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He I D3 line has a unique response to the flare impact on the low solaratmosphere and can be a powerful diagnostic tool for energy transportprocesses. Using images obtained from the recently digitized films of Big BearSolar 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 inversionline (PIL). The impulsive phase of the flare starts with a main elongatedsource that darkens in D3, inside of which bright emission kernels appear atthe time of the initial small peak in hard X-rays (HXRs). These flare coressubsequently evolve into a sharp emission strand lying within the dark halosimultaneously with the main peak in HXRs, reversing the overall sourcecontrast from -5% to 5%. The radiated energy in D3 during the main peak isestimated to be about 10^30 ergs, which is comparable to that carried bynonthermal electrons above 20 keV. Afterwards the flare proceeds along thecircular PIL in the counterclockwise direction to form a dark circular ribbonin 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 suggestthat the present event resembles the so-called black-light flare that isproposed based on continuum images, and that D3 darkening and brighteningfeatures herein may be due to, respectively, the thermal conduction heating andthe direct precipitation of high-energy electrons.

He I D3 Observation of the 1984 May 22 M6.3 Solar Flare