Ionization of a static interplanetary gas and expected emission lines from this gas

Abstract

The degree of ionization of hydrogen and helium in a static interplanetary gas is calculated and the expected intensities of the more intense helium emission lines in the visible and infra-red regions are given as a function of electron density and kinetic temperature. A zodiacal light spectrum recently obtained places an upper limit on the static electron density for given kinetic temperatures. Future high resolution observations of λ 5876 or λ 10,829 A ̊ could give useful upper limits on the static electron densities for kinetic temperatures below 10 5°K. Comments are made on the observations of H α.