A magnetospheric critical velocity experiment: Particle results

Abstract

In March of 1983, a barium injection sounding rocket experiment (the Star of Lima) was conducted to investigate Alfvén's critical ionization velocity (CIV) hypothesis in space. Included in the instrumented payload was a University of California, San Diego (UCSD), particle detection experiment consisting of five retarding potential analyzers. Despite conditions that appeared to be optimal for the critical velocity effect, the particle data, in agreement with optical observations, indicate that a fractional ionization of only ˜5×10−4 was observed, indicating that the conditions required for the effect to occur are still not well understood. However many of the required phenomena associated with the CIV effect were observed; in particular a superthermal electron population was formed at the expense of ion drift kinetic energy in the presence of intense electrostatic waves near the lower hybrid frequency. The amount of ionization produced is plausibly consistent with the observed electron flux, but could also be accounted for by residual solar UV at the injection point. It is shown based on the UCSD data set that one obvious explanation for the low ionization efficiency, namely that the ionizing superthermal electrons may rapidly escape along field lines, can be ruled out.