Solar wind electron interaction with the dayside lunar surface and crustal magnetic fields: Evidence for precursor effects

Abstract

Electron distributions measured by Lunar Prospector above the dayside lunar surface in the solar wind often have an energy dependent loss cone, inconsistent with adiabatic magnetic reflection. Energy dependent reflection suggests the presence of downward parallel electric fields below the spacecraft, possibly indicating the presence of a standing electrostatic structure. Many electron distributions contain apparent low energy (<100 eV) upwardgoing conics (58% of the time) and beams (12% of the time), primarily in regions with non-zero crustal magnetic fields, implying the presence of parallel electric fields and/or wave-particle interactions below the spacecraft. Some, but not all, of the observed energy dependence comes from the energy gained during reflection from a moving obstacle; correctly characterizing electron reflection requires the use of the proper reference frame. Nonadiabatic reflection may also play a role, but cannot fully explain observations. In cases with upward-going beams, we observe partial isotropization of incoming solar wind electrons, possibly indicating streaming and/or whistler instabilities. The Moon may therefore influence solar wind plasma well upstream from its surface. Magnetic anomaly interactions and/or non-monotonic near surface potentials provide the most likely candidates to produce the observed precursor effects, which may help ensure quasi-neutrality upstream from the Moon.

The effect of surface topography on the lunar photoelectron sheath and electrostatic dust transport

“…At locations relatively far from the presence of the crater, we found photoelectron sheath potentials and electric fields typical of that from previous estimations, simulations, and observations (Freeman and Ibrahim, 1975;Halekas et al, 2008;Poppe and Horányi, 2010). Interestingly, these simulations do not show the presence of a non-monotonic potential layer above the surface of the Moon while in the solar wind, as previous simulations and observations have indicated (Poppe and Horányi, 2010;Poppe et al, 2011Poppe et al, , 2012Halekas et al, 2012). We attribute this to the simulation volume to which our computational constraints limit us.…”

A comparison of ARTEMIS observations and particle‐in‐cell modeling of the lunar photoelectron sheath in the terrestrial magnetotail

“…These works also suggested, somewhat un-intuitively, that the non-monotonic potential may in fact be energetically preferred over the monotonic solution. In-situ measurements by the Lunar Prospector (LP) Electron Reflectometer (ER), which orbited the Moon and could sense the lunar surface potential via electron reflectometry, suggested that non-monotonic potentials were indeed present [Halekas et al, 2008b[Halekas et al, , 2012. By comparing with a one-dimensional particle-in-cell simulation, these measurements were confirmed as resulting from nonmonotonic potentials [Poppe and Horányi, 2010;Poppe et al, 2011].…”

First remote measurements of lunar surface charging from ARTEMIS: Evidence for nonmonotonic sheath potentials above the dayside surface

“…Preliminary comparisons of PIC model results with plasma sheet data from LP show very encouraging consistency, lending credence to the existence of nonmonotonic potentials above the dayside surface in at least one plasma regime [ Poppe et al , 2011]. Solar wind data also provide hints of such structures, though LP did not have energy resolution and coverage sufficient to make definitive conclusions [ Halekas et al , 2011]. The terrestrial magnetotail lobes, meanwhile, which have plasma densities so low (<∼0.1 cm −3 ) that spacecraft charge to rather large positive potentials and spacecraft photoelectrons dominate measurements, present a very difficult environment in which to remotely measure surface charging.…”