1. Wei Liu 1, Vahé Petrosian 2, Brian Dennis 1, & Gordon Holman 1 1 NASA Goddard Space Flight Center 2 Stanford University Conjugate Hard X-ray Footpoints in the ‘03/10/29 X10 Flare: Unshearing Motions, Asymmetries, and Correlations

2. Contents 1. Introduction 2. RHESSI observations of the hard X-ray footpoints in the 2003/10/29 X10 flare (a million $ event: Xu+04, Metcalf+04, Liu,C.+05, Krucker+05, Ji+08,…) 2.1 Source motions – footpoint unshearing & loop-top downward motion 2.2 Asymmetries – hard X-ray flux, magnetic field 2.3 Correlation – between hard X-ray flux and magnetic field strength 3. Summary and discussion

3. surface corona surface (movie courtesy of T. Forbes) Original models: Sturrock (1966), Hirayama (1974), Kopp & Pneuman (1976) 1. Introduction: Classic picture of two-ribbon flares (pre-RHESSI) Loop-top X-rays Footpoint X-rays

4. 2. RHESSI X-ray observations of the 2003/10/29 X10 flare X-ray & microwave fluxes

5. Loop-Top: red cntr, +’s FootPoints: cyan cntr, triangles bkgrd: TRACE 195 Footpoint migration (on MDI) 2.1 Source motions

6. Source motions in detail Anti-parallel (to N.L.) motion early on ( also Sakao+98, Masuda+’00 ) Note the rotated “L” shape. (Liu, Petrosian, Dennis, & Holman, ApJ, 2009 March, in press)

7. Source motions (vs. time) Rapid decrease of footpoint shear cotemporary w/ loop-top downward motion (see also Ji, H. et al. 2008)

8. (Liu et al. 2004; see also Sui & Holman 2003, Sui et al. 2004, Holman et al. 2005) More loop-top downward motion: 2003/11/03 X3.9 flare Downward motion

9. Cotemporary Footpoint unshearing & loop-top downward motions – interpretation & Discussion * Ji et al (‘07): lower-lying, less sheared field lines=> less free- energy. Can such topological transition actually take place? * Hudson (‘00): Implosion; * Loop-top downward motion: Longcope, Guidoni, & Linton (’08): gas-dynamic shock heating. c.f. Forbes & Acton (’96): Yohkoh SXR loop shrinkage; Veronig et al. (‘06): Betatron acceleration, don’t expect footpoints to move. No clear explanation yet.

10. 2.2 Asymmetric footpoint hard X-ray fluxes & magnetic field strengths (Cartoon – courtesy of L. Fletcher) slow motionfast motion Strong HXR Weak HXR Different field convergence Different loss-conesDifferent electron precipitating rates & HXR fluxes, I 1 /I 2 ~B 2 /B 1

11. Hard X-rays & magnetic field strengths E-FP X-ray brighter and B-field weaker than W-FP – Consistent with asymmetric mirroring, but not all the time!  Alternative – asymmetric column densities to footpoints?  Need spectral info

12. Spectra of footpoint & loop-top sources E-FP: slightly harder (flatter or smaller spectral index)

13. Spectral index evolution of footpoints – E-FP: consistently harder spectrum, smaller column density from loop-top

14. Effects of asymmetric column densities alone on HXR flux and spectral index –> contradiction (c.f, Falewicz & Siarkowski ‘07) Consistent w/ Saint-Hilaire et al. (‘08)

15. Footpoint Asymmetry: Discussion Mirroring or column density alone – inconsistent w/ data … combined – possible Other transport effects: Non-uniform target ionization (Brown; Kontar) – Yes (E-FP: brighter, harder) Photospheric albedo (Langer & Petrosian, Bai & Ramaty) – No (E-FP: softer) Relativistic beaming (McTiernan & Petrosian) – No (E-FP: softer) Return current – ? Acceleration, asymmetric ( McClements & Alexander ‘05 ) – ? Final answer yet to come from detailed modeling.

16. 2.3 Correlation between hard X-ray flux (log) & magnetic field strength Implications: Stochastic acceleration – particle acceleration rate strongly depends on B; Electric field acceleration – larger electric field => larger electron flux and harder spectrum

17. 3. Summary & Discussion 1. Footpoints unshearing motion & simultaneous loop-top downward motion – c.f, classic flare model. 2. Asymmetric hard X-ray footpoints – asymmetric magnetic mirroring, column densities, and other transport effects. 3. Correlation between hard X-ray flux and magnetic field strength – implication for particle acceleration mechanisms. Preprints (hardcopy & pdf) available here