Republication of: radial movement in sunspots

Abstract

Displacements of the lines of hydrogen and calcium in the neighbourhood of sunspots, indicating violent motions in the line of sight, is a common characteristic of spot disturbances. Such phenomena are frequently observed during periods of active change in spot development, or during the genesis of a spot. These line-shifts rarely affect the spectra of other elements than those of the higher chromospheres. In very violent outbursts, in addition to the hydrogen and calcium lines, those of He, Mg, Na, and some of the enhanced displacements may be either an increase or a decrease of wave-length, and may amount to several Ångström units, indicating movements of approach or recession of several hundred kilometers per second. These movements are seldom maintained for more than a few minutes at a time, and are usually to be found in the immediate neighbourhood of sports, rarely within the umbral area.