Prompt ionization in the CRIT II barium releases

Abstract

Observations of electron and ion distributions inside a fast neutral barium jet in the ionosphere show significant fluxes within 4 km of release, presumably related to beam plasma instability processes involved in the Critical Ionisation Velocity (CIV) effect. Electron fluxes exceeding 5 × 1012/cm2‐str‐sec‐keV were responsible for ionizing both the streaming barium and ambient oxygen. Resulting ion fluxes seem to be consistent with 1–2% ionization of the fast barium, as reported by optical observations, although the extended spatial distribution of the optically observed ions is difficult to reconcile with the in‐situ observations. When the perpendicular velocity of the neutrals falls below critical values, these processes shut off. Although these observations resemble the earlier Porcupine experimental results [Haerendel, 1982], theoretical understanding of the differences between these data and that of earlier negative experiments is still lacking.