Abstract
Decimetric radio events with large numbers of spikes during the impulsive phase of flares have been selected. In the observing range of 100 to 1000 MHz some flares have of the order of 10000 spikes or more. The average half-power bandwidth of spikes has been measured to be only 1.5% of the spike frequency. Since the emission frequency is determined by some source parameter (such as plasma frequency or gyrofrequency) the source dimension must be a small fraction of the scale length. From the flare configuration a typical upper limit of the dimension of 200 km is found. The observed fragmentation in the radio emission cannot be explained by a patchy emission mechanism of a single and much larger source without an additional (and unknown) assumption. It is proposed that the fragmentation already occurs in the exciter. Four events were analyzed in detail and compared to UV, SXR, and HXR data. The density of the loops where the SXR and HXR emission was observed has been measured before the flare. The plasma frequency well agrees with the observed frequency of spikes. The spikes thus originate close to or in the energy release region. It is suggested here that the fragmentation of the exciter is due to a fragmentation of the primary energy release. Each of these 104 ‘microflares’ would release an energy of the order of 1026 erg within 0.05 s.
Published Version
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