A YOHKOH SEARCH FOR BLACK-LIGHT FLARES

Calculations which predict that a phenomenon analogous to stellar nega tive pre-flares could also exist on the Sun were published by Henoux e t al. (1990), and Aboudarham et al., (1990), who showed that at the be ginning of a solar white-light flare (WLF) event an electron beam can cause a transient darkening before the WLF emission starts, under cert ain conditions. They named this event a ''black light flare (BLF). Suc h a BLF event should appear an diffuse dark patches lasting for about 20 seconds preceding the WLF emission, which would coincide with inten se and impulsive hard X-ray bursts. The BLF location would be at (or i n the vicinity of) the forthcoming bright patches. Their predicted con trast depends on the position of the flare on the solar disc and on th e wavelength band of the observation. The Yohkoh satellite provided wh ite-light data from the aspect camera of the SXT instrument (Tsuneta e t al., 1991), at 431 nm and with a typical image interval of 10-12 s. We have studied nine white-light flares observed with this instrument, with X-ray class larger than M6. We have found a few interesting epis odes, but no unambiguous example of the predicted BLF event. This stud y, although the best survey to date, was not ideal from the observatio nal point of view. We therefore encourage further searches. Successful observations of this phenomenon on the Sun would greatly strengthen o ur knowledge of the lower solar atmosphere and its effects on solar lu minosity variations.