VLA Observations of Solar Active Regions - Part Two - Solar Bursts

Abstract

view Abstract Citations (21) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS VLA Observations of Solar Active Regions. II. Solar Bursts Lang, K. R. ; Willson, R. F. ; Felli, M. Abstract The VLA has been used at wavelengths of 6 cm and 20 cm to detect seven solar bursts from two active regions with flux densities between 0. 1 and 6.0 sfu. The regions of burst energy release were resolved with angular sizes between 6" and 12" and brightness temperatures between 1 and 2 × 107 K. The angular sizes of the radio bursts are comparable to those of the soft X-ray kernels, and the brightness temperatures are comparable to the temperatures of the plasma which gives rise to soft X-ray emission. This suggests that we are detecting the radio wavelength counterpart of the flares seen at soft X-ray wavelengths. The two solar bursts observed at 6 cm wavelength occurred within a plage-associated "coronal condensation" in regions away from sunspots and near magnetic neutral lines. The Hα emission associated with one burst was, within the uncertainties of measurement, spatially coincident with the 6 cm burst. The five solar bursts detected at 20 cm wavelength were composed of individual events which had a duration of between 20 s and 60 s, but our time resolution of 10 s precluded the detection of components with shorter duration. Although thermal energies normally detected at soft X-ray wavelengths are inferred from their brightness temperatures, the train of events which make up the 20 cm bursts are similar to the elementary flare bursts detected at hard X-ray wavelengths in other active regions. The observed bursts at 20 cm wavelength can be explained by either the bremsstrahlung or the gyroresonant radiation of a hot, dense plasma with an electron temperature Te ≲ 2 × 107 K and an electron density of Ne ≍ 1010 cm-3. The gyroradiation easily explains the high observed circular polarization while indicating magnetic field strengths of H 90 G in the emitting region; while the bremsstrahlung requires polarization due to propagation effects at frequencies near the gyrofrequency in magnetic fields of strength H ≍ 500 G. We also discuss the implications of VLA observations for studies of the physical nature of the burst emitting sources. Publication: The Astrophysical Journal Pub Date: July 1981 DOI: 10.1086/159042 Bibcode: 1981ApJ...247..338L full text sources ADS |