Physics
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsh43b1528c&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #SH43B-1528
Physics
7509 Corona, 7519 Flares, 7554 X-Rays, Gamma Rays, And Neutrinos, 7594 Instruments And Techniques, 7845 Particle Acceleration
Scientific paper
The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) has observed significant non- thermal emission up to ~70~keV from coronal sources prior to the impulsive phase in some flares, which has interesting implications for particle trapping and flare energy release. Images and spectra during the pre- impulsive phase of the GOES-class X4.8 event on 23~July~2002 suggests that the coronal hard X-ray (HXR) emission is non-thermal, as analysis of the Fe (~6.7~keV) and Fe/Ni (~8~keV) line complexes constrains the thermal component to have little or no thermal continuum emission above ~15~keV. The over-the-limb X3.1 event on 24~Aug~2002 displays similar spectral characteristics, and occultation of the footpoints places the non-thermal HXR emission unambiguously in the corona. RHESSI images of this event show an extended source below ~35~keV and a compact looptop source above ~35~keV, possibly indicating trapping of energetic electrons. Spatially-integrated spectra are well-fit by a broken power-law and the spectral indices above and below the break differ by ~2, suggesting an observation of the transition between thin- and thick-target bremsstrahlung. The collisional lifetime of non-thermal electrons is <10~sec at the inferred densities of ~1011~cm-3 and the rising HXR flux thus suggests continuous particle injection. We apply the technique of Johns &Lin (1992) to invert the pre-impulsive phase photon spectra and recover the original electron energy spectra as a function of time, both for spatially-integrated spectra and source-isolated spectra using imaging spectroscopy. We then compare the inverted spectra with a forward-modeling analysis of the photon spectra using the iron line analysis technique of Caspi &Lin (2006) to constrain the thermal model. We compare the results for both 24~Aug~2002 and 23~Jul~2002, and discuss the implications for electron acceleration and flare plasma heating in the corona during this period.
Caspi Amir
Krucker Sa"m
Lin Robert P.
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