Spectrophotometric study of sunspots

Abstract

After a brief review of some of the earlier work on sunspots the experimental technique employed for the spectrophotometry of sunspots and the results obtained in the case of 5 spots are described. In the case of one of the spots studied on 1951 May 13, the intensity ratio umbra/photosphere decreased from about 32% at λ 6620 A to 15% at λ 5078 A. The lowest value of 22% at λ 6620 A was recorded for the umbral intensity of a spot on May 24. Microphotometer records of two spots are reproduced showing the discontinuous nature of the transition from the umbra to the penumbra first observed visually by Secchi. Tables and curves of I. (λ, T^∗)/I (λ, T) based on Planck's radiation law are given and the effective temperatures of the spots calculated. Odgers' theory of the "bright rings" in sunspots is considered and it is pointed out that the observed small values of excess intensity of the bright rings probably imply that sunspots extend to appreciable depths below the photosphere.