イオンサイクロトロン波動及び斜め伝搬ホイッスラーモード波動粒子相互作用のテスト粒子シミュレーション

概要

イオンサイクロトロン波動及び斜め伝搬ホイッスラーモード波
動粒子相互作用のテスト粒子シミュレーション
Test Particle Simulation on Wave-Particle Interactions of EMIC waves and Obliquely
Propagating Whistler-mode Waves
研究代表者：謝

恰凱

（京都大学生存圏研究所）
yikai_hsieh@rish.kyoto-u.ac.jp

研究分担者：大村善治

（京都大学生存圏研究所）
omura@rish.kyoto-u.ac.jp
担当：計算結果の理論的検討

研究目的 (Research Objective):

The population, motion, and distribution of energetic electrons in the Earth's inner
magnetosphere are strongly affected by wave-particle interactions with whistler mode waves,
resulting in energy increasing/decreasing and pitch angle scattering of these electrons. Several
observations and simulations show evidence of whistler-mode wave-driven electron
precipitation (e.g., Hikishima et al., 2010 Nishimura et al., 2010; Kurita et al., 2016; Miyoshi
et al., 2022) under the parallel propagating assumption. However, electron precipitation
regarding large wave normal angles has been reported (Zhang et al., 2022) but has not yet
been studied by simulation. When the wave normal angle is very close to the resonance cone
angle, the resonance condition should be very different from parallel waves or slightly oblique
waves. This study aims to investigate electron precipitation for broad initial kinetic energies
and equatorial pitch angles interacting with whistler-mode chorus with various wave normal
angles. We checked the differences between precipitation rates and wave normal angles, and
then examined the precipitation mechanism of different nth order resonances.
計算手法 (Computational Aspects)

We applied test-particle simulations to calculate the electron trajectories interacting with a
pair of chorus emissions. With the results of the test particle simulations, we generated the
numerical Green’s function as the modified electron distribution after wave-particle
interactions. We have 12 wave models with 3 amplitude settings and 4 wave normal angle
settings covering parallel chorus waves, slightly oblique chorus waves, and very oblique
chorus waves. The wave models are shown in Fig.1. We simulated wave-particle interactions
in the Earth’s dipole magnetic field at around L=4.5. We input 3600 electrons in a test particle
simulation with electrons with the same initial kinetic energy and the same equatorial pitch
angle. For a Green’s function set, we calculate initial energy K0 from 10 keV to 6 MeV and
initial equatorial pitch angle a0 from 5° to 89°. We submitted 183,600,000 electrons for one
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Green’s function set. Both MPI and OpenMP methods are employed for parallel computing.
After generating the Green’s functions, we integrated electrons inside the loss cone to
calculate the precipitation rates of each initial pitch angle and kinetic energy. Finally, we
compared the precipitation rates of the different wave models.
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研究成果（ Accomplishments

We theoretically derived the equatorial pitch angle scattering rates of resonant electrons and
analyzed them in different wave amplitudes, wave frequencies, wave normal angles, and nth
resonances. We also compared the pitch angle scatter rates to the simulated precipitation rates.
The physical findings are itemized as follows:
1. Amplitude is the most critical factor affecting electron precipitation.
2. In general, low-energy electrons have a higher precipitation rate than high-energy
electrons because the pitch angle scattering rate of n=1 cyclotron resonance is higher for
low-energy electrons than high-energy electrons. (See red parts of Fig. 2 a-d)
3. For large amplitude waves, the precipitation rate of the very oblique chorus waves is
about 1.5 times greater than that of the parallel waves and about 1.2 times greater than
that of the slightly oblique waves due to the active nonlinear trapping via the n = 0
Landau resonance and the n = -1 cyclotron resonance and nonlinear scattering of the n =
2 cyclotron resonance. (See Fig. 2e)
4. In the large amplitude and very oblique case, electrons can precipitate from initial
equatorial pitch angles > 40 deg around 100 keV because of strong nonlinear trapping via
the n = -1 cyclotron resonance. (See Fig. 2d and magenta circles in Fig. 3.)
5. The anomalous trapping effect is much weaker in the oblique cases than in the parallel
case.

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Fig.2 Precipitation rates for electrons interacting with a pair of chorus emissions shown
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Fig. 3 Connection of simulated precipitation rates (left panel) and theoretical pitch angle
scattering rates (right panel).
公表状況（ Publications

‘,‘~

（論文）

1
. Hsieh, Y.-K., & Omura, Y. (2023). Precipitation rates of electrons interacting with
lower-band chorus emissions in the inner magnetosphere. Submitted to J
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（口頭）

1.

2.

3.

4.

5.

6.

7.

Hsieh, Y.-K., Y. Omura, Electron precipitation rates due to nonlinear effects of parallel
and oblique whistler mode waves in the Earth’s inner magnetic field, American
Geophysical Union (AGU) 2022 Fall Meeting, Chicago, USA & Online, Dec 2022.
Hsieh, Y.-K., Y. Omura, Energetic electron precipitation associated with nonlinear
wave-particle interactions between electrons and very oblique chorus waves, The 10th
VLF/ELF Remote Sensing of Ionospheres and Magnetospheres Workshop (VERSIM
2022), Sodankylä, Finland & Online, Nov 2022.
Hsieh, Y.-K., Y. Omura,Energetic electron precipitation induced by very oblique chorus
waves in the Earth s inner magnetosphere, 第 152 回地球電磁気・地球惑星 圏学会相
,
模原, 2022 年11月 .
Hsieh, Y.-K., Y. Omura, Comparison of Energetic Electron Precipitation Induced by
Parallel and Oblique Whistler Mode Chorus Waves, Asia Oceania Geosciences Society
(AOGS) 19th Annual Meeting, Online, Aug 2022.
Hsieh, Y.-K., Y. Omura, Precipitation of resonant electrons interacting with parallel and
oblique whistler mode chorus waves, AT-AP-RASC 2022, Gran Canaria & Online,
May-June 2022.
Hsieh, Y.-K., Y. Omura, Precipitation rates of energetic electrons interacting with parallel
and oblique whistler mode chorus emissions in the magnetosphere, Japan Geoscience
Union (JpGU) Meeting 2022, Chiba, Japan & Online, May 2022.
Hsieh, Y.-K., Y. Omura, Energetic electron loss process associating with oblique chorus
emissions in the outer radiation belt, EGU General Assembly 2022, Vienna & Online,
May 2022.

（ポスター）

1.

Hsieh, Y.-K., & Omura, Y., Energetic electron precipitation driven by oblique whistler
mode chorus emissions: Test particle simulation and Green’s function method, The 14th
International School for Space Simulations (ISSS-14), Online,Sep 2022.
2. Hsieh, Y.-K., & Omura, Y., Pitch angle scattering rates and precipitation rates of energetic
electrons interacting with lower-band chorus emissions during nonlinear interactions with
multiple resonances, Asia Oceania Geosciences Society (AOGS) 19th Annual Meeting,
Online, Aug 2022.
（受賞）

1.

Hsieh, Y.-K., Outstanding Presentation Awards for Early Career Scientists of the 10th
VLF/ELF Remote Sensing of the Ionosphere and Magnetosphere (VERSIM) workshop.
2022 Nov. 11th. ...

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