The exposure of the Moon to the Earth’s plasmasheet Mike Hapgood STFC Rutherford Appleton Laboratory

CONTEXT Moon crosses magnetotail around Full Moon –~4 to 5 days per month –Xgse ~ - 60 Re Summary of 50 keV electrons seen by Cluster Sometimes encounters dense hot plasma of the plasmasheet Lunar surface can gather charge in these conditions E.g. observed by Lunar Prospector Plasmasheet

When is Moon in plasmasheet? Z moon ~ Z sheet Z sheet set by dipole tilt –annual ± 4 Re, + in northern summer –also smaller diurnal motion –also variation with Ygse Z moon set by inclination of Moon’s “orbit” –annual motion ± 5.5 Re –phase varies with precession of Moon’s orbit (18.6 year cycle) June Ecliptic plane Moon’s orbit inclined by 5  Nodes precess 360  in 18.6 years AN DN Neutral sheet

Detailed calculation Apply to period , with 1h resolution –70 year run to cover several precession cycles –Include mix of past and future dates Take plasmasheet as | Z moon - Z sheet | ≤ 2 Re Moon position –Inertial position (RA, Dec, R) from IDL Astronomy Library at NASA Goddard –Convert to GSE using local transformation library Plasmasheet location –Use Tsyganenko 1998 neutral sheet model (to X=-100 Re) –Assume V sw =400 km s -1, θ SW =0, By =0 (limited real data – subject for future work)

Example output

Exposure per month raw monthly 25-month mean half-yearly envelopes

Model overview Marked variation in lunar exposure over 18 year precession cycle –Driven by phase difference between Z variation of plasmasheet and of full Moon –In-phase near cycle maximum Best match slightly off maximum (ΔZmoon > ΔZsheet) –Antiphase at cycle minimum Peaks around 1976/1980, 1995/1999, 2013/2017 Are past peaks supported by observations?

What do observations report? No long-term observations But spot observations are suggestive –Lunar Prospector (1998-9) observed upward e- beans indicative of surface charging to several kV associated with PS and SEP (Halekas et al, GRL, 2005 & 2007) –Lunar exosphere (Na) observed during 5 eclipses in , exosphere strongly enhanced in cases close to PS crossings (Wilson et al, GRL, 2006) What about Apollo? –Missions all on dusk flank (First Quarter moon phase) –Little or no overlap with plasmasheet

Why does this matter? Dust transport –dust is major environmental issue for lunar exploration –levitated dust observed by Apollo & precursors (e.g. images, visual reports, surface dust experiments) –electrodynamics is key to dust transport Charging of equipment on surface –Similar to spacecraft charging Risk of discharge on landing –Potential drop over Debye length above surface? –Similar risk exists for aircraft in Earth’s atmosphere?

Next steps Explore impact of By on plasmasheet model –Focus on periods with good IMF data –Improve time resolution of model –Work in progress Look for other data sources: –Anything from Apollo surface measurements? –Geotail and Wind lunar passes –SMART-1, Chandryaan, Lunar Reconnaissance Orbiter Highlights need for better plasmasheet models –Need to model Z variation –Old models from ISEE give X & Y, Cluster gives Z

Conclusions Dynamical properties of Moon’s orbit imply 18-year cycle in lunar charging –Related to cycle of eclipse occurrence –Should consider in long-term mission planning –Experience at minimum (e.g. now) is not a guide to conditions at maximum Needs further work –Explore role of By in model –Search for additional observational data –New measurements to monitor e- flux and charging (but can this be done on a penetrator?) MoonLite concept