Ultraviolet Flare Ribbon Brightenings and the Onset of Hard X-Ray Emission

Abstract

The broadband UV images taken by the Transition Region and Coronal Explorer (TRACE) provide a unique opportunity to observe transition region and chromospheric emission from flare footpoints at high spatial and temporal resolution. In this Letter, we present comparisons of UV flare footpoint evolution with hard X-ray light curves from the Yohkoh Hard X-Ray Telescope and the Compton Gamma Ray Observatory Burst and Transient Source Experiment (BATSE). The nine events analyzed in this Letter all show evidence for flare footpoint brightenings that precede the onset of the hard X-ray emission. Regression analysis between TRACE UV data taken at very high cadence (2-3 s) and hard X-ray light curves, however, shows that the initial hard X-ray burst is positively correlated only with footpoints that show no pre-hard X-ray activity. This indicates that energy release during the preflare and impulsive phases of the flare is occurring on different loops.