13 September 2019

研究集会「太陽地球系物理学分野のデータ解析手法、ツールの理解と応用」

複数衛星観測とシミュレーションデータ解析に基づくプラズマ波動の放射線帯変動への寄与
高橋 直子1, 関 華奈子1, Mei-Ching Fok2, Yihua Zheng2, 三好 由純3, 笠原 慧1,桂華 邦裕1, David Hartley4, 笠原 禎也5, 笠羽 康正6, 東尾 奈々7, 松岡 彩子8,
横田 勝一郎9, 堀 智昭3, 篠原 育8
1. 東京大学大学院理学系研究科
2. NASA Goddard Space Flight Center
3. 名古屋大学宇宙地球環境研究所
4. Department of Physics and Astronomy, University of Iowa
5. 金沢大学総合メディア基盤センター
6. 東北大学大学院理学研究科 惑星プラズマ・大気研究センター
7. 宇宙航空研究開発機構
8. 宇宙科学研究所
9. 大阪大学大学院理学研究科

!1

!2

Radiation belt: MeV-energy particles are trapped• MeV-electron fluxes enhance during the recovery phase of magnetic storm.

[Horne+2013][Modified from Baker+2014]

1 MeV

relativistic(outer belt)

100 keV

Sub-relativistic???

10 keV
Ring current

1 keV
100 eV< 10 eV

Plasmasphere
L=3

Plasma sheet→ Source ofouter belt

Only convection E-field:
up to ~ 200 keV
L=6[Modified from Miyoshi+2012]

Question
How energetic particles accelerate
up to relativistic energies?

Electromagnetic waves contributes
to the particle acceleration

!3Contribution of waves to the particle acceleration
1. Radial diﬀusion by Pc5 ULF waves

MeV-electron flux (at L*~5.3, by RBSP/REPT)• Periodic waves with 1.6-6.7 mHz• Pc5 ULF waves are globally seen.
• Source:- Magnetospheric compression by Psw- Kelvin-Helmholtz instability- ring current plasma by substorm
injection

ULF wave power (1-10 mHz),
by IMAGE ground magnetometer network[Da Silva+2019]

2. Local acceleration by whistler-mode chorus waves (~0.1 fce - fce)
• Chorus waves evolve due to thetemperature anisotropy (Tperp>Tpara)• Source: 10s keV electrons

By Van Allen ProbesChorus
wave power
1.8 MeVflux[Li+2016]

!4

Purpose of this study
Electromagnetic waves play a crucial role in accelerating electrons
up to relativistic energy.
However,…

how much these waves aﬀect the radiation belt variation?
→ still few papers that compare between both contributions• When, where, and how ULF and whistler-mode chorus waves contributeto the MeV-electron flux enhancement?
• Can the physical-based model (i.e., MHD model) reproduce global
distributions of ULF and chorus waves?
• Can simulation parameters predict observed wave activity?

1. Global characteristics of ULF waves: March 2017 magnetic storm
2. ULF waves vs. Whistler-mode chorus waves: May 2017 magnetic storm

!5BATS-R-US + CRCM
[coupling method: Buzulukova et al., 2010; Glocer et al., 2013]
Global magnetosphere

Inner magnetosphere

BATS-R-US

CRCM(Block-Adaptive-Tree Solar-Wind Roe-Type Upwind Scheme)(Comprehensive Ring Current Model)• Ideal MHD equations(continuity, momentum, Maxwell’s equations)• Cartesian grid
• Huge size of simulation box
32 RE (upstream) - 224 RE (downstream)[Powell et al., 1999; Toth et al., 2012]• Bounce-averaged Boltzmann equation→ temporal variation in the phase spacedensity of ring current species• Assumption of gyrotropic & anisotropic
conditions for the pressure tensor• Magnetospheric electric field isself-consistently solved.

magnetic field
temperaturedensity

Pressure• connect to the ionosphere (2.5 RE)→ Ionospheric potential is estimated byassuming the dipole field, whichmaps back to the magnetic equator.
[Fok et al., 2001]

!6Initial condition
Solar wind input — For BATS-R-US• Real solar wind parameters• Only IMF Bx is fixed (average).
• The dipole tilt is included.
Particles — For CRCM
• ion: Maxwellian, electron: Kappa• no temperature anisotropy (even boundary)> 8-10 RE: BATSRUS (MHD)
3 RE: inner boundary

3-8 (~10) RE:
CRCM (ring current model)
sunwardResolution of output files• Both time and spatial resolutions are increased only in the inner magnetosphere(|x, y, z| ≦ 15 RE)- BATSRUS (MHD)
5 seconds, 0.15 RE at finest- CRCM (ring current)
10 minutes, 1/3 RE→ to reproduce Pc5 ULF waves (1.7-6.7 mHz)

1. Global characteristics of ULF waves
Reference
Takahashi, N., Seki, K., Teramoto, M., Fok, M.-C., Zheng, Y., Matsuoka, A., et al. (2018).
Global distribution of ULF waves during magnetic storms: Comparison of Arase, groundobservations, and BATSRUS + CRCM simulation. Geophysical Research Letters, 45.
https://doi.org/10.1029/2018GL078857

!8‘Local’ comparison: Simulation vs. Arase
(d)
(a)

(e)
Thesimulation can reproduce the enhancement of ULF waves
(a)
(a)
in(b)
Bphi (frequency: 2-3 mHz). → consistent with Arase

Arase satellite

(f)
(b)
(c)
(b)
(a)
(g)
(c)
(c)
(b)
(d)
(h)
(d)
(c)
(d)
(e)
(i)
(e)
(d)
(e)
(f)

Simulation

(j)
(f)
(e)
(g)
(f)
(g)
(g)
(h)
(f)

Discrepancies• smaller amplitude of ULF waves→ large numerical dissipation• No higher-frequency waves(10-30 mHz)→ low temporal resolution ofsolar wind input parameters(~1 min)

(h)
(g)
(h)
(i)
(i)
(h)
(i)
(j)
(j)
(i)
(j)

*background: 15-min running average

!9‘Global’ comparison: Relative ULF activity
main

recovery

L=3.5–7.0

Pc5-range ULF wave activity
main

pre-storm levelrecoverysimulation

GMAG

highhigh↓↓(relatively) lowhighpressurepressure+VSW & AE

MLAT=54o–68o

relate to…
pre-storm level

The simulated ULF wave activity isstrongly aﬀected by the solar winddynamic pressure.

* ULF wave intensity:normalized by 3-h average during the pre-storm

!102. ULF waves vs. Whistler-mode chorus waves
Target event: May 2017 magnetic storm→ Suitable for understanding global characteristics of waves
simulation timeArase(apogee: post-midnight)

RBSP-B(apogee: dusk)

field lines: TS04

!11

Electron flux

Plasma waves

Observation: L-value dependence

dotted line: minimum of Dst index

Arase: use only outbound-pass data due to high inclinationin the inbound pass

• ULF: (main phase and) early recovery phase• chorus: late recovery phase

Observation:
Temperature anisotropy vs. chorus wave power

From MEP-e/Arase: Energy-dependent structure is seen.
• L~5.0 - 5.5: 10-30 keV• L~3.5 - 4.0: 30-64 keV!12

!13Simulation: 2D map@magnetic equator*satellite position:
trace using TS04

RBSP-B

Anisotropy(Tperp/Tpara-1)

Arase
RBSP-B[mV2/m2][mV2/m2]

Arase[mV2/m2]

ULF power (Ephi)

!1430-64 keV electrons

10-30 keV electrons

Can simulation predict observed chorus waves?

Energy-dependent structure isseen.
• L~5.0 - 5.5: 10-30 keV• L~3.5 - 4.0: 30-64 keV→ Consistent with observation
Recovery phase• Quite low electron flux
at low L-shell (why?)

dotted line: minimum of Dst index
Threshold: Energy density = 10-10 keV/cm3

!15

Summary
Physics-based simulation can qualitatively reproduce
global characteristics of plasma waves.
For Pc5 ULF waves (= MHD waves)…
• Small wave power in the simulation→ Large numerical dissipation• Higher-frequency waves cannot be reproduced in the simulation.
→ low temporal resolution of solar wind input parameters (~1 min)• No enhancement of simulated ULF wave power is seen during the main phase.
→ Simulation strongly aﬀects to the solar wind dynamic pressure variation.
For whistler-mode chorus waves (=VLF waves)…
• Energy-dependent characteristics are seen.
• At low L-shell (L~4), the energy density of electrons is quite smaller thanobservation.