On the state of ionization of the local interstellar medium

The abundance ratio of neutral hydrogen to helium, as deduced from interplanetary observations of Lyman-alpha and He 584 Å radiation by Mariner 10, is significantly lower than the cosmic abundance ratio of these elements, thus showing that the local interstellar medium (LISM) is partly ionized. The effect of various sources of ionization — especially electron impact and EUV photon flux — on hydrogen and helium is discussed. It is shown that the observational data on the temperature of the LISM, on the diffuse EUV flux and on the neutral and electron densities in the nearby interstellar medium (NISM) are not all compatible. However, if the diffuse EUV flux below 912 Å as deduced from the preliminary analysis of Voyager observations is not representative, then it is easy to reconcile all observations. In this case an important source of ionization of the LISM would be electron impact, yielding an ionization degree of about 50% for the hydrogen component.