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) 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 ,t he 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 H a 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 (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 H a images. The velocity-time curve of the microwave source shows a good correlation with that obtained from the two Ha kernels. It is well accepted that solar flares are due to the sudden release of magnetic energy by magnetic reconnection (Priest & Forbes 2000). In the classical two-dimensional "CSHKP" reconnection model of a two-ribbon flare (Carmichael 1964; Sturrock 1966; Hirayama 1974; Kopp & Pneuman 1976), op- positely directed magnetic field lines are stretched by eruption to form a vertical current sheet where reconnection occurs. According to this model, the two footpoints (FPs) of a flare, residing in areas of opposite magnetic polarities, are expanding outward and away from each other as the flare proceeds (S ÿ vestka & Cliver 1992). Furthermore, it is well recognized that flare ribbon expansion is the chromospheric signature of progressive magnetic reconnection in the corona, in which new field lines reconnect at higher and higher altitudes. This picture, together with a simplified MHD reconnection model, yields a simplified method of measuring the electric field in the recon- nection region. The product of the horizontal velocity of the ribbon expansion and local longitudinal magnetic field is a measure of an effective electric field strength along the current sheet (Forbes & Lin 2000 and references therein). The value of the effective electric field is usually taken as a measure of the reconnection rate. Meanwhile, the behaviors of hard X-ray (HXR) and/or Ha FPs are often used to infer the reconnection scenario during flares. From the high-cadence and high spatial resolution obser- vation of a C9.0 flare, Qiu et al. (2002) inferred the macroscopic electric field from the velocity of flaring kernels' motion and the longitudinal magnetic field. The temporal variation of the electric field inferred from one type of kernel motion is found