Non-monotonic potentials above the day-side lunar surface exposed to the solar radiation

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• Potential above the day-side lunar surface in different plasma environments is found.
• The solar wind velocity is taken into account in the electron distribution.
• For any photoelectron density the surface potential is minimal for a slow solar wind.
• In the plasma sheet the lunar potential depends strongly on the plasma temperature.

Basic equations describing stable non-monotonic altitude profiles of the electric potential arising near the Moon׳s surface due to the joint action of solar ultraviolet radiation and interactions with the plasma environment are obtained for two cases: the surface is in the solar wind and the surface is exposed to the terrestrial plasma sheet. The influence of the solar wind on the non-monotonic potential is investigated in a wide range of drift velocities for different values of the photoelectron density. It is found that for any photoelectron density the surface potential reaches its minimum value for a slow solar wind. This effect is most pronounced for the lunar regolith regions not enriched with hydrogen. When the Moon is exposed to both solar radiation and the terrestrial plasma sheet, the surface potential and the potential minimum are calculated as functions of the ion and electron temperatures for different values of the photoelectron density. It is shown that both potentials depend strongly on the temperature of plasma sheet populations, particularly in the range where the ratio of the ion temperature to the electron temperature is less than three.