Fast Time Structure during Transient Microwave Brightenings: Evidence for Nonthermal Processes

Transient microwave brightenings (TMBs) are small-scale energy releases fromthe periphery of sunspot umbrae, with a flux density two orders of magnitudesmaller than that from a typical flare. Gopalswamy et al (1994) first reportedthe detection of the TMBs and it was pointed out that the radio emissionimplied a region of very high magnetic field so that the emission mechanism hasto be gyroresonance or nonthermal gyrosynchrotron, but not free-free emission.It was not possible to decide between gyroresonance and gyrosynchrotronprocesses because of the low time resolution (30 s) used in the data analysis.We have since performed a detailed analysis of the Very Large Array data withfull time resolution (3.3 s) at two wavelengths (2 and 3.6 cm) and we can nowadequately address the question of the emission mechanism of the TMBs. We findthat nonthermal processes indeed take place during the TMBs. We presentevidence for nonthermal emission in the form of temporal and spatial structureof the TMBs. The fast time structure cannot be explained by a thermodynamiccooling time and therefore requires a nonthermal process. Using the physicalparameters obtained from X-ray and radio observations, we determine themagnetic field parameters of the loop and estimate the energy released duringthe TMBs. The impulsive components of TMBs imply an energy release rate of 1.3x 10^22 erg/s so that the thermal energy content of the TMBs could be less than10^24 erg.Comment: 15 pages (Latex), 4 figures (eps). ApJ Letters in press (1997