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Observational Evidence of Particle Acceleration Associated with Plasmoid Motions

We report a strong association between the particle acceleration and plasma motions found in the 2010 August 18 solar flare. The plasma motions are tracked in the extreme-ultraviolet (EUV) images taken by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory and the Extreme UltraViolet Imager (EUVI) on the Solar Terrestrial Relation Observatory spacecraft Ahead, and the signature of particle acceleration was investigated by using Nobeyama Radioheliograph data. In our previous paper, we reported that in EUV images many plasma blobs appeared in the current sheet above the flare arcade. They were ejected bidirectionally along the current sheet, and the blobs that were ejected sunward collided with the flare arcade. Some of them collided or merged with each other before they were ejected from the current sheet. We discovered impulsive radio bursts associated with such plasma motions (ejection, coalescence, and collision with the post flare loops). The radio bursts are considered to be the gyrosynchrotron radiation by nonthermal high energy electrons. In addition, the stereoscopic observation by AIA and EUVI suggests that plasma blobs had a three-dimensionally elongated structure. We consider that the plasma blobs were three-dimensional plasmoids (i.e. flux ropes) moving in a current sheet. We believe that our observation provides clear evidence of particle acceleration associated with the plasmoid motions. We discuss possible acceleration mechanisms on the basis of our results.