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Abstract
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. Here 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.
Highlights
The abrupt motion of the photospheric flux during a solar flare is thought to be a back reaction caused by the coronal field reconfiguration
By means of a set of 45 s cadence full-disk continuum intensity images and 12 min cadence vector magnetograms taken by the Helioseismic and Magnetic Imager (HMI)[25] telescope on board the Solar Dynamic Observatory (SDO), we characterize the sudden reversal in the rotation of a part of a sunspot during an X1.6 flare
The multiwavelength extreme ultraviolet observations from the Atmospheric Imaging Assembly Telescope (AIA)[26] aboard the SDO show the flare ribbon in the 1,600 Å passband (Fig. 1a) and the post-flare loop in the 94 Å passband (Fig. 1b,c)
Summary
The abrupt motion of the photospheric flux during a solar flare is thought to be a back reaction caused by the coronal field reconfiguration. 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. 13) have reported sudden, shear-relaxing motions in the course of the flares These motions may be driven by a horizontal Lorentz force that can be deduced from the abrupt changes of the photospheric magnetic field[14]. We note that such shear-relaxing motions may have a role in the impulsive variations of the helicity transport. We suggest that the reversal in the rotation of the sunspot indicates that the increasing twist is transported downward across the photosphere
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