FEATURES OF THE DEVELOPMENT OF A CIRCULAR SOLAR FLARE

Abstract

We present the results of the analysis ofmorphology and evolution of the circular solar flare usingH-alpha images. H-alpha filtergrams were obtained withthe Meudon spectroheliograph. The active region NOAA9087 had a complex multipolar magnetic field configura-tion. New magnetic fluxes emerged during the evolution ofthis flare-productive active region. The high flare and surgeactivity was observed in the active region.According to Solar Geophysical Data (SGD) the3N/M6.4 class solar flare occurred on July 19, 2000 at06:37 UT, peaked at 07:23 UT and lasted 2.5 hours. Twobright kernels appeared near large positive-polarity sunspotat the beginning of the flare. In a few minutes bright kernelsoccurred in the center of the active region near polarity in-version line. Space solar observatory Yohkoh detected ahard X-ray (HXR) coronal source in the 13.9-22.7 keV and22.7-32.7 keV energy bands in this location.New kernels appeared in the southern and eastern partsof the active region at the boundaries of the chromosphericnetwork. They brightened sequentially clockwise, whichmay indicate a slipping reconnection. Magnetic reconnec-tion was observed in the main phase of the flare in the east-ern part of the active region. In the late flare phase arcadeof post-reconnection EUV loops connected the main flareribbon with the place of repeated reconnection. Additionalheating may be required for the explanation of the longflare decay phase.Flare ribbons of the circular shape were formed. Thecomplex magnetic configuration of the studied active re-gion and circular shape of the ribbons suggest that it had afan-spine magnetic topology with null points. Possibly,flare ribbons are the locations of intersections of the fanquasi-separatrix layer with the chromosphere. They ap-peared as a result of heating or particle beam moving alonga quasi-separatrix layer from a source in the corona.