  He I D3 line has a unique response to the flare impact on the low solar
atmosphere and can be a powerful diagnostic tool for energy transport
processes. Using images obtained from the recently digitized films of Big Bear
Solar Observatory, we report D3 observation of the M6.3 flare on 1984 May 22,
which occurred in an active region with a circular magnetic polarity inversion
line (PIL). The impulsive phase of the flare starts with a main elongated
source that darkens in D3, inside of which bright emission kernels appear at
the time of the initial small peak in hard X-rays (HXRs). These flare cores
subsequently evolve into a sharp emission strand lying within the dark halo
simultaneously with the main peak in HXRs, reversing the overall source
contrast from -5% to 5%. The radiated energy in D3 during the main peak is
estimated to be about 10^30 ergs, which is comparable to that carried by
nonthermal electrons above 20 keV. Afterwards the flare proceeds along the
circular PIL in the counterclockwise direction to form a dark circular ribbon
in D3, which apparently mirrors the bright ribbons in Halpha and He I 10830 A.
All these ribbons last for over one hour in the late gradual phase. We suggest
that the present event resembles the so-called black-light flare that is
proposed based on continuum images, and that D3 darkening and brightening
features herein may be due to, respectively, the thermal conduction heating and
the direct precipitation of high-energy electrons.
