Traces of the dynamic current sheet during a solar flare

High-cadence and high-resolution time sequences of far Hα off-band images provide a unique tool to study the evolution of the fine structure of flare kernels. The fine structure contains important information on flare topology and the triggering mechanism. In this Letter, we concentrate on the rapid changes of the relative positions of two conjugate flare footpoints. In order to carry out this study with the highest physical precision, we use r _c = Σr_jI_j/ΣI_j (I _j is the Hα brightness at r_j) to compute the centroid of an Hα bright kernel region caused by solar flares. Using this, we probe the fine temporal structures connected to the distance between the centroids of two conjugate kernels of an M2.3 flare. The flare, which occurred on 2002 September 9 in NOAA Active Region 0105, was observed at Big Bear Solar Observatory at the far off-band center wavelength of H α. - 1.3 Å, with a cadence of ∼40 ms. The flare was also observed by RHESSI. The time profile of the separation distance shows an excellent anticorrelation to that of the hard X-ray (HXR) emissions in 25-50 keV, which exhibit a number of separate spikes (the linear Pearson correlation coefficient is found to be ∼-0.83). The separation between the two centroids decreases at the rising periods of four HXR spikes, then it increases after the peak time of the flare to show the expected separation motion. The most obvious decreasing, which occurred during the first HXR peak, was confirmed by corresponding images. This implies that during the impulsive phases, the energy transported from the corona is deposited increasingly inwardly between the two kernels. This new and perhaps surprising tendency for the energy deposition can be explained as being caused by current sheet pinch motions, which, at the same time, enhance the magnetic energy reconnection rate to produce the observed HXR spikes.