Upper bound estimates of anomalous ion production in space-based critical ionization velocity experiments

The critical ionization velocity (CIV) is an anomalous ionization mechanism first proposed by Alfven. Experiments have confirmed the existence of a critical velocity in laboratory experiments, but sounding rocket experiments have been ambiguous as to the existence of the critical velocity in the ionosphere. The purpose of this paper is to produce upper bound estimates of anomalous ion production in space-based experiments of the critical ionization velocity. The analysis relies on the results of implicit particle-in-cell simulations and a simple rate model to predict the number of ions produced as a neutral cloud traverses a point in space. The model assumes a point release of neutral gas in the ionosphere which is meant to represent a typical sounding rocket experiment. The results of the model suggest why strong evidence of CIV is not observed in space-based experiments. Space-based experiments require the anomalous ionization process to be initiated through seed ionization provided by charge exchange or other mechanisms. This seed ionization process is too slow in space experiments to ignite CIV. The results also indicate that some of the space based results can be accounted for by assuming a large barium-oxygen charge exchange cross section instead of invoking anomalous ionization mechanisms.