HARD X-RAY AND ULTRAVIOLET OBSERVATIONS OF THE 2005 JANUARY 15 TWO-RIBBON FLARE

Abstract

In this paper, we present comprehensive analysis of a two-ribbon flare\nobserved in UV 1600{\\AA} by Transition Region and Coronal Explorer and in HXRs\nby Reuven Ramaty High Energy Solar Spectroscopic Imager. HXR (25-100 keV)\nimaging observations show two kernels of size (FWHM) 15?? moving along the two\nUV ribbons. We find the following results. (1) UV brightening is substantially\nenhanced wherever and whenever the compact HXR kernel is passing, and during\nthe HXR transit across a certain region, the UV count light curve in that\nregion is temporally correlated with the HXR total flux light curve. After the\npassage of the HXR kernel, the UV light curve exhibits smooth monotonical\ndecay. (2)We measure the apparent motion speed of the HXR sources and UV ribbon\nfronts, and decompose the motion into parallel and perpendicular motions with\nrespect to the magnetic polarity inversion line (PIL). It is found that HXR\nkernels and UV fronts exhibit similar apparent motion patterns and speeds. The\nparallel motion dominates during the rise of the HXR emission, and the\nperpendicular motion starts and dominates at the HXR peak, the apparent motion\nspeed being 10-40 km s-1. (3) We also find that UV emission is characterized by\na rapid rise correlated with HXRs, followed by a long decay on timescales of\n15-30 minutes. The above analysis provides evidence that UV brightening is\nprimarily caused by beam heating, which also produces thick-target HXR\nemission. The thermal origin of UV emission cannot be excluded, but would\nproduce weaker heating by one order of magnitude. The extended UV ribbons in\nthis event are most likely a result of sequential reconnection along the PIL,\nwhich produces individual flux tubes (post-flare loops), subsequent non-thermal\nenergy release and heating in these flux tubes, and then the very long cooling\ntime of the transition region at the feet of these flux tubes.\n