速報“ Fast Magnetic Reconnection via J

ets and Current Microsheets”

by P. G. Watson & I. J. D. Craig

2003, ApJ, 590, L0000(in press)

Abstract

• Numerical simulations of highly nonlinear magnetic reconnection provide evidence of ultrathin current microsheets.

• These small-scale sheets are formed by strong jets from a primary large-scale current layer.

• The size of the secondary microsheet is determined by the resistivity.

• This scaling suggests that microsheets may provide fast reconnection sites in the solar corona.

Introduction(1/3)

• Fast reconnection occurs in solar corona, in which the reconnection rate is independent of resistivity.

• The purpose of the letter is to point out that the exhaust region of a large-scale current layer can provide the external source for small-scale secondary reconnection events.

Introduction(2/3)

• Numerical simulations show that ejecta from the primary sheet act rather like a turbo mechanism, enhancing dissipation by supplying high-pressure collimated jets to sustain the microsheet.

Shibata et al. 1994

Introduction(3/3)

• The microsheet is very much smaller than primary sheet, its length being controlled by the narrow exhaust jet of the primary current layer.

• Such microsheets have the potential to act as extremely short-lived localized sites of energy release (e.g., X-ray bright points) in magnetically complex

plasmas such as the solar corona.

Reconnection Model • Simulations:

Heerikhuisen, Craig & Watson(2000),

Watson & Craig(2001),

　 Hirose, Litvinenko, Shibata, Tanuma et al.(in prep.)

• Analytic models:

Craig & Henton(1995),

Craig & Fabling (1996),

Craig & Watson(2000)

Craig & Henton 1995

Hirose, Litvinenko, Shibata, Tanuma et al. (in prep.)

The Reconnection Simulations

• Initial condition:

• Including resistivity and viscousity

• Simulation region: -1<x,y<1

/)cos(

/)sin()sin(

ˆ),,(

ˆ),,(

x

yx

ztyxB

ztyxv

Initial Condition

Prim

ary current sheet

Results

Secondary current sheet

Jet

Jet

Results

• Primary sheet: Fast reconnection Saturation of sheet Sweet-Parker

• Secondary sheet: Fast reconnection

Typical model α=1, ε=0.3

(Resistivity is uniform: eta=ν=0.0001)

Results(V and B)

Results(Current)

Primary sheet

Secondary sheet

Results(J v.s. Time)

Secondary sheet

Primary sheet

Dependence of Results on Resistivity

fast

slowfast

Much thinner than primary one

Discussion and Conclusions• Secondary (small-scale) current sheet is created by t

he collision between two reconnection jets.• Fast reconnection can occurs in the secondary sheet

even after the fast reconnection stops in the primary one.

• Although the bulk of energy release probably occurs in the primary structure, microsheets powered by primary ejecta could well account for localized hot spots within the plasma(e.g., X-ray-bright points associated with solar flares).