Double peak quasi-periodic pulsations in a circular-ribbon flare

Abstract

We study quasi-periodic pulsations (QPPs) during the impulsive phase of the C8.3 flare SOL2002-08-06T01:43. The shape of an extended 5.7 GHz source is similar to a tadpole with the head located above the region of a negative magnetic polarity, surrounded by positive polarity patches and with a remote tail source. The flare configuration includes bright extreme ultraviolet (EUV) ropes with footpoints near the boundary of the negative magnetic field region and it can be identified as a circular ribbon flare. We use simultaneous observations carried out by the Siberian Solar Radio Telescope at 5.7 GHz, the Nobeyama Radio Heliograph (NoRH) at 17 and 34 GHz, the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)/HXR, and the Transition Region and Coronal Explorer (TRACE) imaging in the extreme ultraviolet. The flare HXR emission is produced by a compact source located at the south periphery of the Negative Magnetic field Region (NMR). The QPPs are observed during a one-minute interval after the start of the impulsive phase, when this HXR source appeared. The remote source is detected on the variation maps of the of the brightness temperature at 17 GHz and is located at the end of tadpole tail about 60 arcsec eastward. More than a dozen cotemporal HXR and microwave pulses with timescales from 1.5 s up to about 8 s were observed in the flare kernel. At 5.7 GHz, the pulses are more prominent near the remote source where they are highly polarized and generated by the electron beams propagating from the flare kernel. The main tone of the QPP periodicity corresponds to the oscillations with a period of 8 s and is accompanied by the variations in the hardness of nonthermal electrons, that is, in the efficiency of the acceleration mechanism. The second intensity harmonic (about a 3-s period) appears due to a double peak structure of the QPP event. Such pulse shapes suggest oscillations of the current sheet during the loop coalescence as a modulation mechanism of the flare energy release.