The exposure of the Moon to the Earth’s plasmasheet

1. The exposure of the Moon to the Earth’s plasmasheet Mike Hapgood STFC Rutherford Appleton Laboratory (M.Hapgood@rl.ac.uk)

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

3. When is Moon in plasmasheet? • Zmoon ~ Zsheet • Zsheet set by dipole tilt • annual ± 4 Re, + in northern summer • also smaller diurnal motion • also variation with Ygse • Zmoon 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 Neutral sheet Ecliptic plane AN DN Moon’s orbit inclined by 5 Nodes precess 360 in 18.6 years

4. Detailed calculation • Apply to period 1960-2030, with 1h resolution • 70 year run to cover several precession cycles • Include mix of past and future dates • Take plasmasheet as | Zmoon - Zsheet| ≤ 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 Vsw=400 km s-1, θSW=0, By =0 (limited real data – subject for future work)

5. Example output

6. Exposure per monthraw monthly25-month meanhalf-yearly envelopes

7. 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?

8. 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 1993-2000, 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

9. 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?

10. 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

11. 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