Microwave Study of a Circular Ribbon Flare

We present a microwave study of a circular ribbon flare. Data obtained are the 17/34 GHz maps from the Nobeyama Radioheliograph (NoRH) of SOL2014-12-17T04:51 along with (E)UV and magnetic data from the Solar Dynamics Observatory (SDO). Major results and interpretations are as follows: (1) the first activation of the circular ribbons comes in the form of a gradual increase of the 17 GHz flux without a counterpart at 34 GHz, about 13 min before the impulsive phase, which we interpret as representing thermal preheating. (2) The first sign of nonthermal activity occurs in the form of stepwise flux increases at both 17 and 34 GHz about 4 min before the impulsive phase, which is likely of nonthermal activation. (3) The main impulsive flare produces a sign flip of the 17 GHz polarization state, which implies a breakout type magnetic eruption. (4) The 17 GHz flux shows quasi-periodic pulsations with a period around 1.5 min before and during the flare. While the preflare pulsation is more obvious in total intensity at one end of the flare loop, the flare pulsation is more obvious in the polarized intensity at the region near the inner spine. We interpret the latter as evidence for torsional Alfven waves generated at the magnetic reconnection. (5) The 17/34 GHz fluxes show a secondary maximum at 25 min after the impulsive phase maximum. These fluxes are more enhanced in the flare looptop than in the footpoints, which has an implication on the nature of the EUV late phase. Our results show that microwave emission of a circular ribbon flare provides new information on the activities in the fan-spine structure.