Ultraviolet flare ribbon brightenings and the onset of hard X-ray emission

The broadband UV images taken by the Transition Region and Coronal Explorer (TRACE) provide a unique opportunity to observe transition region and chro mospheric emission from flare footpoints at high spatial and temporal resol ution. In this Letter, we present comparisons of UV flare footpoint evoluti on with hard X-ray light curves from the Yohkoh Hard X-Ray Telescope and th e Compton Gamma Ray Observatory Burst and Transient Source Experiment (BATS E). The nine events analyzed in this Letter all show evidence for flare foo tpoint brightenings that precede the onset of the hard X-ray emission. Regr ession analysis between TRACE UV data taken at very high cadence (2-3 s) an d 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 impuls ive phases of the flare is occurring on different loops.