ON THE OCCURRENCE OF BLUE ASYMMETRY IN CHROMOSPHERIC FLARE SPECTRA

We present observations of optical spectra of a flare in which blue li ne asymmetry was seen for more than 4 min close to the flare onset. Th e maximum blue asymmetry coincided with the maximum of a hard X-ray an d microwave burst. We discuss possible interpretations of the blue asy mmetry and conclude that the most plausible one is electron-beam heati ng with return current. Although this process predicts downflows in th e lower transition region and upper chromosphere, its ultimate effect on the line profiles can be blue asymmetry: the upper layers moving aw ay from us absorb the radiation of the red peak thus lowering its inte nsity in comparison to the blue one.