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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?
• 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
Ecliptic plane
Moon’s orbit inclined by 5
Nodes precess 360 in 18.6 years
AN
DN
Neutral sheet
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)
Example output
Exposure per monthraw 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 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
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