Abstract
Abstract. 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 h per month at a minimum of the eighteen-year cycle rising to 40 h 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 2012–2019.
Highlights
The growing interest in lunar exploration necessitates a better understanding of the operating environment at the lunar surface
We estimate the likelihood of Moon-plasmasheet encounters and show, for the first time, that this is strongly modulated over an 18 year cycle, driven by the precession of the Moon’s orbit
The model described here shows, for the first time, that the exposure of the Moon to the plasmasheet is strongly modulated over an eighteen-year cycle driven by the precession of the Moon’s apparent orbit around the Earth
Summary
The growing interest in lunar exploration necessitates a better understanding of the operating environment at the lunar surface. There is growing observational evidence that the lunar surface can acquire negative potentials of several kilovolts (i.e. relative to the potential some Debye lengths above the surface) when exposed to strong fluxes of energetic electrons. Such potentials are a threat to operation of devices on the surface and may play an important role in dust transport. We estimate the likelihood of Moon-plasmasheet encounters and show, for the first time, that this is strongly modulated over an 18 year cycle, driven by the precession of the Moon’s orbit This modulation is consistent with existing observations and is an important context for interpreting those observations. The specification of the lunar charging environment must take account of the 18-year cycle and in particular include awareness of high-risk periods when encounters are highly likely – the being in 2012–2019
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