Helium Abundance from Optical Spectra of Quiescent Prominences

Abstract

In 1971 one of the authors of the present investigation tried to determine the abundance ratio of helium to hydrogen and found it to be 5 ~ 8% by number (Hirayma, 1971). Quite recently Heasley and Milkey (1978) reported it to be 10 ± 2.5%. This paper describes another attempt to determine the abundance of helium, from the observations using the 25 cm coronagraph at Mt. Norikura for 11 quiescent prominences during 1971-1977. We concentrated our effort on the observation of the faintest part of the prominence because it is much easier to interpret. Figure 1 shows the intensity ratio of Hel 3888 to the hydrogen Balmer H8line plotted against the total intensity of the H7line. We can see that the fainter part of the prominence, i.e. the left part, has generally stronger emission in helium. We infer from this figure that the upper limit to the intensity ratio of helium to hydrogen is attained at a value of 0.7 ± 0.05.