K emission-line widths and the solar chromosphere

Abstract

Closely spaced microphotometer tracings parallel to the dispersion of one excellent frame of a K-line time sequence have been utilized for a study of the nature of the K_2v , K_2R intensities in the case of the solar chromosphere. The frequency of occurrence of the categories of intensity ratio I_K2v/I_K2R are as follows: I_K2v > I_K2R = 45.3 per cent; I_K2v = I_K2R = 4.7 per cent; I_K2v < I_K2R = 25 per cent; I_K2R = 0 = 22.3 per cent; I_K2v = 0 =0.7 per cent. Two types of absorbing components are postulated to explain the pattern of observed K_2v , K_2R intensity ratios. One component with minor Doppler displacements acting on the normal K₂₃₂ profile, where K_2V > K_2R, produces the cases K_2v ≫ K_2R , K_2v = K_2R , K_2v < K_2R . The other component arises from 'dark condensations' which are of size 3500 kms as seen in K_2R. They have principally large down flowing velocities in the range 5-8 km/sec and are seen on K₃ spectroheliograms with sizes of about 5000 kms, within the coarse network of emission. These 'dark condensations' give rise to the situation K_2R = 0. K₂-line widths are measured for all tracings where K_2v , K_2R are measurable simultaneously. The distribution curve of these widths is extremely sharp. The K₂ emission source is identified with the bright fine mottles visible on the surface. Evidence for this interpretation comes from the study of auto-correlation functions of K₂ intensity variations and the spacing between the bright fine mottles from both spectrograms and spectroheliograms. The life time of the fine mottling is 200 sec. The supergranular boundaries which constitute the coarse network come in two intensity classes. A low intensity network has the fine mottles as its principal contributor to the K emission. When the network is bright, the enhancement is caused by increased K emission due to the accumulation of magnetic fields at the supergranule boundary. The K₂ widths of the low intensity supergranular boundary agree with the value found for the bright mottles. Those for the brighter network are lower than this value, similar to the K₂ widths as seen in the active regions. It is concluded that bright fine mottling is responsible for the relation, found by Wilson and Bappu, between K emission line widths and absolute magnitudes of the stars. The paper discusses the solar cycle equivalents that stellar chromospheres can demonstrate and indicates a possible line of approach for successful detection of cyclic activity in stellar chromospheres.