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 (∼40 ms) Hα 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 Hα Fine Structure Telescope at the GanYu Solar Station of the Purple Mountain Observatory. For this flare, a pair of conjugate Hα 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 Hα 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 Hα observations. Both of these time profiles show a good correlation with RHESSI HXR light curves in the higher energy range (≳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 Hα images. The velocity-time curve of the microwave source shows a good correlation with that obtained from the two Hα kernels.