Hα red asymmetry of solar flares

Abstract

The evolutional characteristics of the red asymmetry of Hα flare line profiles were studied by means of a quantitative analysis of Hα flare spectra obtained with the Domeless Solar Telescope at Hida Observatory. Red-shifted emission streaks of Hα line are found at the initial phase of almost all flares which occur near the disk center, and are considered to be substantial features of the red asymmetry. It is found that a downward motion in the flare chromospheric region is the cause of the red-shifted emission streak. The downward motion abruptly increases at the onset of a flare, attains its maximum velocity of about 40 to 100 km s-1 shortly before the impulsive peak of the microwave burst, and rapidly decreases before the intensity of Hα line reaches its maximum. Referring to the numerical simulations made by Livshits et al. (1981) and Somov et al. (1982), we conclude that the conspicuous red-asymmetry or the red-shifted emission streak of Hα line is due to the downward motion of the compressed chromospheric flare region produced by the impulsive heating by energetic electron beam or thermal conduction.