The Abrupt Rotational Motions Of The Sunspots During The Solar Flares

The abrupt motion of the photospheric flux during a solar flare is thought to be a back reaction caused by the coronal field reconfiguration.However,the type of motion pattern and the physical mechanism responsible for the back reaction has been uncertain.In this theise,we investigate the various abrupt photospheric motions produced by the flare and the associated impulsive changes in the rate of transportation of the magnetic helicity across the photosphere.Using observations from the Helioseismic and Magnetic Imager on board the Solar Dynamic Observatory(SDO),we show that the direction of a sunspot's rotation is reversed during an X1.6 flare using observations from the Helioseismic and Magnetic Imager.A magnetic field extrapolation model shows that the corresponding coronal magnetic field shrinks with increasing magnetic twist density.This suggests that the abrupt reversal of rotation in the sunspot may be driven by a Lorentz torque that is produced by the gradient of twist density from the solar corona to the solar interior.These results support the view that the abrupt reversal in the rotation of the sunspot is a dynamic process responding to shrinkage of the coronal magnetic field during the flare.Using observations from SDO/HMI,we found one counterclockwise and two clockwise vortex flows on the photosphere in the NOAA active region 12371 during the flare SOL2015-06-22T18:23(M6.5).The counterclockwise vortex was located on the footpoint of the erupting hot channels observed by the Atmospheric Imaging Assembly(AIA)Telescope on board SDO.The two clockwise vortices resided on either side of the polarity inversion line.At these vortices,the impulsive and irreversible change in the photospheric vector magnetic field were detected.The resulting change in the photospheric Lorentz force provides a torque in each vortex,which has the same direction with each vortex.A magnetic field extrapolation model shows that the coronal field starting from the two clockwise vortices suffered significant shrinkage during the changeover period of the photospheric field.Moreover,some of the modeled field rooted in the counterclockwise vortex displays a pronounced expansion during the flare.These results suggest that the clockwise vortices could result from the contraction of the magnetic field lines during the flare,while the counterclockwise vortex may be attributed to the expansion of the eruptive flux rope as observed in the AIA images.Using the 135-second cadence of the photospheric vector data provided by SDO/HMI,we examined the time-evolution of the transportation of magnetic helicity across the photosphere during 16 flares with the energy class lower than M5.0.During the flare in 4 out of 16 events,we found the impulsive helicity flux with the sign opposite with that before the flare.This indicates that even the flare with less energy could result in anomalistic transportation of the magnetic helicity across the photosphere.Accompanying with the impulsive helicity flux,the poynting flux across the photosphere evolved from positive to negative.This indicates that the magnetic helicity and energy was injected into the solar interior during the flare.In each of the 4 events,the impulsive change in the helicity flux was always mainly contributed by abrupt change in horizontal velocity field on a sunspot located near the flaring neutral line.All of these sunspots were swept by the flare ribbon and connected by the post-flare loops,indicating that motions of the sunspots could be a result of the magnetic reconnection during the flare.The velocity field on each sunspot shows either an obvious vortex patten or an shearing patten relative to the another magnetic polarity.Both the rotational and shear motions tended to relax the magnetic twist and shear in the corona.During these flares,abrupt change in the in Lorentz force acting on these sunspots were found.The resultant force and torque applied on the sunspot produced by the change in the Lorentz force always had the same direction with the rotational motion and shearing motion of these sunspots.These results support the view that the impulsive helicity transportation during the flare could be resulted from the photospheric motions of magnetic flux driven by the change in the Lorentz force applied on the photosphere.