White-light flares are defined as those flares that produce significant elahancement of emission in the visible light continuum. The source of energy for this emission has not yet been identified with several possibilities being suggested: heating of the lower chromosphere by some mechanical or magnetic means, or by soft X-ray or extreme ultraviolet radiation from coronal loops being absorbed in the lower chromosphere and re-emitted in the visible.
Using non-LTE radiative transfer calculations for hydrogen and helium in a simple model atmosphere we show that EUV (2 < 912/~) radiation cannot be the main energy source for white-light flares. Estimates of the observed energy emitted in the visible and the EUV indicate that there may be enough energy in the EUV to account for the white light flare with this mechanism. Using enhancements in the wavelength region below 912 ~ of up to ~7 โข l09 ergs cm -2 s-1 ster-1 (~5 โข 105 times the estimated quiet Sun radiation field) to represent flare EUV emission from above we investigated the non-LTE level populations for hydrogen and helium and the lower atmospheric heating resulting from this radiation. The basic result is that the opacities in the Lyman continuum and the helium I and II continua are so much larger than even the enhanced opacity in the visible hydrogen continuum that the EUV radiation is absorbed before it can have a significant effect in the visible light continuum. However, the EUV radiation can cause a significant enhancement of Ha emission.