Optical observations on critical ionization velocity experiments in space

Summary form only given. A number of critical ionization velocity (CIV) experiments have been performed in space. CIV has been observed in laboratory experiments, but experiments in space have been inconclusive. Most space experiments have used barium which ionizes easily, and with emission lines from both neutrals and ions in the visible optical observations can be made from the ground. High initial ionization in some barium release experiments has been claimed due to CIV. However, a number of reactions between barium and the ambient plasma have been suggested as more likely processes. Currently the most popular process in this debate is charge exchange with O/sup +/. This process has a large cross section, but the question is whether it is large enough. The cross section for charge exchange with calcium should be even larger, but in a double release of barium and calcium (part of the NASA Combined Release and Radiation Effects Satellite release experiments) most ionization was observed from the barium release. Moreover, if charge exchange is the dominant process, the amount of ionization should relate to the oxygen ion density, and that does not appear to be the case.