The Boundary of the Solar Chromosphere

Abstract

IN analysing photographs of the flash spectrum it is customary to measure the lengths of the chromospheric arcs on the negative, and to deduce therefrom the heights to which various elements rise in the chromosphere. The H and K lines of calcium are always found to rise the highest, and their extent fixes the boundary between the chromosphere and the corona. One would very much like to know photometrically whether the intensity of H and K light is really falling off rapidly at this apparent boundary; or whether it fades out slowly and extends appreciably beyond. Since 1897 the view seems to have prevailed that it does not. In that year Young wrote: “The photograph also seems to make it certain that hydrogen, helium, and calcium, though brilliantly conspicuous upon the plate in the images of the prominences, are entirely absent from the corona, a result agreeing with that deduced from similar photographs made in 1893, but only recently published. It is quite clear that the earlier observations (referred to on pages 261 and 262 of ‘The Sun’) were misleading from the fact that the apparatus did not sufficiently guard against the effects of illumination of the air by light from the prominences” (Astrophysical Journal, 6, 155). By illumination of the air is meant scattering of light in the earth's atmosphere.