Do Impulsive Solar-Energetic-Electron (SEE) Events Drive High-Voltage Charging Events on the Nightside of the Moon?

Abstract

When the Earth’s moon is in the supersonic solar wind, the darkside of the Moon and the lunar plasma wake can be very dangerous charging environments. In the absence of photoelectron emission (dark) and in the absence of cool plasma (wake), the emission or collection of charge to reduce electrical potentials is difficult. Unique extreme charging events may occur during impulsive solar-energetic-electron (SEE) events when the lunar wake is dominated by relativistic electrons, with the potential to charge and differentially charge objects on and above the lunar surface to very-high negative electrical potentials. In this report the geometry of the magnetic connections from the Sun to the lunar nightside are explored; these magnetic connections are the pathways for SEEs from the Sun. Rudimentary charging calculations for objects in the relativistic-electron environment of the lunar wake are performed. To enable these charging calculations, secondary-electron yields for impacts by relativistic electrons are derived. Needed lunar electrical-grounding precautions for SEE events are discussed. Calls are made 1) for future dynamic simulations of the plasma wake in the presence of time-varying SEE-event relativistic electrons and time-varying solar-wind magnetic-field orientations and 2) for future charging calculations in the relativistic-electron wake environment and on the darkside lunar surface.

Characteristics of Lunar Surface Electrons Inferred From ARTEMIS Observations: 1. Backscattered Electrons

“…However, there exists a more fundamental physical difference between the two, in that secondary electrons are electrons liberated from the surface by the incident electrons, while BSE are a fraction of the incident electrons that are reflected back to space. Depending on the specific environment, the lunar surface is typically positively charged in the sunlit hemisphere, from <∼10 V in the solar wind (e.g., Borovsky & Delzanno, 2021; Halekas et al., 2008, 2011b) to tens to hundreds of volts in the magnetotail lobes (Harada et al., 2013, 2017; Pedersen, 1995), and negatively charged in darkness, up to negative hundreds of volts (Halekas et al., 2002, 2008, 2011b) or sometimes kilovolts during solar energetic particle events (Halekas et al., 2007, 2009b). Inside Earth's magnetotail plasma sheet, non‐monotonic sheath potentials can occur above the dayside surface (Collier et al., 2017; Halekas et al., 2011a; A. Poppe et al., 2011; A. R. Poppe et al., 2012).…”