SSRT - Research results on fast active processes
Investigations of microwave subsecond bursts (spikes) have been carried out at the SSRT since 1992. Over 30 bursts with a subsecond time structure were recorded. The following results were obtained.
Microwave spikes are observed in ~ 15 % of bursts recorded. Typically, spike sources coincide with sources of the main burst. The emission flux of the spikes, covered by the observations, was typically about one or two tens of s.f.u. The maximum flux value was determined by the energetics of the event and reached several thousand s.f.u.
Spikes are categorized, according to their duration, into two classes, with the average values 50 and 150 ms. They were observed druing the burst growth and decay phases alike, and predominantly in groups rather than uniformly7.
The apparent size of the spike sources can exceed 1'. It was found that their size depends on the distance from the solar disk center, which is accounted for by the scattering of radio emission from a compact source in the lower corona. The values of the flux and size of the spike sources correspond to their brightness temperatures which exceed considerably the burst continuum temperature8.
Distribution of the recorded spikes in duration
Dependence of the observed size of the spike sources on the distance from the solar disk center. Solid curve - calculated T. Bastian - values of the apparent size of a point source as it is expanding due to the scattering of emission from turbulent irregularities in the corona
In the cases that were investigated, spikes were generated in regions with a moderate magnetic field and a high plasma density. The spikes observed at 5.7 GHz are most likely a separate class of flare phenomena than elementary events making up a microwave burst. Most probably, a plasma coherent mechanism is responsible for their generation. Plasma inhomogeneities at magnetic loop tops observed in soft X-ray rays are essential for the generation of microwave spikes9,10.