RELATIVE IONIZATIONS IN THE NEAREST INTERSTELLAR GAS

We compare CLOUDY predictions for the equilibrium ionization in the in terstellar cloud surrounding the solar system with pick-up ion data. T he incident radiation held includes contributions from hot stars, the emission from the conductive cloud boundary and the diffuse EUV backgr ound. To within the observational uncertainties, CLOUDY predictions fo r the ratios n(He degrees)/n(O degrees), n(N degrees)/n(O degrees), n( Ne degrees)/n(O degrees), and n(He degrees)/n(Ne degrees) are consiste nt with pick-up ion data, provided that O degrees and N degrees are fi ltered by similar to 50% in the heliopause region and the outer helios phere as predicted by others. Thus, the steady-state ionization model and assumed radiation held appear approximately valid. However, the yo uth and low intervening column density towards the Vela pulsar leave o pen the possibility that the parent supernova explosion similar to 10, 500 years ago, and 200 pc distant, may also have affected LISM ionizat ion, although the mechanism is uncertain. Support for this last possib ility is provided by the apparent signature of the Vela explosion in t he terrestrial geological record.