太陽系科学研究系

浅村 和史

アサムラ カズシ  (Kazushi Asamura)

基本情報

所属
国立研究開発法人宇宙航空研究開発機構 宇宙科学研究所 太陽系科学研究系 准教授
学位
理学修士
博士(理学)

J-GLOBAL ID
200901058783588460
researchmap会員ID
1000292024

学歴

 2

論文

 219
  • A. Nagatani, Y. Miyoshi, K. Asamura, L.. M. Kistler, S. Nakamura, K. Seki, Y. Ogawa, I. Shinohara
    Geophysical Research Letters 51(18) 2024年9月16日  査読有り
  • S. Imajo, Y. Miyoshi, Y. Kazama, K. Asamura, I. Shinohara, K. Shiokawa, Y. Kasahara, Y. Kasaba, A. Matsuoka, S.‐Y. Wang, S. W. Y. Tam, T.‐F. Chang, B.‐J. Wang, C.‐W. Jun, M. Teramoto, S. Kurita, F. Tsuchiya, A. Kumamoto, K. Saito, T. Hori
    Journal of Geophysical Research: Space Physics 129(9) 2024年9月12日  査読有り
    Abstract The Arase satellite observed the precipitation of monoenergetic electrons accelerated from a very high altitude above 32,000 km altitude on 16 September 2017. The event was selected in the period when the high‐angular resolution channel of the electron detector looked at pitch angles within ∼5° from the ambient magnetic field direction, and thereby was the first to examine the detailed distribution of electron flux near the energy‐dependent loss cone at such high altitudes. The potential energy below the satellite estimated from the observed energy‐dependence of the loss cone was consistent with the energy of the upgoing ion beams, indicating that ionospheric ions were accelerated by a lower‐altitude acceleration region. The accelerated electrons inside the loss cone carried a significant net field‐aligned current (FAC) density corresponding to ionospheric‐altitude FAC of up to ∼3μA/m2. Based on the anisotropy of the accelerated electrons, we estimated the height of the upper boundary of the acceleration region to be >∼2 RE above the satellite. The height distribution of the acceleration region below the satellite, estimated from the frequency of auroral kilometric radiation, was ∼4,000–13,000 km altitude, suggesting that the very‐high‐altitude acceleration region was separated from the lower acceleration region. Additionally, we observed time domain structure (TDS) electric fields on a subsecond time scale with a thin FAC indicated by magnetic deflections. Such a TDS may be generated by the formation of double layers in the magnetotail, and its potential drop could significantly contribute (∼40%–60%) to the parallel energization of precipitating auroral electrons.
  • Masahito Nosé, Keisuke Hosokawa, Reiko Nomura, Mariko Teramoto, Kazushi Asamura, Yoshizumi Miyoshi, Takefumi Mitani, Takeshi Sakanoi, Taku Namekawa, Takeshi Kawano, Yoshihiro Iwanaga, Shunichi Tatematsu, Masafumi Hirahara, Alexa Halford, Mykhaylo Shumko, Marc R. Lessard, Kristina Lynch, Nicholaos Paschalidis, Allison N. Jaynes, Matthew G. McHarg
    Journal of Geophysical Research: Space Physics 129(6) 2024年5月31日  査読有り
    Abstract We made observations of magnetic field variations in association with pulsating auroras with the magneto‐impedance sensor magnetometer (MIM) carried by the Loss through Auroral Microburst Pulsations (LAMP) sounding rocket that was launched at 11:27:30 UT on 5 March 2022 from Poker Flat Research Range, Alaska. At an altitude of 200–250 km, MIM detected clear enhancements of the magnetic field by 15–25 nT in both the northward and westward components. From simultaneous observations with the ground all‐sky camera, we found that the footprint of LAMP at the 100 km altitude was located near the center of a pulsating auroral patch. The auroral patch had a dimension of ∼90 km in latitude and ∼25 km in longitude, and its major axis was inclined toward northwest. These observations were compared with results of a simple model calculation, in which local electron precipitation into the thin‐layer ionosphere causes an elliptical auroral patch. The conductivity within the patch is enhanced in the background electric field and as a result, the magnetic field variations are induced around the auroral patch. The model calculation results can explain the MIM observations if the electric field points toward southeast and one of the model parameters is adjusted. We conclude that the pulsating auroral patch in this event was associated with a one‐pair field‐aligned current that consists of downward (upward) currents at the poleward (equatorward) edge of the patch. This current structure is maintained even if the auroral patch is latitudinally elongated.
  • Masafumi Hirahara, Yoko Fukuda, Yusuke Ebihara, Kanako Seki, Takeshi Sakanoi, Kazushi Asamura, Taku Takada, Atsushi Yamazaki, Yasumasa Kasaba, Hirobumi Saito
    Journal of Geophysical Research: Space Physics 129(5) 2024年5月15日  査読有り
    Abstract We present the simultaneous and conjugated auroral emission and particle data obtained by a low‐altitude polar‐orbiting micro‐satellite, Reimei, for elucidating their latitudinal distributions and variations in the nightside auroral oval. Here are reported a few notable examples of the Reimei observations with high time and spatial resolutions, namely ∼120 msec. and ∼1.2 km × 1.2 km for multispectral auroral images and 40 msec. for energy‐pitch angle distributions of electrons and ions with energies of 10 eV–12 keV, respectively. The auroral images show various fine‐scale auroral activities characterized by the following types of auroral forms and variations: faint bands, streaming multiple arcs, shearing arcs, and vortices/curls, which are typical of the latitudinal properties of auroras. The particle analyzer simultaneously observed various properties of electron energy‐pitch angle and latitudinal distributions, and their temporal variations, each of which corresponds to a type of the auroral activities. Their features are summarized below. Reimei repetitively observed inverted‐V signatures of low‐energy (<1 keV) field‐aligned electrons in addition to the higher‐energy (several keV) diffuse electrons in low‐latitude auroral oval. In more active regions at higher latitudes, the dominant energy flux responsible for the multiple‐arc emissions was carried by the well‐known inverted‐V electron precipitation. The rapidly rotating vortices or so‐called curls of fine‐scale discrete auroras near the poleward boundary of the auroral oval were closely associated with the significant energy fluxes of spiky field‐aligned electron bursts with energy‐time dispersions produced by dispersive Alfvén waves.
  • K.‐H. Kim, C.‐W. Jun, J.‐W. Kwon, J. Lee, K. Shiokawa, Y. Miyoshi, E.‐H. Kim, K. Min, J. Seough, K. Asamura, I. Shinohara, A. Matsuoka, S. Yokota, Y. Kasahara, S. Kasahara, T. Hori, K. Keika, A. Kumamoto, F. Tsuchiya
    Journal of Geophysical Research: Space Physics 129(5) 2024年5月6日  査読有り
    Abstract This is the first report of significant energization (up to 7,000 eV) of low‐energy He+ ions, which occurred simultaneously with H‐band electromagnetic ion cyclotron (EMIC) wave activity, in a direction mostly perpendicular to the ambient magnetic field. The event was detected by the Arase satellite in the dayside plasmatrough region off the magnetic equator on 15 May 2019. The peak energy of the He+ flux enhancements is mostly above 1,000 eV. At some interval, the He+ ions are energized up to ∼7,000 eV. The H‐band waves are excited in a frequency band between the local crossover and helium gyrofrequencies and are close to a linear polarization state with weakly left‐handed or right‐handed polarization. The normal angle of the waves exhibits significant variation between 0° and 80°, indicating a non‐parallel propagation. We run a hybrid code with parameters estimated from the Arase observations to examine the He+ energization. The simulations show that cold He+ ions are energized up to more than 1,000 eV, similar to the spacecraft observations. From the analysis of the simulated wave fields and cold plasma motions, we found that the ratio of the wave frequency to He+ gyrofrequency is a primary factor for transverse energization of cold He+ ions. As a consequence of the numerical analysis, we suggest that the significant transverse energization of He+ ions observed by Arase is attributed to H‐band EMIC waves excited near the local helium gyrofrequency.

MISC

 133

共同研究・競争的資金等の研究課題

 21