Magnetic Development of Flaring Regions at Centimeter Wavelengths

Extract

It has been known for many years that the flare build-up manifests at centimeter wavelengths (2-6 cm), in the form of increased intensity and increased polarization of the active region. The flare-associated bursts originate in these intense sources, and the probability of occurrence of bursts increases with the increasing intensity of these narrow bright regions. With the availability of arc-second resolution using the VLA it has been possible to study the nature of this build-up from two-dimensional synthesized maps over short periods before the start of a flare. For a hard x-ray associated impulsive 6 cm burst observed on June 25, 1980 (Kundu, Schmahl, and Velusamy 1981), we produced several 15-minute synthesized maps in total intensity (I) and polarization (V) just before the flare onset (Kundu 1981). Figure 1 shows the central 11x16 regions of 15 minute synthesis maps over the period 14:45-15:45 UT. As can be seen from these Figures, the region is very complex, consisting of numerous components many of which are bipolar. These components have brightness temperatures of 6-9x10⁶ K during the hour before the flare. The burst source was located close to the neutral line of these oppositely polarized regions near B. The burst maximum is identified, with a “+” and the burst extent averaged over the period 1551-1600 UT is shown by the dotted contour in the last map.