Some open problems for magnetic reconnection in solar flares

Transcription

1 Some open problems for magnetic reconnection in solar flares Bernhard Kliem Astrophysical Institute Potsdam 1. Introduction 2. Reconnection outflow termination shock 3. Supra-arcade downflows 4. Impulsive bursty reconnection and spiky emissions 5. Reconnection and particle acceleration 6. Reconnection in current sheet above erupting flux Page 1 of 23 INI MRT Programme Workshop, Cambridge, 10 August 2004

2 1 Brief overview of current flare models Eruptive (ejective) events: CMEs, prominence/filament eruptions, eruptive flares Opening:?? Main phase: Standard (CSHKP) model Confined (non-ejective) events: Failed filament/prominence eruptions (same initiation mechanism) Confined ( impulsive / compact ) flares (probably different process, e.g.: loop-loop reconnection) Page 2 of 23

3 Initiation/Opening Tether cutting: reconnection in core field Magnetic breakout: reconnection above arcade Flux rope catastrophe: catastrophe of flux rope & reconnection below Flux rope instability: MHD instability, e.g., kink Page 3 of 23 Evolution toward standard model in all cases.

4 Main phase: standard model (CSHKP) Page 4 of 23 McKenzie (2002), Yohkoh-10 proc. Widely accepted (after 12 years of Yohkoh and SOHO data) Some aspects still unclear or debated, e.g.: Energy partition between particle acceleration and direct plasma heating Mechanisms of very efficient particle acceleration Existence of standing fast-mode termination shock Some aspects of the reconnection outflow

5 2 The fast-mode termination shock(s) in the reconnection outflow jets Page 5 of 23 B, u, and j are shown; β = 0.1 Forbes 1986, ApJ 305, 553 The shock has been sugested to be the site of the main particle acceleration (e.g., Tsuneta & Naito).

6 Expect shock for low β p CS = p + B2 = (1 + β) B2 2µ 0 2µ 0 β = ρ = ρ 0 p B 2 /2µ 0 u out = V A ( ) 1/2 ( ) 1/2 γp γ(1 + β) C s = = V A ρ 2 Page 6 of 23 u out > C s β < 2/γ 1 = 0.2

7 Termination shock even without obstacle TS Page 7 of 23 B, u, and T are shown; β = 0.03 Innes & Tóth 1999, Solar Phys. 185, 127 There may as well be a shock in the upper reconnection outflow jet.

8 Page 8 of 23 A = fast mode termination shock; β = 0.15 Shimizu & Ugai 2003, Phys. Plasmas 10, 921

9 A radio signature of the termination shock? Aurass et al. 2002, A&A 384, 273; Aurass & Mann 2004, ApJ, in press Page 9 of 23 Klassen et al (Type II burst catalogue)

10 Any signature of the termination shock in the EUV? Page 10 of 23 TRACE Fe XII 195 Å (1.5 MK) + Fe XXIV 192 Å (> 15 MK) 2002 April 21 flare Inflows come to rest smoothly. Simply aggregation? Does strong inhomogeneity prevent shock formation?

11 3 Supra arcade downflows Downflows = reconnection outflow jet Page 11 of 23 Dense cool blobs or voids? Voids: retracting reonnected loops (standard model), but of low density Origin of wriggles? see McKenzie 2000, Solar Phys. 195, 381

12 Are wriggles caused by Kelvin-Helmholtz Instability? Page 12 of 23 However: effect may be caused by multiple reflections of initial perturbation at boundaries of simulation box. (Highly varying reconnection rate due to anomalous resistivity model.) S. Tanuma, personal comm.

13 Wriggles naturally expected if downflows are cool blobs Page 13 of 23 (Simulation yet to be done.)

14 4 Impulsive bursty reconnection and spiky emissions 1992 Oct 5 eruptive flare: quasi-periodic radio emissions, anticorrelated with hard X-rays; τ radio 1.3 s Page 14 of 23 HXR spikes: τ HXR 0.7 s e.g.: 1991 June 15 X flare:

15 Kliem et al. 2000, A&A 360, 715 (see also Scholer & Roth 1987) Page 15 of 23 Dynamics of η an (x, t) important quasi-periodic reconnection pulses Does collisionless reconnection offer this possibility?

16 Page 16 of May 27, TRACE 195 Å Complexity is an alternative Quasi-periodicity?

17 5 Reconnection and particle acceleration Page 17 of 23? Number problem? Energy content of waves? Formation? Time scales

18 The electron number problem Electron flux (E > 20 kev, in X flare) s 1 Current sheet inflow: Ṅ = 2n u in L 2 CS 2 ( ) s 1 for n = cm 3, u in = ( )V A, V A = 3900 km s 1 (B = 100 G), L CS = cm. Whole current sheet probably just OK, but X-line region alone insufficient by orders of magnitude! Page 18 of 23

19 Stochastic acceleration f t W T t = 1 p 2 p = k [ p 2 D(p) f ] p [ k 2 D k (k 2 W T ) ] γw T + S t = ( ) s t = ( ) s Page 19 of 23 Miller et al. 1997, JGR 102, Wave energy injection at 10 7 ( ) cm Does reconnection pump sufficient energy into waves? Does turbulent cascade occur in reconnection outflow jets?

20 6 Reconnection in current sheet above erupting flux? Page 20 of 23 Φ = 5π

21 Remote brightenings and dimmings Page 21 of 23 Manoharan et al. 1996, ApJ 468, L73

22 Remote gamma-ray source in 2002 July 23 X flare Page 22 of 23 RHESSI (& TRACE; SOHO/MDI) Lin et al ApJL; Hurford et al ApJL

23 Astonishing reconnection (across 20 netwok cells) Page 23 of 23 Yohkoh/SXT: Huge interconnecting loop early in solar cycle Farnik et al. 1999, Solar Phys. 187, 33