Radio Spectral Imaging of Another Likely Solar Flare Termination Shock Event

Solar flare termination shocks are believed to be produced by super-magnetosonic reconnection outflows impinging upon dense, reconnected flare loops. They have been suggested as one of the viable mechanisms for accelerating electrons and ions to high energies. Although such shocks have been indicated to exist in numerical experiments, observational evidence remains rare. Using radio dynamic spectroscopic imaging of a long-duration C1.9 flare obtained by the Karl G. Jansky Very Large Array (VLA), Chen et al. 2015 (Science, 350, 1238) found that a type of decimetric coherent bursts, referred to as “stochastic spike bursts”, were likely the radio signature of nonthermal electrons interacting with myriad density fluctuations at the front of a flare termination shock. Here we report another spike burst event recorded during the gradual phase of a long-duration M8.4-class eruptive flare on 2012 March 10 by VLA in 1—2 GHz. VLA radio spectroscopic imaging of the spikes shows that, similar to the case of Chen et al. 2015, the burst centroids form an extended structure in the corona located well above the flare arcade. We combine RHESSI X-ray imaging, extreme ultraviolet imaging observations of the flare from two vantage points, one from the Solar Dynamics Observatory (SDO) against the disk and another from the Solar Terrestrial Relations Observatory Ahead (STEREO-A) from the limb, as well as radio imaging to elucidate the location of spike bursts in 3 dimensions. The results show that this structure is located in the above-the-looptop region where a termination shock presumably forms. In the close vicinity of this structure, we also find the presence of a diffuse supra-arcade fan structure where multitudes of plasma downflows are observed from both SDO and STEREO-A. Our observations provide another piece of evidence that supports the interpretation of such stochastic spike bursts as radio emission from solar flare termination shocks.