[1] Dust grains on the lunar surface are exposed to UV radiation and solar wind plasma and can collect electrical charges, leading to their possible lift-off and transport in the presence of near-surface electric fields. Motivated by the long-standing open questions about the physics of electrostatic lunar dust transport, we investigated the dynamics of dust grains on a conducting surface in a laboratory plasma. The dust used in these experiments was a nonconducting JSC-Mars-1 sample with particle size of less than 25 microns. We found that dust grains placed on a conducting surface, which is biased more negatively than its floating potential, charge positively, and an initial pile spreads to form a dust ring. Dust particles were observed to land on insulating blocks, indicating the height of their hopping motion. The measured electrostatic potential distribution above the dust pile shows that an outward pointing electric field near the edge of the pile is responsible for spreading the positively charged grains. A nonmonotonic potential dip was measured in the sheath above an insulating patch, indicating a localized upward electric field causing the dust lift-off from the surface. Faraday cup measurements showed that the grains near the boundary of the dust pile collect more charge than those closer to the center of the dust pile and can be more readily lifted and moved in the radial direction, leading to the formation of a spreading ring.

Electric field fluctuations such as those due to plasma waves in Earth's magnetosphere may modulate photoelectrons emitted from spacecraft surface, causing fluctuations in spacecraft potential. We experimentally investigate such photoelectron-mediated spacecraft potential fluctuations. The photoelectric charge of a spacecraft model is found to increase with increasing applied electric field as more photoelectrons escape the spacecraft model surface and dissipates with a decrease in the electric field through collection of ambient plasma electrons. When the applied electric field is driven to oscillate at a frequency lower than the response frequency of the spacecraft model, the surface potential follows the electric field oscillations. The spacecraft model maintains an approximately constant potential if the electric field oscillations are driven at a much higher frequency. When a high-frequency electric field modulated by a low-frequency envelope is applied, rectified oscillations in the potential of the spacecraft model are observed. Our experimental results indicate that photoelectron-mediated wave rectifications must be taken into account when spacecraft potential fluctuations are used to infer plasma density structures.

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