Wei Liu1, Vahé Petrosian2, Brian Dennis1, & Gordon Holman1

1 NASA Goddard Space Flight Center2 Stanford University

Conjugate Hard X-ray Footpoints in the ‘03/10/29 X10 Flare: Unshearing Motions,

Asymmetries, and Correlations

Contents1. 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

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

2. RHESSI X-ray observations of the 2003/10/29 X10 flare

X-ray & microwave fluxes

Loop-Top: red cntr, +’sFootPoints: cyan cntr, trianglesbkgrd: TRACE 195

Footpoint migration (on MDI)

2.1 Source motions

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)

Source motions (vs. time)

Rapid decrease of footpoint shear cotemporary w/ loop-top downward motion

(see also Ji, H. et al. 2008)

(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

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.

2.2 Asymmetric footpoint hard X-ray fluxes & magnetic field strengths

(Cartoon – courtesy of L. Fletcher)

slow motion fast motion

Strong HXR

Weak HXR

Different field convergence

Different loss-cones

Different electron precipitating rates & HXR fluxes, I1/I2~B2/B1

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

Spectra of footpoint & loop-top sources

E-FP: slightly harder (flatter or smaller spectral index)

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

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)

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.

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

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