ARTEMIS observations of lunar pickup ions: Mass constraints on ion species

Abstract

Observations of heavy ions of lunar origin give important information regarding lunar exospheric processes, especially with respect to exospheric particle abundance and composition. Electrostatic analyzers without a time‐of‐flight section provide highly sensitive, absolute density detection but without mass discrimination. Here we place constraints on lunar ion species through inference of the average ion mass using such instruments. The technique is based on the plasma quasi‐neutrality requirement, an independent electron density measurement, and the fact that electrostatic analyzers underestimate the ion density by the square root of the ion mass. Applied to a case of such observations by ARTEMIS in the terrestrial lobe reported by Poppe et al. (2012), our technique suggests an average mass of 28 amu for lunar pickup ions. This result, consistent with the lower limit of 24 amu derived in the Poppe et al. model, suggests that the observed ions were most likely Al+ and Si+. The technique is also refined and applied to a more complicated event with a series of heavy ion surges in the plasma sheet, to show the spatial and/or temporal dependence of the observed lunar ion species. The technique is particularly timely given the planned conjunctions and coordinated lunar studies by NASA's Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) and Lunar Atmosphere and Dust Environment Explorer missions.