Quasi-Periodic Radio Bursts Associated with Fast-mode Waves near a Magnetic Null Point

Abstract

This paper presents an observation of quasi-periodic rapidly-propagating waves observed in the AIA 171/193 \AA~ channels during the impulsive phase of an M1.9 flare occurred on 7 May 2012. The instant period was found to decrease from 240~s to 120~s, the speed of the wave fronts was in the range of ~664-1416 km/s. Almost simultaneously, quasi-periodic bursts with similar instant periods, ~70~s and ~140~s, occur in the microwave emission and in decimetric type IV, and type III radio bursts, and in the soft X-ray emission. The magnetic field configuration of the flare site was consistent with a breakout topology, i.e., a quadrupolar field along with a magnetic null point. The quasi-periodic rapidly-propagating wavefronts of the EUV emission are interpreted as a fast magnetoacoustic wave train. The observations suggest that the fast-mode waves are generated during the quasi-periodic magnetic reconnection in the cusp-region above the flare arcade loops. For the first time, we provide the evidence of a tadpole wavelet signature at about 70--140~s in decimetric (245/610~MHz) radio bursts, along with the direct observation of a coronal fast-mode wave train in EUV. In addition, at AIA 131/193 \AA\ we observed quasi-periodic EUV disturbances with the periods of 95~s and 240~s propagating downward at the apparent speed of 172-273 km/s. The nature of these downward propagating disturbances is not revealed, but they could be connected with magnetoacoustic waves or periodically shrinking loops.