ARTEMIS observations of lunar pick‐up ions in the terrestrial magnetotail lobes

Abstract

We report observations by the twin‐probe mission ARTEMIS of pick‐up ions of lunar origin obtained during times when the Moon was within the terrestrial magnetotail lobes. These ions were detected as two separate focused beams above the dayside lunar surface. 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 suggest that the sources of these two electric fields are (a) the near‐surface electric field due to the lunar photoelectron sheath and (b) the electric field generated by the magnetotail lobe convection velocity. 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.