H-alpha red asymmetry of solar flares

Physics

Scientific paper

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106

Chromosphere, H Alpha Line, Red Shift, Solar Flares, Solar Spectra, Emission Spectra, Plasma Acceleration, Solar Flux Density, Solar Radio Bursts, Solar Velocity, Spectrophotometry

Scientific paper

The evolutional characteristics of the red asymmetry of H-alpha flare line profiles were studied by means of a quantitative analysis of H-alpha flare spectra obtained with the Domeless Solar Telescope at Hida Observatory. Redshifted emission streaks of H-alpha 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 redshifted emission streak. The downward motion abruptly increases at the onset of a flare, attains its maximum velocity of about 40 to 100 km/s shortly before the impulsive peak of the microwave burst, and rapidly decreases before the intensity of H-alpha line reaches its maximum. Referring to the numerical simulations made by Livshits et al. (1981) and Somov et al. (1982), it is concluded that the conspicuous red-asymmetry or the redshifted emission streak of H-alpha line is due to the downward motion of the compressed chromospheric flare region produced by impulsive heating by energetic electron beam or thermal conduction.

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