Hα, Extreme‐Ultraviolet, and Microwave Observations of the 2000 March 22 Solar Flare and Spontaneous Magnetic Reconnection

The evolution of a GOES class X1.1 solar flare, which occurred in NOAA Active Region 8910 on 2000 March 22, is discussed using observations from the Owens Valley Solar Array (OVSA), Big Bear Solar observatory (BBSO), Transition Region and Coronal Explorer (TRACE), and the Michelson Doppler Imager (MDI) on board Solar and Heliospheric Observatory (SOHO). During the impulsive phase, a set of coronal loops are visible in the TRACE 171 A ˚ (� 1 � 10 6 K) wavelength band, which is confined to a small volume in the center of the large ��� -type active region. This is rapidly followed by the emergence of bright Hribbons that coincide with the EUV emission. Radio images show a single source encompassing the Hribbons at 5 GHz, but at higher frequencies a double source is seen within the area bounded by the compact Hand EUV emissions. We interpret the observation under the idea of the confined flare in contrast with the more com- monly cited, eruptive flare. We use a schematic magnetic reconnection geometry based on the MDI magneto- gram to suggest that the EUV loops show some parts of a separatrix, and that the radio and Hsources coincide with the whole part of the separatrix and its footpoints, respectively. First of all, it explains why this flare lacks the separating motion of Hribbons, a signature for eruptive flares. Second, the very short dura- tion of microwave bursts in spite of the large amount of soft X-ray flux is explicable under this scenario, since energy release via spontaneous reconnection in a confined magnetic structure can be very rapid. Third, the confined magnetic geometry is also considered favorable for preserving chromospheric evaporation and plasma turbulence as inferred from the OVSA microwave spectrum. In addition, a coronal mass ejection as detected in the LASCO coronagraph after this flare is briefly discussed in relation to the above flare model. Subject headings: MHD — radiation mechanisms: nonthermal — Sun: flares — Sun: magnetic fields — Sun: radio radiation — Sun: UV radiation