EIT crinkles as evidence for the breakout model of solar eruptions

We present observations of two homologous flares in NOAA Active Region 8210 occurring on 1998 May 1 and 2, using EUV data from the EUV Imaging Telesco pe (EIT) on board the Solar and Heliospheric Observatory, high-resolution a nd high-time cadence images from the soft X-ray telescope on Yohkoh, images or fluxes from the hard X-ray telescope on Yohkoh and the BATSE experiment on board the Compton Gamma Ray Observatory, and Ca XIX soft X-ray spectra from the Bragg crystal spectrometer (BCS) on Yohkoh. Magnetograms indicate that the flares occurred in a complex magnetic topology, consisting of an e merging flux region (EFR) sandwiched between a sunspot to the west and a co ronal hole to the east. In an earlier study we found that in EIT images, bo th flaring episodes showed the formation of a crinkle-like pattern of emiss ion ("EIT crinkles") occurring in the coronal hole vicinity, well away from a central "core field" area near the EFR-sunspot boundary. With our expand ed data set, here we find that most of the energetic activity occurs in the core region in both events, with some portions of the core brightening sho rtly after the onset of the EIT crinkles, and other regions of the core bri ghtening several minutes later, coincident with a burst of hard X-rays; the re are no obvious core brightenings prior to the onset of the EIT crinkles. These timings are consistent with the "breakout model" of solar eruptions, whereby the emerging flux is initially constrained by a system of overlyin g magnetic field lines, and is able to erupt only after an opening develops in the overlying fields as a consequence of magnetic reconnection at a mag netic null point. In our case, the EIT crinkles would be a signature of thi s pre-impulsive phase magnetic reconnection, and brightening of the core on ly occurs after the core fields begin to escape through the newly created o pening in the overlying fields. Morphology in soft X-ray images and propert ies in hard X-rays differ between the two events, with complexities that pr eclude a simple determination of the dynamics in the core at the times of e ruption. From the BCS spectra, we find that the core region expends energy at a rate of similar to 10(26) ergs s(-1) during the time of the growth of the EIT crinkles; this rate is an upper limit to energy expended in the rec onnections opening the overlying fields. Energy losses occur at an order of magnitude higher rate near the time of the peak of the events. There is li ttle evidence of asymmetry in the spectra, consistent with the majority of the mass flows occurring normal to the line of sight. Both events have simi lar electron temperature dependencies on time.