Wave Activities and Electron
Acceleration near the Separatrices ofMagnetic Reconnection
Keizo Fujimoto
Division of Theoretical Astronomy, NAOJ

Waves in MRX Region: Obs.

f [kHz]EC (Electron cyclotron: Whistler) → ElectromagneticLangmuir (plasma oscillation)
ESW (Electrostatic Solitary Wave) → Electrostatictime[Viberg et al., GRL, 2013]
MR2014@Tokyo, Japan2

Waves in MRX Region: Observations
Whistler waves[Tang et al., GRL, 2013]

 Previous simulations wereunable to reproduce them.
𝜔𝜔𝑟𝑟 ≈ 0.3𝜔𝜔𝑐𝑐𝑐𝑐
 Generation mechanisms?

 Their roles in reconnection？

MR2014@Tokyo, Japan3

Electron Energetics: Observations
Energy[Asano et al, JGR, 2008]

4 – 10keV

Reconnection region
 High energy electrons aroundseparatrices
 “Flat-top” electrons

MR2014@Tokyo, Japan4

Previous Simulation of Waves in MRX[Fujimoto & Machida, JGR, 2006]
For anti-parallel reconnection
Langmuir waves + Electron heating at PSBL

B

Electron 2-stream instability
MR2014@Tokyo, Japan

Parallel electron heating
5

Previous Simulation of Waves in MRX[Fujimoto & Sydora, GRL, 2008]

Whistler waves in the pile-up region of B-fieldTimeξB

ξB

B

Temperature anisotropy (Te⊥/Te|| > 1) generates whistlers.
Electrons are scattered to the parallel direction.
MR2014@Tokyo, Japan6

Purpose of This Study To understand the generation mechanisms of the waves
around the separatrices of anti-parallel reconnection, and To clarify the roles of the waves in reconnection, inparticular, in electron acceleration.
Recent PIC simulations have shown the signature ofelectrostatic waves in the separatrix region.
[Lapenta+, GRL, 2011; Egedal+, Nat. Phys., 2012]

However, the corresponding instabilities have not been
identified clearly.

MR2014@Tokyo, Japan

7

Strategy of Our PIC Simulation
𝑣𝑣𝑡𝑡𝑡,𝑒𝑒 ≪ 𝑉𝑉𝑒𝑒,𝑜𝑜𝑜𝑜𝑜𝑜
for waves be active
Diffusion region(𝑣𝑣𝑡𝑡𝑡,𝑒𝑒 ≡𝑉𝑉𝑒𝑒,𝑜𝑜𝑜𝑜𝑜𝑜 ~𝑉𝑉𝐴𝐴𝐴𝐴= 𝑣𝑣𝑡𝑡𝑡,𝑒𝑒 ⁄ 𝛽𝛽𝑒𝑒
−1/2𝑉𝑉𝑒𝑒,𝑜𝑜𝑜𝑜𝑜𝑜 ~𝑉𝑉𝐴𝐴𝐴𝐴 = (𝑚𝑚𝑖𝑖 ⁄𝑚𝑚𝑒𝑒 )1/2 (𝑛𝑛𝑏𝑏 ⁄𝑛𝑛0 )More realistic parameters
mi/me: 100 → 400
nb/n0: 0.2～0.3 → 0.04 AMR-PIC
Long-time evolution :Periodic → Open boundary

2𝑇𝑇𝑒𝑒 /𝑚𝑚𝑒𝑒 )𝑉𝑉𝐴𝐴0 ∝ 1⁄ 𝛽𝛽𝑒𝑒

MR2014@Tokyo, Japan

Np： ~ 1010
Memory：~ 1TB8

Wave Activities Around Separatrices[Fujimoto, GRL, 2014]E||Ion-frame Ey

MR2014@Tokyo, Japan

9

Inflow

Wave Activities

ESESEM

V|| > 0 Weak waves
 Local strong
acceleration of electron Intense wave activities Electron heating
MR2014@Tokyo, Japan

10

Wave Generation Mechanisms Linear analyses𝝎𝝎 = 𝝎𝝎𝒓𝒓 + 𝒊𝒊𝜸𝜸

Buneman Inst.
Vb > vth,eLower-hybrid𝜔𝜔𝑟𝑟 ~𝜔𝜔𝑝𝑝𝑝𝑝

Electron-electron 2-stream instability
MR2014@Tokyo, Japan𝜔𝜔𝑟𝑟≈ 0.3𝜔𝜔𝑐𝑐𝑐𝑐

Beam-driven whistler
instability
11Role of the Waves
Buneman

Electron-electron
2-stream instability
Beam-driven
whistler instability

a few 100eV Flat-top~5keVMR2014@Tokyo, Japan

Non-thermalcomponent

12Electron Acceleration Mechanism
Acceleration due tolocal ES potential
ElectronIon

δH112−𝑒𝑒𝜙𝜙𝐴𝐴 ~ 𝑚𝑚𝑒𝑒 𝑉𝑉𝐴𝐴𝐴𝐴= 𝑚𝑚𝑖𝑖 𝑉𝑉𝐴𝐴222𝑟𝑟𝑔𝑔𝑔𝑔 ~𝛿𝛿𝐻𝐻 ~ 𝑐𝑐 ⁄𝜔𝜔𝑝𝑝𝑝𝑝 ⇒ 𝑣𝑣𝑡𝑡𝑡,𝑖𝑖 ~𝛼𝛼𝑉𝑉𝐴𝐴

1𝑚𝑚𝑒𝑒 𝑣𝑣||2 ≈ 𝑒𝑒𝜙𝜙02

𝛼𝛼 = 𝐵𝐵𝑖𝑖 /𝐵𝐵𝑒𝑒 >1−𝑒𝑒𝜙𝜙𝐶𝐶 ~ 𝛼𝛼 2 − 1 𝑚𝑚𝑖𝑖 𝑉𝑉𝐴𝐴2 /2< 0MR2014@Tokyo, Japan13

Electron Acceleration Mechanism
At point C𝑽𝑽𝒆𝒆|| ≈Ve||

Dynamo𝟐𝟐𝟐𝟐𝝓𝝓𝑪𝑪 ⁄𝒎𝒎𝒆𝒆~ 𝜶𝜶𝟐𝟐 − 𝟏𝟏 𝑽𝑽𝑨𝑨𝑨𝑨∝ 𝜷𝜷𝒆𝒆 −𝟏𝟏/𝟐𝟐Inflow

Δ𝜙𝜙 > |𝜙𝜙𝐴𝐴 |MR2014@Tokyo, Japan14

Summary[Fujimoto, GRL, 2014]

The generation mechanismsof the waves in the separatrix
regions have been identified
for anti-parallel reconnection.
Key parameters are
realistically low plasma beta.
The waves are responsible for “flat-top” and non-thermalelectrons.
The waves are useful to diagnose the electron dynamics in thereconnection region by means of on-going and/or up-coming
satellite observations.
Guide-field cases will be investigated as a next step.
MR2014@Tokyo, Japan15