Causes of white-light flares

White-light flares generally exhibit two distinct phases, the impulsive and the main phase. Spectral characteristics of the optical and X-ray emissions associated with these two phases are reviewed. Events show no Balmer discontinuity, or only a small one, during the main phase are examined in detail. It is concluded that intense 1-10 A X-ray emission may heat the upper photosphere sufficiently to produce H(-) continuum emission. The effects of a high-temperature, high-pressure coronal flare plasma upon the chromosphere and photosphere are examined with the aid of recent model atmosphere calculations. It is concluded that white-light flares lacking Balmer discontinuities are due to the extreme narrowing of the chromosphere under the high-temperature and high-pressure conditions prevailing in flares rated M5 and above on the GOES X-ray scale. 23 references