Magnetic Reconnectionin Large and Fully Kinetic System

Keizo Fujimoto
Space Physics Seminar on Dec. 04, 2006

Introduction of myself
March, 2003 Master of Science at Kyoto University, JapanMarch, 2006 Ph.D. of Science at Kyoto University, JapanApril, 2006 -Research Fellow of Japan Society for the Promotion ofScience (JSPS)

Currently I belong to ….
Department of Physics, University of Alberta, Canada.
(Supervised by Prof. Rick Sydora)
National Institute of Information and Communications Technology(NiCT), Japan. (Supervised by Dr. Takahiro Obara)

Space Physics Seminar at UofA

Introduction
Non-MHD processes can affectMHD-scale dynamics.

(Cowley, 1985)

Ion and electron-scale processes
are coupling and determine theMHD-scale structure.

Space Physics Seminar at UofA

Introduction
Main issues on magnetic reconnection in solar flares andmagnetospheric substorm
1.

What is the mechanism to quickly initiate reconnection?
When and where can reconnection be triggered in thesystems of interest?

2.

What is the mechanism to support a quasi-steady fast reconnection?
Is it possible to explain the energy release associated withsolar flares and magnetospheric substorm by reconnection
alone?

3.

How and where can plasmas be accelerated and heated?
What is the energy transport processes associated withreconnection?
Space Physics Seminar at UofA

Introduction
What is the fast reconnection?
Fast reconnection: Ey ~ 0.1 VA0B0(VA0, B0: Alfven velocity and magnetic field in the lobes.)¾ Within 10 minutes, about 15 Re of magnetic flux in lobes can bereconnected in the Earth magnetotail.
¾ Within 50 seconds, most of the magnetic field in the flux tube with an area
1018 cm2 and a length 109 cm can be reconnected in the solar flares.

Space Physics Seminar at UofAIntroduction
Quasi-steady models of fast reconnection
9 MHD model:[Petschek, 1964]

9 Kinetic model （GEM Reconnection Challenge, 2001）：
Inside the electron diffusion region

Outside the electron diffusion region

Due to periodic
boundaries

Inclusion of the Hall term is sufficientcondition for fast reconnection. [e.g.,Birn et al., 2001][Hesse et al., 2001]

However, steady-state reconnection is not achieved.
Space Physics Seminar at UofA

Motivation
It is necessary to perform a large-scale kinetic simulation of magnetic
reconnection in order to check whether or not the Hall effects are
sufficient for a steady-state fast reconnection.

However, it is still difficult to conduct large-scale simulations using
conventional particle-in-cell (PIC) codes because of limited computerresources.
In this study, I have constructed a new EM PIC code using adaptive
meshes and succeeded to perform efficient kinetic simulations.

Space Physics Seminar at UofAEM code with adaptive mesh refinement (AMR) techniqueRestriction in full particle code:
Magnetotail Lobe

[Birdsall & Langdon, 1995]

Grid spacing can becoarser in the lobe region
than in the plasma sheet.
Central Plasma SheetSpace Physics Seminar at UofAEM code with adaptive mesh refinement (AMR) technique
AMR techniqueParticle splitting algorithm
Δr = ΔL/N1/2

N: Number of particles in the cell.

x1,2 = x0 ± Δr; x3,4 = x0
y1,2 = y0; y3,4 = y0 ± Δr
V1,2,3,4 = V0;
q1,2,3,4 = q0/4;
m1,2,3,4 = m0/4

Space Physics Seminar at UofAEM code with adaptive mesh refinement (AMR) techniqueSpace Physics Seminar at UofA

Time evolution of the reconnection rate

Space Physics Seminar at UofAWhy do the reconnection rate decrease?

Space Physics Seminar at UofA

How can the electron diffusion region be elongated?

[Fujimoto, Phys. Plasmas, 2006]

mi/me=1
mi/me=100

Curvatureof field line
Electron current
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Comparison with observations(Simulation result)
Consistent with observations. （e.g., Asano et al., 2004）

Expected mechanism to enhance Ey
Buneman-type instability
can be excited.

Electronpressure tensor

Electron Anomalous
resistivity
inertiaSpace Physics Seminar at UofA

Summary and Conclusions
We have investigated the time evolution of magnetic reconnection in large
and fully kinetic systems. Our kinetic code employs the AMR and particle
splitting techniques.
1. Inclusion of the Hall term is NOT sufficient for quasi-steady fastreconnection.
The polarization field, which is caused by the inertia difference
between ions and electrons, elongates the electron diffusion region,
so that quasi-steady reconnection is not achieved.
2. Inclusion of the Hall term is NOT necessarily required for fast reconnection.
Even in the system without the Hall effects (mi/me = 1), fast reconnection
can occur.

Space Physics Seminar at UofA

Summary and Conclusions
Buneman-type instability is expected to give an anomalous resistivity,
enhancing the reconnection rate. However, it is not clear how much it
affects the system.

We need to extend the present 2-D code to the 3-D code and investigate theeffects of the anomalous resistivity on magnetic reconnection.

Space Physics Seminar at UofA