Periods and Frequency Drifts of Groups of the Decimetric Spikes in Two Solar Flares

Abstract We studied the radio emission occurring as narrowband decimetric spikes observed during the 10 May 2022 and 26 August 2022 flares. In the radio spectra, these spikes were distributed in groups that occurred quasi-periodically with the periods 5.1 s in the 10 May 2022 flare and 9.1 s in the 26 August 2022 flare. In some parts of these groups, even subgroups of spikes distributed with the quasi-periods of 0.19 s (10 May 2022 flare), and 0.17 s and 0.21 s (26 August 2022 flare) were found. Some of these subgroups even drifted to higher or lower frequencies, which was observed for the first time. At the time of the dm-spikes observation, a pair of reconnecting loops are identified in the SDO/AIA EUV observations of the 10 May 2022 flare, one of which is interpreted as belonging to a small erupting filament. We propose that these loops reconnect in the dynamic quasi-periodic regime (the period 0.19 s) and this reconnection is modulated by an oscillation of one of the interacting loops (the period 5.1 s). Accelerated electrons from this process are trapped in reconnecting plasma outflows, and thus the drifting groups of spikes are generated. The 26 August 2022 flare is a complex event with several systems of bright loops; nevertheless, it also shows a disintegrating erupting filament similar to the 10 May 2022 flare, meaning that the dm-spikes are likely generated by similar reconnection processes.