Revised interstellar neutral helium/hydrogen density ratios and the interstellar UV-radiation field

The data deduced from the LV-spectroscope on the Copernicus satellite strongly suggest that the most important ionization source in interstellar space near the solar system is a UV radiation field originating from B-stars. Adopting this hypothesis, the ionization state of several elements in the interstellar medium observed by Copernicus has been used to determine the required radiation field. From this, the degree of ionization of elements that could not be observed by Copernicus is estimated. It is shown that this interpretation of the Copernicus data can be made consistent with neutral interstellar hydrogen densities inferred from extraterrestrial Lα observations and with electron densities deduced from pulsar dispersion measures. Furthermore, it is shown that the ratio of neutral interstellar helium to neutral interstellar hydrogen is likely to be 2 to 3 times as large as the cosmic abundance ratio of these elements. The possibility that this ratio is about 10 times as large, meaning equal interstellar neutral hydrogen and helium densities near the solar system, cannot be ruled out. It would, however, require an interstellar radiation temperature near 9000 K. A comparison of the intensity of the interplanetary back scattered He 584 A and the H 1216 A radiation would lead to a direct determination of this ratio provided the solar radiation at these lines is known. (Auth.)