Solar Burst Observations at Centimeter Wavelengths

Abstract

The centimeter -wave bursts are simple: they are characterized by a rapid rise in intensity and a slower decline. The burst-radiation is in general smooth, usually free of details in time and frequency and is a partially polarized broadband continuum. It starts almost immediately after an associated Hα flare and originates from a source of small angular size (< 1′ arc). The smooth continuum emission of centimeter-wave bursts has been accounted for by synchrotron radiation from fast electrons accelerated during flares. I shall not attempt to give here a full description of the observational characteristics of centimeter bursts, rather I shall concentrate on certain specific features relevant to the understanding of the generating mechanism of these bursts. More than 20 years ago, centimeter bursts were studied with a spatial resolution as good as ∼ 1′ arc. Even with this resolution the presence of small scale structures on scales of 1′–4′ were observed in cm-λ bursts sources. Since that time both spatial and time resolution have steadily improved; we now have a spatial resolution of ″ and time resolution of ∼ 10 ms. Such resolutions have revealed smaller scale structures of a few arc seconds and more complexity in burst sources. At the same time soft x-ray observations, particularly from Skylab, with resolution better than 5″ (e.g. Kahler et al. 1975; Vorpahl et al. 1975) have given new information about the flare phenomenon. Because of the close association of soft x-ray and cm-λ burst sources, these observations are particularly important for a proper understanding of the generating mechanism of cm-λ bursts, acceleration of electrons and the origin of solar flares.