Ultra-violet footpoints as tracers of coronal magnetic connectivity and restructuring during a solar flare

Abstract

Context. The bright, compact ultraviolet sources that appear in flare ribbons are interpreted as sites of energisation of the chromosphere, most likely by electron beams from the corona. Previously we have developed an algorithm to track these compact sources in observations by the Transition Region and Coronal Explorer (TRACE), recording position and intensity. We now exploit this further. Aims. We aim at identifying conjugate footpoint pairs by cross-correlating the TRACE 1600 A lightcurves in one particular event – the 2002-July-17 M 8.5 flare. We also seek the spatial relationship between the magnetic flux transfer (reconnection) rate, well-connected locations, and energy input by electrons. Methods. We performed wavelet a trous filtering on the UV light curves, followed by a linear cross-correlation, to identify wellcorrelated pairs. We used RHESSI data to determine the locations of strong electron beam input. Results. Maps of footpoint pairs were produced in which we can identify well-separated locations that have well-correlated 1600 A light curves. The time lag between credible conjugate footpoint brightenings can be a few seconds. The flare magnetic connectivity is found to evolve with time. RHESSI hard X-ray sources are found where the flux transfer rate is highest. Conclusions. We propose that the correlated footpoints are in fact conjugate pairs that are magnetically linked. In some instances, this linkage may be via a coronal null. The time lag in many cases is consistent with excitation by relativistic particles, but correlations with a longer time lag may suggest excitation by waves.