VLA Observations of Solar Active Regions. II. Solar Bursts

Details VLA Observations of Solar Active Regions. II. Solar Bursts VLA Observations of Solar Active Regions. II. Solar Bursts

Jul 1981

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981apj...247..338l&link_type=abstract

Astrophysical Journal, Vol.247, P. 338, 1981

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18

Scientific paper

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.

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