Barium ionisation mechanisms in the CRRES G-1 and G-11b releases

Summary form only given. The G-11b chemical release experiment on the Combined Release and Radiation Effects Satellite (CRRES) was conducted in darkness below the solar UV terminator to test the critical ionization velocity (CIV) hypothesis. The quadrupole ion mass spectrometer (QIMS) aboard CRRES measured fluxes of barium ions from this darkness release that were only a factor of ten smaller than the G-1 release measurements in full sunlight. Possible mechanisms for this significant barium ionization in darkness include CIV, charge exchange with O/sup +/, collisional ionization, and associative ionization. The author has evaluated the relative contributions of the collisional mechanisms by constructing a simple model of barium ion fluxes at the spacecraft position and by comparing the model predictions to the measured fluxes. The large fluxes of barium ions from the darkness release seem to be consistent with measurements of the charge transfer cross section. A collective plasma ionization mechanism such as CIV does not seem to necessary to explain the large barium ion fluxes observed.