Quasi‐periodic Pulsations in a Solar Microwave Burst

Abstract

Quasi-periodic pulsations in solar flares can provide important information on physical conditions in flaring regions. In this paper, we study a microwave burst that showed deep quasi-periodic pulsations. The most dramatic feature of this event has been discussed by Asai and coworkers. In the second of four bursts during the. are, strongly modulated pulsations appear in radio images from the eastern end of a long loop and in hard X-rays from the western end of the loop. We show, in addition, that (1) at least five distinct radio sources with very different time profiles can be identified, including emission from the long loop connecting the modulated radio and X-ray sources; (2) substructure is also present in the radio emission from the eastern end of the long loop during the first burst of the. are, but with timescales shorter than in the second burst; (3) radio modulations are seen at the western end of the loop during the second burst but at a level some 20 times weaker than at the eastern end; (4) these radio modulations at the western end of the loop, like the hard X-ray modulations at the same location, appear to lead the modulations at the eastern end by about 0.5 s, but all have the same period. The period of the modulation can be explained by MHD oscillations of the loop approximate to120" long connecting the sources: both oscillations that change the magnetic field strength in the loop, such as propagating fast-mode waves, and torsional oscillations that change the direction of the magnetic field in the loop can explain the observed properties of the modulation of the radio emission. An impulsive reconnection episode is a plausible source of oscillating fast-mode waves and is consistent with some other aspects of the event. However, it is difficult to reconcile the strength of the radio modulations at the eastern end of the loop with their delay relative to the emissions at the western end, where the modulation is observed to be much weaker. If the electrons originate at the western end where the main energy release seems to occur, and the modulation is imposed on them there before they propagate 10(10) cm to the other end of the loop, any effects due to a spread in electron energies or pitch angles would lead to a spread in propagation times that should smooth out the modulation of the radio emission from the remote source, as should any trapping of electrons in the loop: yet the radio emission from the eastern end of the loop shows much stronger modulation than the radio and hard X-ray emission from the western end of the loop.