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The solar vacuum ultraviolet (VUV; < 200 nm) radiation induces photoemission from the surface of the Moon, Mercury, and other airless celestial bodies. This process charges the surface positively and generates a near-surface photoelectron plasma sheath. Solar activity is highly variable in the 70-100 nm range where the photoelectron yield peaks. Available data for high-resolution solar UV spectra and lunar grain photoelectric yield are combined to calculate the variation of photoelectron emission with solar activity. The results show that extreme solar flare conditions can increase photoelectron emission several fold. The consequence of higher photoelectron current is increased surface potential and stronger electric field that is likely to increase the electrostatic mobilization of lunar dust.
There is much evidence to show that lunar “horizon glow” and “streamers” observed at the terminator are caused by sunlight scattered by dust grains originating from the surface. The dust grains and lunar surface are electrostatically charged by the Moon’s interaction with the local plasma environment and the photoemission of electrons due to solar UV and X-rays. This effect causes the like-charged surface and dust particles to repel each other, and creates a near-surface electric field. Previous models have explained micron-sized dust observed at ˜10 cm above the surface, by suggesting that charged grains “levitate” in the local electric field; however this cannot account for observations of 0.1 μm-scale grains at ˜100 km altitude. In order to explain the high-altitude dust observations, we propose a dynamic “fountain” model in which charged dust grains follow ballistic trajectories, subsequent to being accelerated upward through a narrow sheath region by the surface electric field. These dust grains could affect the optical quality of the lunar environment for astronomical observations and interfere with exploration activities.