Reconnection and helicity in a solar flare

Using X-ray images, Ha images, and vector magnetograms, we have studied the evolution of the coronal structure and magnetic field of NOAA Active Region 7154 during 1992 May 5-12. A two-ribbon 4B/M7.4 flare associated with an H alpha filament eruption was observed on May 8, 15:13-19:16 UT. An interesting feature of the region was a long, twisted X-ray structure, which formed shortly before the flare and disappeared after it, being replaced by a system of unsheared postflare loops. Neither the X-ray nor Ha morphology nor the photospheric magnetic field shows any indication of gradual buildup of nonpotential energy prior to the flare. Rather, the long structure appears to result from the reconnection of two shorter ones just tens of minutes before the filament eruption and flare. Using vector magnetograms and X-ray morphology, we determine the helicity density of the magnetic field using the force-free held parameter cc. The observations show that the long structure retained the same helicity density as the two shorter structures, but its greater length implies a higher coronal twist. The measured length and a value combine to imply a twist that exceeds the threshold for the MHD kink instability in a force-free cylindrical flux tube. We conclude that theoretical studies of such simple models, which have found that the MHD kink instability does not lead to global dissipation, do not adequately address the physical processes that govern coronal magnetic fields.