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In this Letter, we give a detailed analysis to the M3.3 class flare thatoccurred on August 17, 2013 (SOL2013-08-17T18:16). It presents a clear pictureof mutual magnetic interaction initially from the photosphere to the corona viathe abrupt rapid shearing motion of a small sunspot before the flare, and thensuddenly from the corona back to the photosphere via the sudden retractionmotion of the same sunspot during the flare impulsive phase. About 10 hoursbefore the flare, a small sunspot in the active region NOAA 11818 started tomove northeast along a magnetic polarity inversion line (PIL), creating ashearing motion that changed the quasi-static state of the active region. Afilament right above the PIL was activated following the movement of thesunspot and then got partially erupted. The eruption eventually led to the M3.3flare. The sunspot was then suddenly pulled back to the opposite direction uponthe flare onset. During the backward motion, the Lorentz force underwent asimultaneous impulsive change both in magnitude and direction. Its directionalchange is found to be conformable with the retraction motion. The observationprovides direct evidence for the role of the shearing motion of the sunspot inpowering and triggering the flare. It especially confirms that the abruptmotion of a sunspot during a solar flare is the result of a back reactioncaused by the reconfiguration of the coronal magnetic field.

Sunspot shearing and sudden retraction motion associated with the 2013 August 17 M3.3 Flare