The low dissipation regions are predicted to persist in permanently shadowed regions of the Moon (or other airless bodies). Unlike on the dayside of the Moon where the accumulated charge on astronauts and rovers is largely dispersed by photoelectric and charge exchange interactions, the plasma-depleted nightside and permanently shadowed regions of the Moon lack a reliable source of electrical grounding. Since the surface of the Moon is electrically insulating, the main source of electrical grounding is the ambient plasma. In addition, the resulting environment poses a possible challenge to human explorers and their equipment, owing to the inability to dissipate electrostatic charge that accumulates by triboelectric surface interaction (Farrell et al., 2008b Jackson et al., 2011, 2015). The interest in this orographic plasma wake effect is motivated by a number of secondary processes, including dust transport (Poppe et al., 2012 Stubbs et al., 2006), surface sputtering (Killen et al., 2012), and nonuniform distribution of volatile materials (Farrell et al., 2010). The present paper provides an analytic description of the steady-state plasma wake structure in a lunar crater, immediately downstream from a topographical obstruction. Similar plasma wakes arise in comparable airless environments such as asteroids (Poppe et al., 2015 Zimmerman et al., 2014), the moons of Mars (Farrell et al., 2017), and around spacecrafts (Ergun et al., 2010). Experimental evidence of this effect was first observed by the Lunar Prospector (Halekas et al., 2002). As the solar wind flows over the varying topography of the lunar surface, it is predicted to form nonneutral plasma wakes which result in the buildup of local space charge, surface charge, and electrostatic fields (Farrell et al., 2007, 2008a, 2010 Zimmerman et al., 2011).
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