FAST TIME STRUCTURE DURING TRANSIENT MICROWAVE BRIGHTENINGS - EVIDENCE FOR NONTHERMAL PROCESSES

Transient microwave brightenings (TMBs) are small-scale energy release s from the periphery of sunspot umbrae with a flux density 2 orders of magnitude smaller than that from a typical flare, Gopalswamy et al, f irst reported the detection of the TMBs, and it was pointed out that t he radio emission implied a region of very high magnetic field so that the emission mechanism has to be gyroresonance or nonthermal gyrosync hrotron, but not free-free emission. It was not possible to decide bet ween gyroresonance and gyrosynchrotron processes because of the low ti me resolution (30 s) used in the data analysis. We have since performe d a detailed analysis of the Very Large Array data with full time reso lution (3.3 s) at two wavelengths (2 and 3.6 cm), and we can now adequ ately address the question of the emission mechanism of the TMBs. We f ind that nonthermal processes indeed take place during the TMBs. We pr esent evidence for nonthermal emission in the form of temporal and spa tial structure of the TMBs. The fast time structure cannot be explaine d by a thermodynamic cooling time and therefore requires a nonthermal process. Using the physical parameters obtained from X-ray and radio o bservations, we determine the magnetic field parameters of the loop an d estimate the energy released during the Th?Bs, The impulsive compone nts of TMBs imply an energy release rate of similar to 1.3 x 10(22) er gs s(-1), so the thermal energy content of the TMBs could be less than similar to 10(24) ergs.