RECONNECTION AND HELICITY IN A SOLAR-FLARE

Using X-ray images, Ha images, and vector magnetograms, we have studie d the evolution of the coronal structure and magnetic held of NOAA Act ive Region 7154 during 1992 May 5-12. A two-ribbon 4B/M7.4 flare assoc iated with an H alpha filament eruption was observed on May 8, 15:13-1 9:16 UT. An interesting feature of the region was a long, twisted X-ra y structure, which formed shortly before the flare and disappeared aft er it, being replaced by a system of unsheared postflare loops. Neithe r the X-ray nor Ha morphology nor the photospheric magnetic field show s any indication of gradual buildup of nonpotential energy prior to th e flare. Rather, the long structure appears to result from the reconne ction of two shorter ones just tens of minutes before the filament eru ption and flare. Using vector magnetograms and X-ray morphology, we de termine the helicity density of the magnetic field using the force-fre e held parameter cc. The observations show that the long structure ret ained the same helicity density as the two shorter structures, but its greater length implies a higher coronal twist. The measured length an d a value combine to imply a twist that exceeds the threshold for the MHD kink instability in a force-free cylindrical flux tube. We conclud e that theoretical studies of such simple models, which have found tha t the MHD kink instability does not lead to global dissipation, do not adequately address the physical processes that govern coronal magneti c fields.