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[54.03] The hard X-ray spectral structure of flare ribbons

L. Fletcher (University of Glasgow), H. S. Hudson, S. Krucker (SSL/UCB), J. A. Pollock (University of Glasgow)

We examine the spatial distribution of hard X-ray spectral parameters in flares exhibiting the classic two-ribbon structure using RHESSI observations. The flares studied include July~15 and July~17, 2002, and October~29, 2003. We confirm the existence of a tendency for the localization of the hard X-ray sources into dominant bright ``footpoint'' regions which do not show ribbon structure as extensive as that seen in H\alpha or UV~images. As a part of the study we characterize the ribbons photometrically in the EUV as observed by TRACE, confirming earlier results that find complicated relationships between EUV and hard X-rays. We seek an empirical explanation for the restricted hard X-ray footpoints in terms of a spatial analog of the well-known ``soft-hard-soft'' morphology: the regions of weaker hard X-ray emission correspond to steeper X-ray energy spectra and hence to softer electron precipitation spectra. This relationship may be as predicted by the 1D radiation hydrodynamics models of flaring loops.

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Bulletin of the American Astronomical Society, 36 #2
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