Shadowed particle distributions near the Moon

Recent Wind spacecraft observations of broadband ion acoustic noise and Langmuir waves near the lunar wake suggest that the electron dynamics play an important role in the solar wind-Moon interaction. As the solar wind flows past the Moon, electrons and ions impacting the lunar surface are removed from their respective distribution functions. We extend a guiding center model of the solar wind velocity distribution into two dimensions and include separate electron and ion distributions. While the ion density shadow is predominantly in the antisunward direction, the electrons have density shadows that extend roughly along the direction of the interplanetary magnetic field lines tangent to the Moon. The charge imbalance created by the topological difference of these density distributions must induce space charge fields in the solar wind plasma; these may drive currents that generate the observed ion acoustic noise. Unstable positive slope features in the electron distribution function arise naturally in this scenario. Langmuir waves would then be generated by a time-of-flight cutoff distribution as in the terrestrial electron foreshock. This model is not self-consistent, as it does not include space charge effects, but serves to point out the importance of considering a kinetic description of the solar wind-Moon interaction.