  This paper shows how the exposure of the Moon to the Earth's plasmasheet is
subject to decadal variations due to lunar precession. The latter is a key
property of the Moon's apparent orbit around the Earth - the nodes of that
orbit precess around the ecliptic, completing one revolution every 18.6 years.
This precession is responsible for a number of astronomical phenomena, e.g. the
year to year drift of solar and lunar eclipse periods. It also controls the
ecliptic latitude at which the Moon crosses the magnetotail and thus the number
and duration of lunar encounters with the plasmasheet. This paper presents a
detailed model of those encounters and applies it to the period 1960 to 2030.
This shows that the total lunar exposure to the plasmasheet will vary from 10
hours per month at a minimum of the eighteen-year cycle rising to 40 hours per
month at the maximum. These variations could have a profound impact on the
accumulation of charge due plasmasheet electrons impacting the lunar surface.
Thus we should expect the level of lunar surface charging to vary over the
eighteen-year cycle. The literature contains reports that support this: several
observations made during the cycle maximum of 1994-2000 are attributed to
bombardment and charging of the lunar surface by plasmasheet electrons. Thus we
conclude that lunar surface charging will vary markedly over an eighteen-year
cycle driven by lunar precession. It is important to interpret lunar
environment measurements in the context of this cycle and to allow for the
cycle when designing equipment for deployment on the lunar surface. This is
particularly important in respect of developing plans for robotic exploration
on the lunar surface during the next cycle maximum of 2012-19.
