速報 “Fast Magnetic Reconnection via Jets 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

Initial Condition Primary current sheet

Results Jet Secondary current sheet 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) Secondary sheet Primary sheet

Results(J v.s. Time) Secondary sheet Primary sheet

Dependence of Results on Resistivity slow fast fast Much thinner than primary one

Discussion and Conclusions Secondary (small-scale) current sheet is created by the 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).