Comparison of theory and in situ observations for electron and ion distributions in the near wake of the explorer 31 and AE-C satellites

Measurements of electron density, plasma potential, and mean ion mass from the Explorer 31 satellite and measurements of ion current, plasma potential, and ion composition from the Atmosphere Explorer C (AE-C) satellite were used in a comparative study with theory regarding the charged particle distribution in the near wake of an ionospheric satellite. The theoretical wake model of Parker (1976) has been used in the study. It is shown that theory and experiment agree fairly well in the angle-of-attack range between 90 and 135°. In that angular range even the neutral approximation (which treats ions as if they were neutral particles thus ignoring the influence of the electric field) gives fair agreement with the measurements. In the maximum rarefaction zone (145 < θ < 180°), however, the theoretical model overestimates the measured ion depletion (AE-C measurements) by several orders of magnitude. A similar conclusion is drawn from the comparison between theory and the Explorer 31 electron measurements where the theory also significantly overestimates the electron depletion. The study indicates that the discrepancies are mainly due to the use of a steady-state theory and of a single ion equation (using a mean ion mass). It is recommended that improved agreement between theory and experiment be obtained by the use of the timedependent Vlasov-Poisson equations with separate equations for the various ion species.