Jun Chae-Woo, Shoji Masafumi, Nakamura Satoko, Kitahara Masahiro, Teramoto Mariko, Kurita Satoshi, Hori Tomoaki

SCIENTIFIC REPORTS 11 巻 ( 1 ) 頁： 1610 2021年1月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Scientific Reports

Bright, discrete, thin auroral arcs are a typical form of auroras in nightside polar regions. Their light is produced by magnetospheric electrons, accelerated downward to obtain energies of several kilo electron volts by a quasi-static electric field. These electrons collide with and excite thermosphere atoms to higher energy states at altitude of ~ 100 km; relaxation from these states produces the auroral light. The electric potential accelerating the aurora-producing electrons has been reported to lie immediately above the ionosphere, at a few altitudes of thousand kilometres1. However, the highest altitude at which the precipitating electron is accelerated by the parallel potential drop is still unclear. Here, we show that active auroral arcs are powered by electrons accelerated at altitudes reaching greater than 30,000 km. We employ high-angular resolution electron observations achieved by the Arase satellite in the magnetosphere and optical observations of the aurora from a ground-based all-sky imager. Our observations of electron properties and dynamics resemble those of electron potential acceleration reported from low-altitude satellites except that the acceleration region is much higher than previously assumed. This shows that the dominant auroral acceleration region can extend far above a few thousand kilometres, well within the magnetospheric plasma proper, suggesting formation of the acceleration region by some unknown magnetospheric mechanisms.

DOI： 10.1038/s41598-020-79665-5

An Ephemeral Red Arc Appeared at 68° MLat at a Pseudo Breakup During Geomagnetically Quiet Conditions 査読有り

Oyama S., Shinbori A., Ogawa Y., Kellinsalmi M., Raita T., Aikio A., Vanhamaeki H., Shiokawa K., Virtanen I., Cai L., Workayehu A. B., Pedersen M., Kauristie K., Tsuda T. T., Kozelov B., Demekhov A., Yahnin A., Tsuchiya F., Kumamoto A., Kasahara Y., Matsuoka A., Shoji M., Teramoto M., Lester M.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS 125 巻 ( 10 ) 2020年10月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Geophysical Research: Space Physics

Various subauroral optical features have been studied by analyzing data collected during periods of geomagnetic disturbances. Most events have been typically found at geomagnetic latitudes of 45–60°. In this study, however, we present a red arc event found at geomagnetic 68° north (L ≈ 7.1) in the Scandinavian sector during a period of geomagnetically quiet conditions within a short intermission between two high-speed solar wind events. The red arc appeared to coincide with a pseudo breakup at geomagnetic 71–72°N and a rapid equatorward expansion of the polar cap. However, the red arc disappeared in approximately 7 min. Simultaneous measurements with the Swarm A/C satellites indicated the appearance of the red arc at the ionospheric trough minimum and a conspicuous enhancement of the electron temperature, suggesting the generation of the arc by heat flux. Since there are meaningful differences in the red arc features from already-known subauroral optical features such as the stable auroral red (SAR) arc, we considered that the red arc is a new phenomenon. We suggest that the ephemeral red arc may represent the moment of SAR arc birth associated with substorm particle injection, which is generally masked by bright dynamic aurorae.

DOI： 10.1029/2020JA028468

その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2020JA028468

Plasma and Field Observations in the Magnetospheric Source Region of a Stable Auroral Red (SAR) Arc by the Arase Satellite on 28 March 2017 査読有り

Inaba Yudai, Shiokawa Kazuo, Oyama Shin-ichiro, Otsuka Yuichi, Oksanen Arto, Shinbori Atsuki, Gololobov Artem Yu, Miyoshi Yoshizumi, Kazama Yoichi, Wang Shiang-Yu, Tam Sunny W. Y., Chang Tzu-Fang, Wang Bo-Jhou, Yokota Shoichiro, Kasahara Satoshi, Keika Kunihiro, Hori Tomoaki, Matsuoka Ayako, Kasahara Yoshiya, Kumamoto Atsushi, Kasaba Yasumasa, Tsuchiya Fuminori, Shoji Masafumi, Shinohara Iku, Stolle Claudia

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS 125 巻 ( 10 ) 2020年10月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Geophysical Research: Space Physics

A stable auroral red (SAR) arc is an aurora with a dominant 630 nm emission at subauroral latitudes. SAR arcs have been considered to occur due to the spatial overlap between the plasmasphere and the ring-current ions. In the overlap region, plasmaspheric electrons are heated by ring-current ions or plasma waves, and their energy is then transferred down to the ionosphere where it causes oxygen red emission. However, there have been no study conducted so far that quantitatively examined plasma and electromagnetic fields in the magnetosphere associated with SAR arc. In this paper, we report the first quantitative evaluation of conjugate measurements of a SAR arc observed at 2204 UT on 28 March 2017 and investigate its source region using an all-sky imager at Nyrölä (magnetic latitude: 59.4°N), Finland, and the Arase satellite. The Arase observation shows that the SAR arc appeared in the overlap region between a plasmaspheric plume and the ring-current ions and that electromagnetic ion cyclotron waves and kinetic Alfven waves were not observed above the SAR arc. The SAR arc was located at the ionospheric trough minimum identified from a total electron content map obtained by the GNSS receiver network. The Swarm satellite flying in the ionosphere also passed the SAR arc at ~2320 UT and observed a decrease in electron density and an increase in electron temperature during the SAR-arc crossing. These observations suggest that the heating of plasmaspheric electrons via Coulomb collision with ring-current ions is the most plausible mechanism for the SAR-arc generation.

DOI： 10.1029/2020JA028068

Statistical study of EMIC Pc1-Pc2 waves observed at subauroral latitudes 査読有り

Kwon Jong-Woo, Kim Khan-Hyuk, Jin Ho, Kwon Hyuck-Jin, Jee Geonhwa, Shiokawa Kazuo, Connors Martin

JOURNAL OF ATMOSPHERIC AND SOLAR-TERRESTRIAL PHYSICS 205 巻 2020年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Atmospheric and Solar-Terrestrial Physics

Although the occurrence rate of electromagnetic ion cyclotron (EMIC) waves is high in the outer magnetosphere (L > 7), it has been suggested in the past that a steep plasma density gradient region of the plasmapause is a preferred location for the generation of EMIC waves. To examine spectral properties of the EMIC waves occurred near the nominal location of the plasmapause (L = 4–5), we focus on Pc1-Pc2 waves observed at subauroral latitude Athabasca station (magnetic latitude: ~62°, and L ~ 4.6). A statistical study of 10,494 wave samples identified from Athabasca data for 2007–2008 reveals the following wave characteristics. (1) Wave frequencies are higher in the postmidnight-to-dawn sector and lower in the late afternoon sector. (2) They mostly appear to be in frequency band between helium and oxygen gyrofrequencies (i.e., He-band) calculated from the dipole field model magnetic field intensity at L = 4.6. (3) The occurrence rate of Pc1-Pc2 waves has a peak in the prenoon sector at 0900–1100 LT under quiet geomagnetic conditions (Kp ≤ 1), but is peaked in the afternoon sector under moderate and disturbed geomagnetic conditions (Kp ≥ 2). (4) The Pc1-Pc2 waves observed at Athabasca were composed of a mixture of left-hand, right-hand, and linearly polarized waves. By comparing previous and recent studies with these observations, we suggest that the subauroral latitude Pc1-Pc2 waves are associated with EMIC waves generated near the plasmapause and discuss the EMIC wave properties in a region of cold and dense plasmas containing heavy ions in the inner magnetosphere.

DOI： 10.1016/j.jastp.2020.105292

Equatorial Plasma Bubble Occurrence Under Propagation of MSTID and MLT Gravity Waves 査読有り

Takahashi H., Wrasse C. M., Figueiredo C. A. O. B., Barros D., Paulino I., Essien P., Abdu M. A., Otsuka Y., Shiokawa K.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS 125 巻 ( 9 ) 2020年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Geophysical Research: Space Physics

We observed a signature of equatorial plasma bubbles (EPBs) followed by a medium-scale traveling ionospheric disturbance (MSTID) crossing the solar terminator over the geomagnetic equator during the period of post-sunset rise (PSSR) of the F layer. Simultaneous observations of the EPBs and the MSTIDs by ground-based Global Navigation Satellite System receivers and OI 630-nm imagers, ionospheric parameters by ionosondes, and mesosphere–lower thermosphere (MLT) gravity waves (MLT-GWs) by OH airglow imagers have been carried out in the equatorial and low-latitude region of Brazil. On 16–17 September 2015, MSTIDs with very fast horizontal phase speeds, propagating toward northeast (NE), were observed over the latitude region of 0–30°S in the afternoon to evening time zone. It was geomagnetically quiet time (Kp: 2–3). An EPB development was observed after one of the MSTIDs crossed the solar terminator at 22:00 UT. Large-scale MLT-GWs with slower phase speeds were also observed both at São João do Cariri (7.5°S, 35.0°W) propagating from southwest (SW) to NE and Cachoeira Paulista (22.7°S, 45.0°W) propagating northward. This is to report on what we consider as a signature of direct seeding of EPBs by an MSTID that was likely induced by a secondary GW from tropospheric deep cloud convections to the south. This study highlights a possible dynamical process from the troposphere to ionosphere via this vertical coupling process over the 2,500 km of horizontal distance.

DOI： 10.1029/2019JA027566

Spatial Extent of Quasiperiodic Emissions Simultaneously Observed by Arase and Van Allen Probes on 29 November 2018 査読有り

Martinez-Calderon C., Nemec F., Katoh Y., Shiokawa K., Kletzing C., Hospodarsky G., Santolik O., Kasahara Y., Matsuda S., Kumamoto A., Tsuchiya F., Matsuoka A., Shoji M., Teramoto M., Kurita S., Miyoshi Y., Ozaki M., Nishitani N., Oinats A. V., Kurkin V. I.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS 125 巻 ( 9 ) 2020年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Geophysical Research: Space Physics

Recent availability of a considerable amount of satellite and ground-based data has allowed us to analyze rare conjugated events where extremely low and very low frequency waves from the same source region are observed in different locations. Here, we report a quasiperiodic (QP) emission, showing one-to-one correspondence, observed by three satellites in space (Arase and the Van Allen Probes) and a ground station. The main event was on 29 November 2018 from 12:06 to 13:08 UT during geomagnetically quiet times. Using the position of the satellites we estimated the spatial extent of the area where the one-to-one correspondence is observed. We found this to be up to 1.21 Earth's radii by 2.26 hr MLT, in radial and longitudinal directions, respectively. Using simple ray tracing calculations, we discuss the probable source location of these waves. At ∼12:20 UT, changes in the frequency sweep rate of the QP elements are observed at all locations associated with magnetic disturbances. We also discuss temporal changes of the spectral shape of QP observed simultaneously in space and on the ground, suggesting the changes are related to properties of the source mechanisms of the waves. This could be linked to two separate sources or a larger source region with different source intensities (i.e., electron flux). At frequencies below the low hybrid resonance, waves can experience attenuation and/or reflection in the magnetosphere. This could explain the sudden end of the observations at the spacecraft, which are moving away from the area where waves can propagate.

DOI： 10.1029/2020JA028126

その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2020JA028126

The Solar Wind Density Control on the Prompt Penetration Electric Field and Equatorial Electrojet 査読有り

Nilam B., Ram S. Tulasi, Shiokawa K., Balan N., Zhang Q.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS 125 巻 ( 9 ) 2020年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Geophysical Research: Space Physics

Prompt penetration of convection/overshielding electric fields to equatorial and low latitudes during the southward/northward turnings of interplanetary magnetic field (IMF Bz) have been widely studied in the literature. The other types of penetration electric fields due to sudden changes in the solar wind dynamic pressure, IMF By and during the onset of substorms have also been recently reported. In this paper, we present the exclusive role of solar wind density changes on the prompt equatorial electric field disturbances using the long-term observations of equatorial electrojet (EEJ) from the Indian sector. In response to the sharp increases in the solar wind density, prompt increases/decreases in the EEJ indicating the eastward/westward prompt penetration electric field (PPEF) of ~20 min periods have been consistently observed on the dayside/nightside. The prompt equatorial electric field disturbances of the opposite polarity have also been observed when the density decreases sharply. Further, the polarity of these PPEF disturbances does not show any clear dependency on the direction of IMF Bz and By. This paper is the first report with a statistically significant number of observations on the characteristics of equatorial electric field disturbances in response to the sudden enhancements/decreases in the solar wind density alone on both dayside and nightside. The underlying physical mechanisms for the prompt equatorial electric field disturbances have been discussed in light of enhanced high-latitude convection and additional field-aligned currents due to sudden enhancement of solar wind density.

DOI： 10.1029/2020JA027869

Ionospheric Plasma Density Oscillation Related to EMIC Pc1 Waves 査読有り

Kim Hyangpyo, Shiokawa Kazuo, Park Jaeheung, Miyoshi Yoshizumi, Miyashita Yukinaga, Stolle Claudia, Kim Khan-Hyuk, Matzka Juergen, Buchert Stephan, Fromm Tanja, Hwang Junga

GEOPHYSICAL RESEARCH LETTERS 47 巻 ( 15 ) 2020年8月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Geophysical Research Letters

We report the first observation of plasma density oscillations coherent with magnetic Pc1 waves. Swarm satellites observed compressional Pc1 wave activity in the 0.5–3 Hz band, which was coherent with in situ plasma density oscillations. Around the Pc1 event location, the Antarctic Neumayer Station III (L ~ 4.2) recorded similar Pc1 events in the horizontal component while NOAA-15 observed isolated proton precipitations at energies above 30 keV. All these observations support that the compressional Pc1 waves at Swarm are oscillations converted from electromagnetic ion cyclotron (EMIC) waves coming from the magnetosphere. The magnetic field and plasma density oscillate in-phase. We compared the amplitudes of density and magnetic field oscillations normalized to background values and found that the density power is much larger than the magnetic field power. This difference cannot be explained by a simple magnetohydrodynamic (MHD) model, although steep horizontal/vertical gradients of background ionospheric density can partly reconcile the discrepancy.

DOI： 10.1029/2020GL089000

Oxygen torus and its coincidence with EMIC wave in the deep inner magnetosphere: Van Allen Probe B and Arase observations 査読有り

Nose M., Matsuoka A., Kumamoto A., Kasahara Y., Teramoto M., Kurita S., Goldstein J., Kistler L. M., Singh S., Gololobov A., Shiokawa K., Imajo S., Oimatsu S., Yamamoto K., Obana Y., Shoji M., Tsuchiya F., Shinonara I, Miyoshi Y., Kurth W. S., Kletzing C. A., Smith C. W., MacDowal R. J., Spence H., Reeves G. D.

EARTH PLANETS AND SPACE 72 巻 ( 1 ) 頁： 111 2020年8月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Earth, Planets and Space

We investigate the longitudinal structure of the oxygen torus in the inner magnetosphere for a specific event found on 12 September 2017, using simultaneous observations from the Van Allen Probe B and Arase satellites. It is found that Probe B observed a clear enhancement in the average plasma mass (M) up to 3–4 amu at L = 3.3–3.6 and magnetic local time (MLT) = 9.0 h. In the afternoon sector at MLT ~ 16.0 h, both Probe B and Arase found no clear enhancements in M. This result suggests that the oxygen torus does not extend over all MLT but is skewed toward the dawn. Since a similar result has been reported for another event of the oxygen torus in a previous study, a crescent-shaped torus or a pinched torus centered around dawn may be a general feature of the O+ density enhancement in the inner magnetosphere. We newly find that an electromagnetic ion cyclotron (EMIC) wave in the H+ band appeared coincidently with the oxygen torus. From the lower cutoff frequency of the EMIC wave, the ion composition of the oxygen torus is estimated to be 80.6% H+, 3.4% He+, and 16.0% O+. According to the linearized dispersion relation for EMIC waves, both He+ and O+ ions inhibit EMIC wave growth and the stabilizing effect is stronger for He+ than O+. Therefore, when the H+ fraction or M is constant, the denser O+ ions are naturally accompanied by the more tenuous He+ ions, resulting in a weaker stabilizing effect (i.e., larger growth rate). From the Probe B observations, we find that the growth rate becomes larger in the oxygen torus than in the adjacent regions in the plasma trough and the plasmasphere.[Figure not available: see fulltext.]

DOI： 10.1186/s40623-020-01235-w

その他リンク： http://link.springer.com/article/10.1186/s40623-020-01235-w/fulltext.html

Arase Observation of the Source Region of Auroral Arcs and Diffuse Auroras in the Inner Magnetosphere 査読有り

Shiokawa K., Nose M., Imajo S., Tanaka Y. -M., Miyoshi Y., Hosokawa K., Connors M., Engebretson M., Kazama Y., Wang S. -Y., Tam S. W. Y., Chang Tzu-Fang, Wang Bo-Jhou, Asamura K., Kasahara S., Yokota S., Hori T., Keika K., Kasaba Y., Shoji M., Kasahara Y., Matsuoka A., Shinohara I.

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS 125 巻 ( 8 ) 2020年8月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Geophysical Research: Space Physics

Auroral arcs and diffuse auroras are common phenomena at high latitudes, though characteristics of their source plasma and fields have not been well understood. We report the first observation of fields and particles including their pitch-angle distributions in the source region of auroral arcs and diffuse auroras, using data from the Arase satellite at L ~ 6.0–6.5. The auroral arcs appeared and expanded both poleward and equatorward at local midnight from ~0308 UT on 11 September 2018 at Nain (magnetic latitude: 66°), Canada, during the expansion phase of a substorm, while diffuse auroras covered the whole sky after 0348 UT. The top part of auroral arcs was characterized by purple/blue emissions. Bidirectional field-aligned electrons with structured energy-time spectra were observed in the source region of auroral arcs, while source electrons became isotropic and less structured in the diffuse auroral region afterwards. We suggest that structured bidirectional electrons at energies below a few keV were caused by upward field-aligned potential differences (upward electric field along geomagnetic field) reaching high altitudes (~30,000 km) above Arase. The bidirectional electrons above a few keV were probably caused by Fermi acceleration associated with the observed field dipolarization. Strong electric-field fluctuations and earthward Poynting flux were observed at the arc crossing and are probably also caused by the field dipolarization. The ions showed time-pitch-angle dispersion caused by mirror reflection. These results indicate a clear contrast between auroral arcs and diffuse auroras in terms of source plasma and fields and generation mechanisms of auroral arcs in the inner magnetosphere.

DOI： 10.1029/2019JA027310

Long-Lasting Ground-Satellite High Coherence of Compressional Dayside Pc3-Pc4 Pulsations 査読有り

Kim Gi-Jeong, Kim Khan-Hyuk, Kwon Hyuck-Jin, Shiokawa Kazuo, Takahashi Kazue, Hwang Junga

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS 125 巻 ( 8 ) 2020年8月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Geophysical Research: Space Physics

We present observations of broadband (∼10–50 mHz) Pc3–Pc4 waves on 4 January 2014. The waves were detected on the dayside simultaneously in a compressional component (δBz) at the Radiation Belt Storm Probes A (RBSP-A) in the inner magnetosphere and in the north-south component (δH) on the ground at a low-latitude Bohyun (BOH) station (L = 1.3) during an interval of small interplanetary magnetic field cone angle, suggesting that upstream ultralow frequency (ULF) waves were the source of the magnetospheric Pc3–Pc4 waves. We observed the ground-satellite high coherence between RBSP-A δBz and BOH δH for a prolonged time interval lasting 6.5 hr, which has not been reported previously, during which RBSP-A was on the inbound and outbound legs moving from L = ∼3.3 to ∼6.3. In order to understand the spatial mode structure of the compressional waves, we examined the ground-satellite cross phase for the high-coherence interval and found that the waves observed in and out of the plasmasphere propagated earthward at the average fast-mode speed of ∼700–1,000 km/s. We also observed cross-phase values smoothly changing with the radial distance of the spacecraft across the plasmapause. This indicates that the presence of the plasmapause has little effect on our fast-mode waves propagating into the inner magnetosphere and to the ground low-latitude station.

DOI： 10.1029/2020JA028074

The Long-Lasting QP Emissions Observed On Arase Satellite And Lovozero Station 査読有り

V.B. Belakhovsky, K. Shiokawa, A. Matsuoka, Y. Kasahara, I. Shinohara, Y. Miyoshi, S.-Y. Wang, Y. Kazama, S. Kasahara, S. Yokota

2020 XXXIIIrd General Assembly and Scientific Symposium of the International Union of Radio Science 2020年8月

Asymmetric Development of Auroral Surges in the Northern and Southern Hemispheres 査読有り

Uchida H. A., Kataoka R., Kadokura A., Murase K., Yukimatu A. S., Miyoshi Y., Shiokawa K., Ebihara Y., Hosokawa K., Matsuoka A., Kurita S., Fujita S., Shinohara I

GEOPHYSICAL RESEARCH LETTERS 47 巻 ( 13 ) 2020年7月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Geophysical Research Letters

Simultaneous eastward and westward traveling surges were observed at Tjörnes, Iceland, and Syowa station, Antarctica, respectively. Several remarkable differences were identified. (1) The position of the initial bright spot was shifted by 1.7 to 2.3 MLT between both hemispheres. (2) The surges differ in traveling speed between the eastward traveling surge (6.5 km s−1) and the westward traveling surge (1.3 km s−1). (3) The Arase satellite was located on a geomagnetic field line connecting both ground stations and observed a significant excess in westward component of the magnetic field, which is consistent with the large shifts of the initial bright spots in both hemispheres. (4) The background Hall current flows eastward (Northern Hemisphere) and westward (Southern Hemisphere). The observed north-south asymmetry of the traveling surges suggests that the ionosphere can play an essential role in controlling the fundamental spatiotemporal development of auroras in both hemispheres.

DOI： 10.1029/2020GL088750

Dilatory and Downward Development of 3-m Scale Irregularities in the Funnel-Like Region of a Rapidly Rising Equatorial Plasma Bubble 査読有り

Ram S. Tulasi, Ajith K. K., Yokoyama T., Yamamoto M., Hozumi K., Shiokawa K., Otsuka Y., Li G.

GEOPHYSICAL RESEARCH LETTERS 47 巻 ( 13 ) 2020年7月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Geophysical Research Letters

The equatorial plasma bubbles, once developed, grow nonlinearly into topside ionosphere, and simultaneous secondary instabilities lead to the development of shorter scale irregularities. The altitudinal growth and generation of smaller scale irregularities determine the spatio-temporal occurrence and the intensity of ionospheric scintillations at wide spectrum of radio waves and have significant implications on the GNSS/Satellite Based Augmentation Systems. In this letter, we present a unique equatorial plasma bubble observation from equatorial atmosphere radar that provides hitherto undisclosed evidence for the smaller (3-m) scale irregularities initially developing at higher altitudes and subsequently developing to lower altitudes in a narrow funnel-like structure. The responsible mechanisms for early development of shorter scale irregularities in the topside and their subsequent development at lower altitudes are discussed in light of difference between the time scales of altitudinal growth and cascading rate of secondary instabilities through high-resolution bubble model simulations.

DOI： 10.1029/2020GL087256

　前のページ - 次のページ　

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25