fl221.pevtsov01 Posted: 30-Sep-95 Updated: 26-Apr-96 Events specified: Flare of 8-May-92 15:12 UT
Alexei Pevtsov, Richard Canfield, Harold Zirin
A filament eruption and two-ribbon solar flare of H-alpha importance 4B (M7.4 in X-rays) was observed by SXT and Big Bear Solar Observatory on May 8, 15:12 - 16:10 UT. A feature of particular interest in this region was an ``inverse S'' coronal X-ray loop, which formed before the flare and disappeared after.
Using photospheric vector magnetograms from Mees Solar Observatory over the period 5-12 May 1992, we find that the average magnetic helicity of AR 7154 did not change significantly over this entire period. This shows that buildup of helicity due to photospheric shear motions is not consistent with the data. We speculate that this highly twisted coronal loop increased its length due to the reconnection of two existing flux tubes, and that magnetic helicity conservation resulted in enough total twist over the new longer field line that it was no longer stable against the kink instability. We plan to make an analysis of the force-free equilibrium and use the observed helicity parameters and coronal dimensions to support our speculation.
The following proposals treated the same flare, although these have already disappeared in current proposals of TBB;
fl088.shimizu04 fl096.kozuka01 fl105.kozuka02
Basically, we have finished this study. In next few days we plan to submit our paper ("Reconnection and Helicity in a Solar Flare") to the Astrophisical Journal.
Abstract:
Using X-ray images, H-alpha images, and vector magnetograms, we have studied the evolution of the coronal structure and magnetic field of NOAA AR 7154 during 5 - 12 May, 1992. A two-ribbon 4B/M7.4 flare associated with an H-alpha filament eruption was observed on 8 May, 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 post-flare loops. Neither the X-ray nor H-alpha 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 field parameter alpha. 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 alpha 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.