Time-dependent flare signature in hydrogen balmer lines

In recent studies of Hα flare imaging spectroscopy performed by Leka et al. and de la Beaujardière et al. in 1993, Hα line profiles have been investigated as a function of space to locate the physical process underlying observed spectral profiles. In this Letter, we discuss time evolution of hydrogen Balmer line profiles at a fixed spatial point recorded by the multichannel spectrograph at Nanjing University in China during the 1991 October 27 white-light flare which occurred in NOAA 6891. It is found that the Balmer emission lines with a central reversal show up at the beginning of the Hα flare but change to single-peaked profiles in about 30 s. This result may, according to Canfield et al.'s model in 1993, indicate that the Balmer line enhancement even at one flare site could be caused by two physical processes, namely, initially by the high-energy particle precipitation and subsequently by the high coronal pressure. We also present evidence for downward motion and the Stark broadening effect on the observed Balmer line wings to examine further the physical consequences of the particle precipitation. Time correlation of the Balmer line emission with hard X-ray and continuum brightening is briefly discussed based on the spectral information.