Dual frequency observations of flares with the VLA

Abstract

We describe observations of three flares made at 5 and 15 GHz with the VLA, two subflares near the limb on 1981 November 21 and 22, and an M7.7 flare on 1981 May 8. Even though the time histories of the November flares indicated simple impulsive bursts, the VLA observed no 5 GHz radiation at all from one flare, and from the other, the 15 GHz radiation emanated from a source which was smaller, lower and displaced from the 5 GHz source. Without the spatial information, we would have derived incorrect results from the assumption that photons of different energy (both at X-ray and radio wavelengths) arose from one homogeneous volume.

The 1981 May 8 flare was intense and complex, having two. or more sources at both 5 and 15 GHz. Prior to the peak of the flare, the sources grew in size to > 20″ to 40″, after which they were not visible to the VLA; only (weak) subsources could be seen. These were located between or at the edge of the Hα ribbons and the two hard X-ray sources imaged by the Hinotori. Highly polarized, bursty radiation observed at Toyokawa at 1 and 2 GHz, indicated that an electron-cyclotron maser operated during the flare. We derive 360 to 660 gauss as the maximum field strength in flaring loops.