速報 “ Fast Magnetic Reconnection via Jets and Current Microsheets”

1. 速報“FastMagneticReconnectionviaJetsandCurrentMicrosheets”速報“FastMagneticReconnectionviaJetsandCurrentMicrosheets” by P. G. Watson & I. J. D. Craig 2003, ApJ, 590, L0000(in press)

2. Abstract • Numerical simulations of highlynonlinear magnetic reconnection provideevidence of ultrathin currentmicrosheets. • These small-scale sheetsare formed by strongjets from aprimary large-scale current layer. • The size ofthe secondary microsheet isdetermined by the resistivity. • Thisscaling suggests that microsheetsmay provide fast reconnectionsites in thesolar corona.

3. 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.

4. Introduction(2/3) • Numerical simulations showthat ejecta from theprimary sheet act ratherlike a turbo mechanism,enhancing dissipation by supplyinghigh-pressure collimated jets tosustain the microsheet. Shibata et al. 1994

5. Introduction(3/3) • The microsheet is very muchsmaller than primary sheet, its length beingcontrolled by the narrowexhaust jet of theprimary current layer. • Suchmicrosheets have the potentialto act as extremelyshort-lived localized sites ofenergy release (e.g., X-ray brightpoints) in magnetically complexplasmas such as thesolar corona.

6. 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)

7. Craig & Henton 1995 Hirose, Litvinenko, Shibata, Tanuma et al. (in prep.)

8. The Reconnection Simulations • Initial condition: • Including resistivity and viscousity • Simulation region: -1<x,y<1

9. Initial Condition Primary current sheet

10. Results Jet Secondary current sheet Jet

11. 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)

12. Results(V and B)

13. Results(Current) Secondary sheet Primary sheet

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

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

16. 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).