A superfine structure in solar microwave bursts

Abstract

We have observed in the microwave range (with the radio spectrometer of the Huairu station (Beijing, NAOC) around 3 GHz) the fine structure of solar radio bursts called zebra patterns and fiber bursts (seen drifting on the frequency stripes in emission and in absorption on the background burst continuum emission). In all seven observed bursts we discovered a new effect: zebra stripes have a superfine structure, consisting of numerous fast spikes with duration at a limit of the time resolution of the spectrometer, 8 ms. Since for zebra patterns and microwave spikes different emission mechanisms were proposed, these new observations require us to revise known theories. An alternative model of microwave millisecond spikes is based on the coupling of plasma waves with ion-sound waves : . Since the main features of zebra stripes and fiber bursts are similar, we consider the the zebra pattern of these bursts as whistler manifestations. Whistlers yield a principal contribution in the fine structure radio emission by coupling with Langmuir waves at sum as well as difference frequencies: . Allowance for the conversion of ion-sound waves into whistlers (and inversely in a pulsating regime) enables us to identify the zebra pattern consisting of spikes as a simultaneous manifestation of both those processes ( and ) in radio sources, related to magnetic reconnection above flare regions.