Transport of solar wind plasma onto the lunar nightside surface

Abstract

We present first measurements of energetic neutral atoms that originate from solar wind plasma having interacted with the lunar nightside surface. We observe two distinct energetic neutral atom (ENA) distributions parallel to the terminator, the spectral shape, and the intensity of both of which indicate that the particles originate from the bulk solar wind flow. The first distribution modifies the dayside ENA flux to reach ∼6° into the nightside and is well explained by the kinetic temperature of the solar wind protons. The second distribution, which was not predicted, reaches from the terminator to up to 30° beyond the terminator, with a maximum at ∼102° in solar zenith angle. As most likely wake transport processes for this second distribution we identify acceleration by the ambipolar electric field and by the negatively charged lunar nightside surface. In addition, our data provide the first observation indicative of a global solar zenith angle dependence of positive dayside surface potentials.

First Observation of Transport of Solar Wind Protons Scattered From Magnetic Anomalies Into the Near Lunar Wake: Observations by SARA/Chandrayaan‐1

“…We show that out of these reflected protons from the SPA, a certain fraction of them indeed reaches the nightside, and also, we quantified the flux and energy of such a population. Recent studies have shown that due to the interaction of protons in near wake region with the nightside lunar surface, energetic neutral atoms are produced (Vorburger et al, ), and that the nightside surface charging can be affected due to the emission of secondary electrons (Nishino et al, ). The plasma in the near wake region generate instabilities and significantly affect the electromagnetic environment of Moon (Nishino et al, ).…”

“…The indirect transport include the entry of solar wind scattered from dayside lunar surface to the near wake region (Nishino, Fujimoto, et al, ; Wang et al, ) and also the solar wind scattered from Earth's bow shock (Nishino et al, ). Further, new population of protons is found in the near lunar wake whose source(s) is(are) yet to be identified (Dhanya et al, , ; Vorburger et al, ).…”

“…The indirect transport include the entry of solar wind scattered from dayside lunar surface to the near wake region Wang et al, 2010) and also the solar wind scattered from Earth's bow shock (Nishino et al, 2017). Further, new population of protons is found in the near lunar wake whose source(s) is(are) yet to be identified (Dhanya et al, 2016(Dhanya et al, , 2017Vorburger et al, 2016). 10.1029/2018GL079330 observational evidence so far for such a process, and also, the energy and flux of such a population in near wake are not known. In the following, we demonstrate the first observational evidence for this process.…”

Particles and Photons as Drivers for Particle Release from the Surfaces of the Moon and Mercury

“…In general, ions in a supersonic flow do not have direct access to the near-Moon wake, though a variety of entry mechanisms are discussed (Nishino et al 2009a(Nishino et al , 2009b, Halekas et al 2014a). Thus, the nightside of the Moon is generally subject to much lower (but not completely zero) incident fluxes of ions and electrons compared with those on the dayside when the Moon, which is located in the solar wind and magnetosheath, as has been observed in energetic neutral atom reflection ratios (Vorburger et al 2016). The situation is more complicated in the terrestrial magnetotail, where both sunward and anti-sunward flows commonly exist (Troshichev et al 1999;Øieroset et al 2002).…”

ARTEMIS Observations of Solar Wind Proton Scattering off the Lunar Surface

“…The ions interact with and perturb the ambient solar wind flow (Fatemi et al, 2014;Harada et al, 2014;Halekas et al, 2012Halekas et al, , 2014Halekas et al, , 2017. The ions travel on cycloid trajectories that can bring them thousands of kilometers upstream of the Moon (Futaana et al, 2003;Holmström et al, 2010), or give them access to the lunar wake and the night side surface (Fatemi et al, 2014;Futaana et al, 2010;Nishino et al, 2009;Vorburger et al, 2016).…”