Viability of the Critical Ionization Velocity Concept in Selected Space Situations

Abstract

When a magnetized plasma and a marginally ionized neutral gas penetrate each other, the gas is rapidly ionized and assimilated into common motion with the plasma if a critical relative velocity corresponding to the ionization energy of the neutrals is surpassed. This effect was predicted by Alfven and confirmed in numerous laboratory experiments. Via similarity transformations from the laboratory to typical space conditions the critical velocity mechanism can be shown to be viable in tenuous space plasmas, too. This was verified by observations on the moon, by a recent rocket experiment in the ionosphere, and by numerical calculations of solar wind behaviour under the influence of the interstellar neutral gas drifting through the heliosphere. — Based on these findings, earlier work is reassessed of other authors who invoked the critical velocity effect in connection with the ionization of cometary atmospheres, with regard to a comet-like interaction between Venus and the solar wind, and as a source mechanism for the ionosphere of Jupiter’s satellite Io. Conceivably, critical velocity effects are also involved in the formation of circumstellar shells such as stellar bubbles and planetary nebulae, the deceleration of supernova remnants by interstellar matter, the expansion of H II regions into molecular clouds, and the physics of radio galaxies.