The exposure of the Moon to the Earth’s plasmasheet

Mike Hapgood

STFC Rutherford Appleton Laboratory

(M.Hapgood@rl.ac.uk)

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