Abstract
A study of quasi-periodic fluctuations of solar microwave emission at the preflare stage is presented. Data from daily solar observations by the Nobeyama Radioheliograph at a frequency of 17 GHz are used. Five flares are analyzed based on synthesized two-dimensional radio images. All of the considered events are characterized by preflare fluctuations of microwave emission with periods of one to several tens of minutes. The length of preflare trains is five pulses on average. Also, 129 flares are analyzed from correlation plots. Seventy-five percent of the events have preflare fluctuations of microwave emission with lengths of 2–3 to 60–70 min. Various fluctuation periods have been observed: from several seconds to several minutes. The effect can be caused by an increase in the amplitude of magnetohydrodynamic (MHD) waves propagating in the active region before the onset of a solar flare.
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