We present observations of the 2002 September 30 white-light flare, in which the optical continuum emission near the Hα line is enhanced by ∼10%. The continuum emission exhibits a close temporal and spatial coincidence with the hard X-ray (HXR) footpoint observed by RHESSI. We find a systematic motion of the flare footpoint seen in the continuum emission; the motion history follows roughly that of the HXR source. This gives strong evidence the this white-light flare is powered by heating of nonthermal electrons. We note that the HXR spectrum in 10-50 keV is quite soft with γ ≈ 7 and there is no HXR emission above 50 keV. The magnetic configuration of the flaring region implies magnetic reconnection taking place at a relatively low altitude during the flare. Despite a very soft spectrum of the electron beam, its energy content is still sufficient to produce the heating in the lower atmosphere where the continuum emission originates. This white-light flare highlights the importance of radiative backwarming to transport the energy below when direct heating by beam electrons is obviously impossible.

We perform a multiwavelength study of a two-ribbon flare on 2002 September 29 and its associated filament eruption, observed simultaneously in the H line by a ground-based imaging spectrograph and in hard X-rays by RHESSI. The flare ribbons contain several H-bright kernels that show different evolutionary behaviors. In particular, we find two kernels that may be the footpoints of a loop. A single hard X-ray source appears to cover these two kernels and to move across the magnetic neutral line. We explain this as a result of the merging of two footpoint sources that show gradually asymmetric emission owing to an asymmetric magnetic topology of the newly reconnected loops. In one of the H kernels, we detect a continuum enhancement at the visible wavelength. By checking its spatial and temporal relationship with the hard X-ray emission, we ascribe it to electron-beam precipitation. In addition, we derive the line-of-sight velocity of the filament plasma based on the Doppler shift of the filament-caused absorption in the H blue wing. The filament shows rapid acceleration during the impulsive phase. These observational features are in principal consistent with the general scenario of the canonical two-ribbon flare model.

We investigate the altitude dependence of hard X-ray (HXR) spectra in solar flares, i.e., whether the HXR spectra are related to the altitudes of reconnection sites. We assume that the reconnection altitude can be scaled by the distance between the two conjugate HXR footpoints in the flare. By searching the RHESSI flare list from 2002 to 2004, we find 42 solar flares below X-class that have enhanced 50-100 keV HXR emission and two well-resolved HXR footpoints at the nonthermal peak time. The preliminary results show that there is a weak correlation (∼ -0.31) between the HXR spectral index and the HXR footpoint distance. We further discuss the possible implications.