Converging motion of Hα conjugate kernels:: The signature of fast relaxation of a sheared magnetic field

In this Letter, we present the results from a high-cadence (similar to 40 ms) Ha blue-wing observation of an M1.1-class solar flare, which occurred in NOAA AR 10687 on 2004 November 1. In collaboration with RHESSI, the observation was made with the Ha Fine Structure Telescope at the GanYu Solar Station of the Purple Mountain Observatory. For this flare, a pair of conjugate Ha kernels shows a kind of converging motion during the impulsive phase. After the impulsive phase, there appears a normal separation motion. The motion of one Ha kernel is perpendicular to the magnetic neutral line, while another kernel's converging shows both perpendicular and parallel components. Nevertheless, the shear angle decreases during the converging motion, clearly showing the relaxation of a sheared magnetic field. All of the above features are confirmed with hard X-ray (HXR) footpoints observed by RHESSI. We also obtained the time profiles of the rate of change of the shear angle and the relative velocity of the two kernels with Ha observations. Both of these time profiles show a good correlation with RHESSI HXR light curves in the higher energy range (greater than or similar to 50 keV). This indicates that, during the peak times of the flare, the relaxation process may have occurred rapidly. This event was also observed by the Nobeyama Radio Heliograph (NoRH), showing a single microwave source. Using NoRH maps at 17 GHz with 1 s cadence, we obtained the time profile of the radio source's velocity using the same method that we used with Ha images. The velocity-time curve of the microwave source shows a good correlation with that obtained from the two Ha kernels.