磁気圏における
Magnetic Reconnection
ー MHD的描像から粒子的描像へ －
長井嗣信
東京工業大学
地球磁気圏でのmagnetic reconnection
昼側での磁気リコネクション
夜側での磁気リコネクション
Dungey model (J. W. Dungey, Phys. Rev. Lett., 6, 47, 1961)
Geotail Observations
Sun
Solar Wind
Bow Shock
Magetosheath
Magnetopause
Magnetotail
磁気圏尾部での磁気リコネクションの証拠
Geotail以前
サブストーム（オーロラ爆発） （1960年代より）
Fast Earthward Flows
with Bz > 0
Bz < 0
Fast tailward Flows
with Bz < 0
磁気圏尾部の磁場はダイポール磁場が
引き伸ばされたものだからすべて北向き
Fast Tailward Flows
地球半径の30倍の距離での磁気圏尾部での磁場とプラズマの観測
オーロラ爆発の全天カメラ像
a substorm onset (aurora breakup)
Kadokura (2002)
SIT-TV at Syowa
427.8 nm
人工衛星Geotail による観測 (1992-)
研究テーマ
MHD的磁気リコネクションの確立
Spacecraft
Launch
Orbit
July 22, 1992
30 RE x 10 RE
period
5 days
Magnetic field
Plasma
Geotail
1/16 sec
0.01 nT
12 sec
ion and electron
0 – 40 keV
3D velocity distribution functions
MHD parameters (n, T, V)
Geotail
磁気リコネクションの観測例
High-Speed Tailward Flowing Ions
Bz < 0
Highｌｙ Accelerated Electrons
Vx < 0
Ion
EQ
off EQ
V=3000km/s
Electron
Outflow Ions
Convection
Heated
Inflow Ions
High V
Outflow Ions
XGSM=-28.9 YGSM=5.8 ZGSM=-2.6 RE
boundary
プラズマの3次元速度分布関数の観測
Less Heated
Inflow Ions
Low V
Outflow Ions
磁気圏尾部での磁気リコネクションの観測例
Z
南向き磁場を持つ
反地球向き高速プラズマ流
電子の加熱・加速
太陽の方向 X
Bx > 0
Bx < 0
北半球
南半球
Bx =0
赤道面 （電流層）
Distribution Functions
Magnetic Field Direction B
B
B
V
V Convection
Stationary
Field-aligned flows
V
Convection flows
プラズマ速度分布関数のMHD的描像 B – V
座標系
磁力線に沿う流れ
field-aligned flows
磁力線に垂直な流れ
convection flows (frozen-in)
High-Speed Tailward Flowing Ions
Highｌｙ Accelerated Electrons
Ion
C
V=3000km/s
Electron
Less Heated
Inflow Ions
Low V
Outflow Ions
B
Boundary
off EQ
Heated
Inflow Ions
High V
Outflow Ions
A
Outflow Ions
Convection
EQ
Magnetotail Reconnection Event
High-Speed Tailward Flowing Ions
Highｌｙ Accelerated Electrons
Magnetic Field Direction B
V Convection
Ion
A
Electron
EQ
near the equatorial plane
Outflow Ions
Convection
Magnetotail Reconnection Event
High-Speed Tailward Flowing Ions
Highｌｙ Accelerated Electrons
Tailward convection flows with Bz < 0
Vi > 2500 km/s
Alfven velocity
~2900 km/s
Ion
Electron
EQ
A
near the equatorial plane
Outflow Ions
Convection
Magnetotail Reconnection Event
High-Speed Tailward Flowing Ions
Highｌｙ Accelerated Electrons
Magnetic Field Direction B
V Convection
Ion
B
off the equatorial plane
Electron
Magnetotail Reconnection Event
High-Speed Tailward Flowing Ions
Highｌｙ Accelerated Electrons
Tailward field-aligned Flows with Bz < 0
V > 2800 km/s
Ion
B
off the equatorial plane
Electron
MHD magnetic reconnection simulation (T. Sato, 1979)
磁気リコネクションの観測例
High-Speed Tailward Flowing Ions
Highｌｙ Accelerated Electrons
Ion
C
V=3000km/s
Electron
Less Heated
Inflow Ions
Low V
Outflow Ions
B
Boundary
off EQ
Heated
Inflow Ions
High V
Outflow Ions
A
Outflow Ions
Convection
EQ
Distribution Functions
Magnetic Field Direction B
B
B
V
V Convection
Stationary
V
Field-aligned flows + Convection flows
B
V//
V
V//
Convection flows
with counter-streaming components
V
Magnetotail Reconnection Event
High-Speed Tailward Flowing Ions
Highｌｙ Accelerated Electrons
Tailward convection flows with Bz < 0
Vi > 2500 km/s
Ve > 4000 km/s
ion-electron decoupling
Alfven velocity ~2900 km/s
Ion
Electron
V=2500km/s
A
near the equatorial plane
Outflow Ions
Convection
EQ
Magnetotail Reconnection Event
High-Speed Tailward Flowing Ions
Highｌｙ Accelerated Electrons
Ion
Electron
V=3000km/s
B
Heated
Inflow Ions
High V
Outflow Ions
B
off the equatorial plane
off EQ
Magnetotail Reconnection Event
High-Speed Tailward Flowing Ions
Highｌｙ Accelerated Electrons
Ion
C
V=3000km/s
Less Heated
Inflow Ions
Low V
Outflow Ions
C
boundary
Electron
Boundary
Magnetotail Reconnection Event
High-Speed Tailward Flowing Ions
Highｌｙ Accelerated Electrons
Ion
C
Electron
V=3000km/s
Boundary
Less Heated
Inflow Ions
Low V
Outflow Ions
Inflowing Electrons
Hall Current Electrons
C
boundary
Tailward Escaping Electrons
人工衛星Geotail による観測 (1992-)
研究テーマ
MHD的描像の磁気リコネクションの確立
粒子的描像の磁気リコネクションの世界への発展
Classical MHD steady magnetic reconnection
Sweet-Parker reconnection
reconnection rate
Petscheck reconnection
イオンと電子の運動を考慮した磁気リコネクションモデル
粒子的描像
Geospace Environmental Modeling (GEM) Magnetic Reconnection Challenge
(Birn et al. J. Geophys. Res., 2001)
B. U. O. Sonnerup (1979)
Ion-Electron Decoupling
イオンー電子の二流体による
磁気リコネクションモデル
イオン慣性長程度でのスケールでの物理
Ion
Electron
NOT frozen-in
still frozen-in
Ion-Electron Decoupling
Hall Effect
(non-MHD)
Ion-Electron Decoupling at the li Scale
ion+
electron Magnetic field
+
ion
electron -
electron diffusion region
le
ion diffusion region
li ~ 40 le
ホール電流系の形成
ion+
electron Magnetic field
ホール電流 j
+
ion
electron -
electron diffusion region
le
ion diffusion region
li ~ 40 le
ホール磁場の形成
４重極構造
ion+
electron Magnetic field
+
ion
ホール電流 j
electron -
ホール磁場
By < 0
electron diffusion region
le
ion diffusion region
li ~ 40 le
ホール電場の形成
ion+
electron Magnetic field
E
+
ion
electron -
ExBで紙面向こうむきの
ドリフト
(dawnward motion)
electron diffusion region
le
ion diffusion region
li ~ 40 le
一般化したオームの法則でMHDで無視した項の役割
電子慣性項
le
電子圧力項
ホール項
1/2
li b
異常抵抗項
li
非対角成分
le = c / wpe
5.3/ n 1/2 (/cc)
li = c / wpi
227/ n
1/2
(/cc)
km
km
V. M. Vasyliunas, Rev. Geophys. Space Phys. 1975
Energy = 1 keV
Velocity
Proton
Electron
Proton
Electron
B = 10 nT
Larmor Radius
Period
440 km/s
460 km
6.6 sec
18800 km/s
11 km
0.004 sec
4600 sqrt(E) / B km
66 / B sec
110 sqrt(E) / B km
0.036 / B sec
地球磁気圏尾部での典型的物理量
1 RE = 6371.2 km
磁気圏尾部
幅
厚さ
磁場
密度
温度
地球半径
40 RE
10 RE
20 nT
0.3 /cc
3 keV イオン
磁気リコネクション領域での物理量
プラズマの厚さ
１ イオン慣性長
外部の磁場とプラズマ
20 ｎT
0.01 /cc
Alfvén速度 4000 km/s
ion inertial length
500 km
li = VA / Wi = c / wpi
2D Full Particle Simulations
me/mi
1/100
Particles
33,554,432
Grid Size
512 x 512
Ion Inertial Length
Electron Inertial Length
I. Shinohara
(Av. 128 /grid)
32 grids
3.2 grids
Initial Current Thickness
0.5 li (Harris Current Sheet)
Double-Periodic Boundary Conditions
Results
at time Wi t = 18.0
イオンの運動
イオンの
アルフベン速度
電子の運動
電子の
アルフベン速度
磁場の分布
南北方向Bz
イオンの速度
電子の速度
Magnetotail Reconnection Event
High-Speed Tailward Flowing Ions
Highｌｙ Accelerated Electrons
Tailward convection flows with Bz < 0
Vi > 2500 km/s
Ve > 4000 km/s
ion-electron decoupling
Alfven velocity ~2900 km/s
Ion
Electron
V=2500km/s
Outflow Ions
Convection
EQ
磁場の分布
南北方向Bz
Intense Bz
イオンの速度
電子の速度
MHD
weak Bz
in the outflow region
10 sec
Bz = -36 nT
tail lobe
Bt = 24 nT
the 3-min interval
Bt = 36 nT
the 90-s interval
electron energy spectra
Flux
strong acceleration
of electrons
Energy
thermal
accelerated
strong acceleration of electrons
Currents and By
The Hall current system
Magnetotail Reconnection Event
High-Speed Tailward Flowing Ions
Highｌｙ Accelerated Electrons
Ion
C
Electron
V=3000km/s
Boundary
Less Heated
Inflow Ions
Low V
Outflow Ions
Inflowing Electrons
Hall Current Electrons
Tailward Escaping Electrons
Reconnection Event
High-Speed Ion Flows
Earthward Flows
Tailward Flows
Highly Accelerated Electrons
December 10, 1996
Earthward
Tailward
Hall Current System
Northern hemisphere
Southern hemisphere
Hall Current density ・・・・ 6～13 ｎＡ/m
2
The Hall current loops exist with the double-current structure
in the narrow regions near the separatrix layers.
By is created by the Hall current loop
By = 0.3 Bt lobe
Hybrid simulation
for reconnection
電子流体として外向きの流れ
Vex
M.S. Nakamura et al.
(1998)
外向きの電子流と内向きの電子流の共存
Particle simulation for reconnection
Hoshino (1998)
Magnetotail Reconnection Event
High-Speed Tailward Flowing Ions
Highｌｙ Accelerated Electrons
Ion
C
Dawnward ion drift
V=3000km/s
Electron
Less Heated
Inflow Ions
Low V
Outflow Ions
Dawnward ion drift
イオン
ExB drift
Hall Electric Field
紙面向こう方向に流れながら赤道面方向へ
Boundary
Geotailによる観測により解明された磁気リコネクションの構造
ＭＨＤ的描像
イオンの流入の加速
電子の流入と加速・加熱
磁場を運ぶAlfvenic Flows
粒子的描像
イオンと電子の分離した運動
ホール電流系とそれによる
４重極構造のホール磁場
ホール電場
EH
the Hall electric field
JxB/en
Cluster Observations
Henderson et al., GRL 2006
Edivp
the electric field by div Pe
-div Pe /en
Cluster Observations
Henderson et al., GRL 2006
EH >> Edivp
EH
Edivp
Hall current
Geotail 1996/01/27 Va 2900 km/s n 0.02/cc B 19 nT
Vi -2500 km/s Ve -4000 km/s
j
7.5 nA/m**2
Geotail
Cluster
6-13 nA/m**2
Eh
10 mV/m
2003/08/24
Jx 20 nA/m**2 Bz 2.7 nT
E hall 4.22 mV/m
Henderson
Ez hall 6 mV/m
Ez Pe 1 mV/m
Vdrift 500 km/s
li scale
電子圧力の非対角成分による電場
Geotailで解明された物理過程
ion dynamics
Geotailでは解明できない物理過程
electron dynamics
磁気リコネクション
electron diffusion region で起きる
１．何がdissipationを担うか
電子圧力の非対角成分
電子慣性
ホール項の役割 electron-positron plasmas
２．trigger mechanism
resistive tearing mode
collisionless
electron tearing mode
ion tearing mode
３．reconnection rateは何により決まるか
４．scale of electron diffusion region
short vs. elongated
Trigger Mechanism
Tearing Mode
A one-dimensional currant sheet
a Harris current sheet model
ideal MHD
stable
(frozen-in constraint)
resistive MHD
resistive tearing mode collisional
(Furth, Killeen, and Rosenbluth, 1963)
kinetic
electron tearing mode (electron Landau resonance)
(Coppi et al., 1966)
ion tearing mode (ion Landau resonance)
(Schindler, 1974)
temperature anisotropy Tperp/ Tpara
Difficulty
A magnetotail field configuration
a normal magnetic field Bn
Lembege and Pellat (1982), Pellat et al. (1991), Quest et al. (1996)
Bn
a strong stabilizing effect for electron tearing
Electron stabilization
Galeev and Zelenyi (1976), Lembege and Pellat (1982)
a stabilization effect for ion tearing
electron pitch angle scattering due to magnetic turbulences
Nonlinear ion tearing-like mode
Galeev et al. (1978)
electron effect is uncertain
Cross-scale coupling
Diffusion region
electron scale
Acceleration processes
ion scale
Boundary conditions
Magnetospheric Phenomena
MHD scale
elongated electron diffusion region?
5 le
5-20 li
Secondary island
2D world
super-Alfvenic agyrotropic electron jet
quenching reconnection process
3D world
new instabilities?
Cluster 2003 tail observation
 Cluster separation 200-250 km

Separation can be ≤ c/wpi

Curlometer technique ideal to estimate the current density profile.

Structure within a thin current sheet can be resolved.
 This talk
Thin current sheet crossings (>50 nA/m2)
2003/08/24 1820-1920, 2003/10/01 1940-2040
Discuss:
Spatial/temporal changes in the current sheet structure
Substorm current sheet with fast flow
 Cluster at postmidnight:
Pi2 onset
X=-17,Y=-4,Z=-3 RE
20030824
tailward
flow
earthward flow
growth phase
 Slow CS traversal and current density enhancement before onset
 Multiple neutral sheet crossings during fast flow intervals
69
Current sheet crossings

72
Check whether dBx/dz profile is the same for the two pairs of
the observation  stability of the CS during a crossing
(Coordinates determined from MVA)
Current sheet profile near X-line

Curlometer resolved current profile near reconnection region
1843:17-1843:25
73
1903:28-1903:39
Reconnection observation
Ion and electron decoupling  Hall electric current

Cluster fast (<4 fci-1) current sheet crossing likely
observed Hall current system in a current sheet with (full)
thickness of ~ c/wpi in regions of tailward and Earthward
of X-line(s)
Velec.
Vion
J
electron demagnetized (electron diffusion)
ion demagnetized (ion diffusion)
Cluster Reconnection Event on August 24, 2003
(-16.8, -3.8, 3.3 Re)
Bz
Vx
Ions
Electrons
Nakamura et al. 2006
Earthward
Tailward
Hall Current System
Northern hemisphere
Southern hemispher
Nagai et al. (JGR 2001)
Cluster Observations
Asano et al. 2006
The Hall current loops exist with the double-current structure
in the narrow regions near the separatrix layers.
Geotail Observations
Nagai et al. JGR 200
磁気中性線付近での荷電粒子の運動
S. W. H Cowley 1985
異常抵抗を作るもの
Anomalous Resistivity
波動
Lower Hybrid Waves ?
Shinohara et al. (1998)
SCOPE
Electron scale
The daughter s/c
dedicated to wave-particle
Interaction issue
High-time resolution
Electron measurements
Ion scale dynamics monitors
Ion scale
In planning phase at ISAS/JAXA
Launch ~2017
ESA-ISAS “CrossScale”
Ion-scale shell
～１０００ｋｍ
MHD-scale monitors
SCOPE ele.-scale kernel
～１００ｋｍ
太陽フレアーの磁気リコネクション
１．image
２．直接測定可能な物理量は？
３．imageは直接物理過程を反映しているか？
磁気圏サブストームの磁気リコネクション
１．その場(in situ)での物理量
２．imageは得られない
３．物理過程のどこの物理量を測定しているか？
磁気圏サブストームの磁気Reconnection
新しい段階の研究を進めるための方針
多点での同時観測
（理論との融合）
次期磁気圏探査衛星 SCOPE
Scale COupling of Plasma Environment
1
3
1
親衛星
子衛星
孫衛星
XYZ方向
親の近傍
electron scale
MHD scale
wave correlation
Magnetic reconnection in the magnetosphere
Magnetotail reconnection (Nightside)
1. Symmetrical
(the tail current sheet embedded in the plasma sheet, the tail lobe)
2. Spontaneous (undriven)
3. Accumulation of the magnetic field sin the tail lobes
4. No preference location (localized, thin current sheet, finite Bn)
5. Quasi-steady?
6. Trigger process (substorm onset)
Magnetopause reconnection (Dayside)
1. Asymmetric
(High-density, Intense-magnetic field, turbulence magnetosheath)
2. Forced (driven)
3. High-solar wind pressure (intense sheath flows)
4. IMF interaction (Bz <0, component merging (sub-solar) vs. anti-parallel merging (near cusp))
5. Quasi-steady vs. transient (Flux Transfer Events)
6. Solar wind-magnetosphere interaction
オーロラ領域での編隊飛行観測
ＴＶ－カメラと
粒子計測
斜め上から見て
オーロラの形状だけでなく
鉛直構造も知る
複数の衛星をＧＴＯにいれた
磁場の形状をモニターすると同時に
粒子観測