OSCILLATIONS OF HARD X-RAY FLARE EMISSION OBSERVED BY RHESSI: EFFECTS OF SUPER-ALFVENIC BEAMS?

Recent Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) observations of hard X-ray (>25 keV) emission from several flares show damped oscillations with periods of 2-4 minutes and damping times of several tens of minutes. We analyze the observations of the 2005 January 19 flare that show the quasi-periodic, damped oscillations, and we propose a physical mechanism for this phenomenon. Namely, we suggest that super-Alfvenic beams in the vicinity of the reconnection region lead to the excitation of the oscillations. The oscillations damp as the flows and the reconnecting magnetic fields dissipate during the decay phase of the flare. We show the results of 2.5-dimensional viscoresistive MHD simulations of this mechanism, and we find qualitative agreement with the RHESSI observations. In addition, we find that the flows destabilize the current sheet, which leads to an increased reconnection rate compared to the case without the beams.