High-cadence observations of an impulsive flare

We analyzed high-cadence observations of a C5.7 flare on 1999 August 23 at Big Bear Solar Observatory (BBSO). The observing wavelength was 1.3 Å in the blue wing of Hα, with a cadence of 0.033 s. In addition, the hard X-ray's time profile obtained by the Burst And Transient Source Experiment (BATSE) and BBSO high-resolution magnetograms was compared with our Hα observations to understand in detail the particle precipitation in this event. The important results are as follows: 1. In Hα-1.3 Å, three flare kernels were observed in the early phase of the flare. The flare started in a nonmagnetic area at the magnetic neutral line. This suggests to us that the top of a low-lying loop is the initial energy release site, while the other two kernels are the footpoints of another overlying flare loop, formed after the magnetic reconnection. 2. We analyzed the temporal behavior of the three flare kernels in the impulsive phase when hard X-ray (HXR) emission was significant. We found that during a 7 s period, the Hα-1.3 Å brightenings at one of the footpoints showed a very good temporal correlation with the HXR flux variation. Therefore, from the spatially resolved Hα off-band observations, we identified this flare kernel as the source of HXR emission. 3. From the footpoint which exhibits the best correlation with HXR emission, the Hα-1.3 Å emission shows high-frequency fluctuations on a timescale of a few tenths of a second. The amplitude of the fluctuations is more than 3 times the noise. Such fluctuations are not evident in the other flare kernels which also do not show good correlation with HXR emission. For this reason, we suggest that the observed high-frequency fluctuations may be signatures of temporal fine structure related to the HXR elementary bursts.