Microwave Study of a Solar Circular Ribbon Flare

A circular ribbon flare (CRF) SOL2014-12-17T04:51 is studied using the 17/34 GHz maps from the Nobeyama Radioheliograph along with (E)UV and magnetic data from the Solar Dynamics Observatory. We report the following three findings as important features of the microwave CRF. (1) The first preflare activation comes in the form of a gradual increase of the 17 GHz flux without a counterpart at 34 GHz, which indicates thermal preheating. The first sign of nonthermal activity occurs in the form of stepwise flux increases at both 17 and 34 GHz about 4 minutes before the impulsive phase. (2) Until the impulsive phase, the microwave emission over the entire active region is in a single polarization state matching the magnetic polarity of the surrounding fields. During and after the impulsive phase, the sign of the 17 GHz polarization state reverses in the core region, which implies a magnetic breakout-type eruption in a fan-spine magnetic structure. (3) The 17 GHz flux around the time of the eruption shows quasi-periodic variations with periods of 1-2 minutes. The pre-eruption oscillation is more obvious in total intensity at one end of the flare loop, and the post-eruption oscillation, more obvious in the polarized intensity at a region near the inner spine. We interpret this transition as transfer of oscillatory power from kink mode oscillation to torsional Alfvén waves propagating along the spine field after the eruption. We argue that these three processes are interrelated and indicate a breakout process in a fan-spine structure.