Optical observations of the early (t < 5 s) ion dynamics of the CRRES G1, G9, and G11A releases

Video images of the CRRES Gl, G9, and G1lA barium releases made at 400–500 km altitudes over the Caribbean in July 1991 have been analyzed to identify morphological features formed in the very early phases of the releases with the aim of associating specific physical processes with these features. Three features are presented: the observation that the inner edge of the ion cloud is located some distance from the release point along the orbital track (“skidding”); the formation of an elongated striation at the inner edge of the ion cloud and an ion depleted region behind the striation; and the formation of a flat neutral barium disk perpendicular to the magnetic field B that is centered some tens of kilometers from the release point. We are linking these morphological features with effects associated with the polarization of the initially dense ion cloud created by solar UV illumination. A simple physical model of the release and the polarization process is presented. Model predictions are both qualitatively and quantitatively in good agreement with the observations. The skidding distance appears to be controlled by the background ionospheric density and the size of the release; the structure in the early ion cloud is due to electric fields primarily setup and maintained by the polarization of the densest part of the ion cloud; and the neutral disk is formed by charge exchange between counterstreaming ions and neutrals outside the polarized core. For the three releases analyzed, no additional internal plasma processes nor processes coupling the release to the ionosphere appear to be critical for the formation of the observed features in the initial ion cloud.