Assessing the effects of coronal mass ejections on the sputtered lunar exosphere: the role of solar wind minor ions

As a body without a thick atmosphere or global magnetic field, the Moon is directly exposed to incident ion fluxes from the solar wind and the terrestrial magnetosphere. As ions strike the lunar surface, they sputter individual regolith atoms, thereby contributing one component of the lunar exosphere. Previous work has studied the lunar sputtered exosphere during both nominal solar wind conditions and extreme space-weather activity such as coronal mass ejections (CMEs). These studies have suggested greater-than-tenfold increases in the lunar exospheric density during CME events due to elevated sputtering rates. Here, we analyze the effects that CMEs may have on the production and equilibrium of the sputtered neutral exosphere at the Moon via the use of in-situ solar wind measurements during CMEs. In particular, we investigate the role that heavy, highly charged minor ions in the solar wind may play during CME impacts at the Moon.

For this purpose, we use measurements of the plasma moments and heavy ion composition during CMEs observed by the ACE/SWICS instrument at Sun-Earth L1 over the period of 1998–2011. We extract the solar wind flux and heavy ion composition during the event intervals listed in the publicly available “Richardson and Cane CME list” and convolve the heavy ion fluxes with appropriate sputtering yields for the lunar regolith. Generally, we find that solar wind heavy ions nominally contribute ~5% of the total sputtering yield while extreme events can reach contributions of ~20%. In no cases, however, do solar wind minor ions dominate the sputtering rates at the Moon. Finally, we discuss the implications of this work for our understanding of the full variability of the Moon’s exosphere.