2:45 PM - 3:00 PM
[PPS06-15] Dependence of solar wind driven charging of the lunar cavity surface on upstream plasma conditions and cavity geometry
Keywords:Solar wind plasma, Lunar, Surface charging, Particle simulation
The lunar surface is directly bombarded by space plasma such as solar wind and sunlight, forming an electrostatic environment near the surface. Satellite observations by lunar explorers have suggested that the lunar day-side surface is positively charged. In general, space plasma has the ability to negatively charge solid surfaces, and it has been known that electron emission processes such as the photoelectric effect are essential to maintain the lunar surface at a positive floating potential. On the other hand, several simulation results have shown that lunar geometry such as craters and vertical holes restrict free plasma motion in space and form a unique electrostatic environment depending on surface topography. Similar to these geometric-scale surface features, microcavities formed by rocks and regolith particles at smaller scales are also interesting targets in terms of mass transport by electrostatic energy. At such spatial scales, the inadequate ability of Debye shielding forms a stronger electrostatic field, which is considered to be one of the key factors in the moving and floating of charged regolith particles.
In this study, we have conducted simulations of a lunar cavity with a vertical side equal to or smaller than the Debye length, with solar wind plasma pouring down from the sky, and analyzed the effect of the cavity on the electrostatic environment. The results show that the solar wind plasma flow forms a positive potential in a simple rectangular cavity and can positively charge the cavity up to several hundred volts, which is equivalent to the kinetic energy of ion particles, as the width-depth ratio of the cavity increases. This is thought to be due to the fact that electrons with directionless motion governed by thermal velocity are trapped at the cavity sides and cause electron depletion at the cavity depth, while ions with high straightforward motion governed by bulk velocity reach the cavity depth sufficiently to realize strong positive charge transport. In addition, the effect of the photoelectric effect of sunlight is stronger in the aspect of negative charge transfer between surfaces than in the aspect of negative charge emission from the surface, and photoelectrons flow into the bottom surface via the side surface, thereby mitigating the solar wind-driven positive charging. The above results indicate that not only the surrounding plasma conditions but also the shape of the cavity itself is important for the surface charging of the cavity.
In this presentation, the mechanism of solar wind-driven high potential formation in the cavity will be explained, followed by a discussion of the upstream plasma conditions and the changes in the charging characteristics depending on the cavity geometry, to clarify the conditions under which the solar wind-driven charging process occurs on the Moon.
In this study, we have conducted simulations of a lunar cavity with a vertical side equal to or smaller than the Debye length, with solar wind plasma pouring down from the sky, and analyzed the effect of the cavity on the electrostatic environment. The results show that the solar wind plasma flow forms a positive potential in a simple rectangular cavity and can positively charge the cavity up to several hundred volts, which is equivalent to the kinetic energy of ion particles, as the width-depth ratio of the cavity increases. This is thought to be due to the fact that electrons with directionless motion governed by thermal velocity are trapped at the cavity sides and cause electron depletion at the cavity depth, while ions with high straightforward motion governed by bulk velocity reach the cavity depth sufficiently to realize strong positive charge transport. In addition, the effect of the photoelectric effect of sunlight is stronger in the aspect of negative charge transfer between surfaces than in the aspect of negative charge emission from the surface, and photoelectrons flow into the bottom surface via the side surface, thereby mitigating the solar wind-driven positive charging. The above results indicate that not only the surrounding plasma conditions but also the shape of the cavity itself is important for the surface charging of the cavity.
In this presentation, the mechanism of solar wind-driven high potential formation in the cavity will be explained, followed by a discussion of the upstream plasma conditions and the changes in the charging characteristics depending on the cavity geometry, to clarify the conditions under which the solar wind-driven charging process occurs on the Moon.