Fine structure inside flare ribbons and its temporal evolution

Abstract

Abstract We observed an X2.3 flare, which occurred on 10 April 2001, in Hα with the Sartorius Telescope at Kwasan Observatory, Kyoto University. Thanks to the short exposure time used for the flare, the Hα images showed the fine structure in the flare ribbons. First, we examined the temporal and spatial evolution of the Hα kernels. We identified the conjugate footpoints in each flare ribbon by calculating cross-correlation functions of the light curves of the Hα kernels. We found that these footpoints are really connected by the flare loops seen in extreme-ultraviolet images obtained with the Transition Region and Coronal Explorer . We also followed the evolution of the energy release site during the flare. Then, we compared the spatial distribution of the hard X-ray (HXR) sources with those of the Hα kernels. While many Hα kernels are found to brighten successively in the development of the flare ribbons, the HXR sources are locally confined to some special Hα kernels where the photospheric magnetic field is sufficiently strong. We estimated the energy release rates at each radiation source, and found that they are high enough at the HXR sources to explain the difference in appearance between the Hα and HXR images.