Temperature and Turbulence in the Chromosphere

Abstract

Measures of chromospheric line profiles in high resolution spectrograms obtained at the total solar eclipse of 1952 February 25 have been used to investigate further the question of temperature and turbulence in the solar chromosphere. In trying to deduce a distribution of line-of-sight velocities, arising from either temperature motions or turbulence, two problems are encountered: ( a ) to estimate the chromospheric heights to which the spectra refer, and ( b ) to correct the measures for broadening by self-absorption and Stark effect. Heights have been estimated partly from a comparison of line intensity ratios with those measured by Dr J. Houtgast on slitless spectrograms obtained at the same eclipse, and partly from the time of exposures, using the motion of the Moon relative to the Sun. Broadening by selfabsorption has been estimated in the case of metallic lines by an empirical method. The corrected line widths, together with others of He I , suggest an approximately linear increase of line-of-sight velocity with height, from $${\xi }_{0}\simeq 2\cdot 5$$ km/sec at the base of the chromosphere, to $${\xi }_{0}\simeq 16$$ km/sec at height 2600 km. Taking into account the uncertainty in effective height, these results agree with those obtained in 1940. The hydrogen Balmer lines have been examined in more detail. The earlier members of the series are broadened by self-absorption, while later members are broadened by Stark effect. Both kinds of broadening were first estimated by an empirical method, which led to the conclusion that the temperature of the lower chromosphere does not exceed 10 000 deg. K. The hydrogen line widths were then computed more elaborately by a theoretical method, which showed that the observations could be fitted almost as well by a temperature 6 000 deg. as by 10 000 deg. It seems unlikely that the present data will yield any closer estimate of temperature than this.