IMPLICATIONS OF TEMPORAL DEVELOPMENT OF LOCALIZED ULTRAVIOLET AND HARD X-RAY EMISSION FOR LARGE SOLAR FLARES

Ultraviolet and hard X-ray (HXR) emissions in solar flares provide observational signatures of the interaction of flare-accelerated particles with the chromospheric plasma. Earlier studies have shown clear evidence of temporal and spatial relationships between UV and HXR emission, suggesting a common physical origin. However, more recent spatially resolved case studies suggest significant variations in the spatial distributions of the emission signatures, implying that the large-scale magnetic topology of the flaring region must play a crucial role in the spatial and temporal development of the localized UV and HXR emission. We present here an analysis of spatially resolved HXR emission and localized UV emission sources from 11 M and X class flares with observations from RHESSI and high-cadence 1600 angstrom observations from TRACE. Within each flare we address the overall temporal development of individual UV sources and relate them to associated impulsive bursts within the HXR flare profile. We find, for these large flares, that in the initial impulsive bursts of flare activity, the majority of the temporally correlated emission evolves in a series of localized co-spatial sources along the UV ribbons consistent with previous two-dimensional reconnection pictures. However, observations of impulses late in flares, show significant departures from the traditional co-spatial/co-temporal picture. The new results include extended UV ribbons with no corresponding HXR emission and marked spatial separations between temporally correlated sources of UV and HXR emission. In seven of the multi-burst events, we observe the development of independent sets of UV and HXR sources corresponding to the individual impulses seen in the temporal profile. The observed separations and the spatial development of co-temporal emission in multi-burst events emphasize the importance of a complex and time varying magnetic topology in shaping the observed emission distributions in both wavelengths. In four of the events, we observe late developing UV sources which show no relationship with the HXR emission. In these events, the emission sources show a strong relationship with lower energy, more spatially extended X-ray emission believed to be of thermal origin. This suggests that, in the later phase of these complex flares, emission from thermal processes comes to dominate non-thermal processes in the production of the UV emission.