SM13B-2372

Electron dynamics
surrounding the X line
in asymmetricmagnetic reconnection

Seiji ZENITANI
Hiroshi Hasegawa
Tsugunobu Nagai

Magnetic reconnection in near-Earth space
Electron-scale measurement with MMS

Burch+ 2016 Science

We try to understand electron-physics
signatures in magnetopause reconnection
Burch+ 2016 Space Sci. Rev.

2D particle-in-cell (PIC) simulation• Asymmetry in plasma properties• Density variation

n1 : n2 = 3 : 1• Magnetic variation

B1 : B2 = 1 : 3• Ion-electron mass ratio• mi : me = 25 : 1

MagnetosphereZZYXMagnetosheathXElectron-physics signatures near the X line(c) E’
Ey0 = [E + V e ⇥ B]y(E + v e ⇥ B)y > 0

1. Sphere-side
1zX
33. Outflow region

2. Sheath-side
2x(E + v e ⇥ B)y < 0

4. Electron mixing
5. Guide-field effects

Sphere-side: Crescent-shaped VDF
Electron velocity distribution function

MMS observation (Burch+ 2016 Science)(c) E’y1z
32x

See also
Hesse+ 2014 GRL
Bessho+ 2016 GRL
Shay+ 2016 GRL

Sphere-side: Crescent-shaped VDF
Sheath-origin
meandering electronsBSphere-origin
core component

https://commons.wikimedia.org/wiki/File:Rogalik.jpg

Meandering electronxySphere-side: Crescent-shaped VDF
Color and radius =Crossing numbersof Bx-reversal• Generic crescent formula(see also Bessho+ 2016)

Sheath-side: Nonadiabatic electrons(c) E’
Eyy0 = [E + V e ⇥ B]y• Curvature parameter

1(Buchner & Zelenyi 1989 JGR)

z32x

(E + v e ⇥ B)y < 0⌘sRc⇢

Magnetic
curvature radiusElectronLarmor radius(g) Box 2parameters- curv. parameter
Curvaturein Region 2Nonadiabatic (k<2.5)• ρ ~ Rc
• Chaotic motion• We no longer expect
the ExB drift!

E + v e ⇥ B 6= 0

Sheath-side: Nonadiabatic electrons(c) E’
Eyy0 = [E + V e ⇥ B]y• Curvature parameter1(Buchner & Zelenyi 1989 JGR)

z32x

(E + v e ⇥ B)y < 0132

⌘sRc
⇢

Magnetic
curvature radiusElectronLarmor radius

Sharp magnetic
curvature in 3D

Outflow region: Magnetic topology(c) E’
Eyy0 = [E + V e ⇥ B]y(g) Curvatureparameter(K) parameterEnsemble
curvature

1N axisz3z
32x• After an appropriate 3D rotation,
the sharpest curvature is locatedat the BL-reversal (≠ Bx-reversal)

x

0.4
0.2
0.0BNBLBM-0.2-0.4-1.0 -0.8 -0.6 -0.4 -0.2 0.0z

Bx reversal

0.2

BL reversal

0.4Outflow region: Poincaré map analysis(c) E’
Eyy0 = [E + V e ⇥ B]y(g) Curvatureparameter(K) parameter
Ensemble
curvature

1N axisz3z
32x
Nonadiabaticx
BAdiabatic

Buchner & Zelenyi 1991 Adv. Space. Res.

Electron mixing(Brute-force way to identify the EDR)

Quantifying electron mixing
Mf (t0 , t)

+ t

“Forward” mixing

01Quantifying electron mixing

Quantifying electron mixing

Quantifying electron mixingt

1“Backward” mixing

Mb (t0 ,0t)

Quantifying electron mixing
Important region forreconnection physics

01

10See also Haller 2015 Annu. Rev. Fluid Mech

Finite-time mixing fraction (FTMF)
MR (t0 ,t) ⌘ M
Mff(t(t00,, t)t) ⇥ Mb (t0 , t)t0 : time of interestt : an electron-scale time interval
Mf (t0 ,t)

Mb (t0 ,t)

MR (t0 , t)

Finding appropriate Δt
MR (t0 ,t) ⌘ M
Mff(t(t00,, t)t) ⇥ Mb (t0 , t)t0 : time of interestt : an electron-scale time interval
t = 0.1

MR (t0 , t)t = 3.0

t = 1.0 ⌦ci 1 = 25 ⌦ce1
Sufficiently smallt that satisfies 2⇡⌦ce1 ⌧tMixing measure
Zenitani+ 2017 JGR

MR (t0 , t)vsEnergy dissipation

Asymmetric RX

Zenitani+ 2011 PRL

De ' j · (E + v e ⇥ B)

Asymmetric RX with a guide-fieldBy

Guide-field reconnection: VDFs

Sphere-origin

Vy

Vz
Sheathorigin
Vx

Vx

Vx

Vx

• Sphere/sheath-origin electrons are separate in VDFs (Hesse+ 2017 PRL)Guide-field reconnection: electron orbits

Vy

Vx• Signatures are enhanced byparallel acc. near the X lineVz

Vx

Guide-field reconnection: Shannon entropy

Xpi log piipi : probability in the
3D velocity space• High-entropy sphere-side layer with unmixed electrons

Online viewer (public version)• http://rish.kyoto-u.ac.jp/~zenitani/DFX/• http://rish.kyoto-u.ac.jp/~zenitani/MRA/• Antiparallel reconnection• Asymmetric reconnection

Summary• 2D PIC simulation ofmagnetopause-type asymmetric reconnection• Electron-physics signatures– Sphere-side : Crescent-shaped distribution functions– Sheath-side : Nonadiabatic/nonideal layer– Outflow-region : Nonadiabatic/nonideal layer near the BL-reversal• Electron mixing– We have introduced finite-time mixing fraction (FTMF)– It marks the energy-dissipation region• Guide-field reconnection– We have confirmed Hesse (2017)’s finding of unmixed electrons– Shannon entropy marks the sphere-side unmixed layer• Reference– Zenitani, Hasegawa, & Nagai, J. Geophys. Res. Space Physics, 122, 7396-7413,
doi:10.1063/1.4963008 (2017)