ARTEMIS observations of lunar pick-up ions in the terrestrial magnetotail

Since lunar orbit insertion in the summer of 2011, the two ARTEMIS spacecraft have made repeated measurements of lunar-based pick-up ions within the terrestrial magnetotail lobes. These ions are often detected as two separate focused beams above the dayside lunar surface, although they can sometimes be detected farther from the Moon. Analysis of these beams has shown that they possess both field-aligned and field-perpendicular velocities, implying the presence of electric fields both parallel and perpendicular to the magnetotail lobe magnetic field. We use the energy and pitch angle spectra to constrain the source locations and compositions of these ions, and conclude that exospheric ionization of the neutral exosphere is the dominant lunar pick-up ion production mechanism in the tail lobes. We also present evidence that one sub-population of these ions are coming from a discrete source on the lunar surface, possible a geologic vent. Finally, we present preliminary results from particle-tracing models of pick-up ion dynamics in the terrestrial magnetotail in order to help constrain the neutral source distribution(s) for these pick-up ions.