IONIZATION PROCESSES IN THE LOCAL INTERSTELLAR-MEDIUM - EFFECTS OF THE HOT CORONAL SUBSTRATE

We present detailed photoionization calculations for the local interstellar cloud in which the Sun is embedded. We consider the EUV radiation field with contribution from discrete stellar sources and from a thermal bremsstrahlung-radiative recombination spectrum emitted from the surrounding 106 K coronal substrate. We establish lower limits to the fractional ionization of hydrogen (XH) and helium (XHe+) of 0.17 and 0.30, respectively. The high He ionization fraction results primarily from very strong line emission below 500 Å originating in the surrounding coronal substrate, while the H ionization is dominated by the EUV radiation from the discrete stellar sources. The dual effects of thermal conduction and the EUV spectrum of the 106 K plasma on ionization in the cloud skin are explored. The EUV radiation field and Auger ionization have insignificant effects on the resulting ionic column densities of Si IV, C IV, N V, and O VI through the cloud skin. Our calculations show that the abundances of these species are dominated by collisional ionization in the thermal conduction front. Because of a low charge exchange rate with hydrogen, the ionic column density ratios of N(C III)/N(C II) and N(N II)/N(N I) are dominated by the EUV radiation field in the local interstellar medium (LISM). These ratios should be important diagnostics for the EUV radiation field and serve as surrogate indicators of the interstellar He and H ionization fraction respectively. Spacecraft such as Lyman which is designed to obtain high resolution spectral data down to the Lyman limit at 912 Å could sample interstellar lines of these ions.