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[PPS09-11] Photoelectron dynamics and associated charging characteristics evolved on the rough surfaces of the Moon
Keywords:Moon, Solar Wind Plasma, Surface charging, Particle simulation, Photoelectron
The surfaces of the Moon and other solid celestial bodies with thin atmospheres and no intrinsic magnetic field are directly bombarded by space plasma particles such as solar wind. As a result, the sunlit surface of the celestial body becomes electrically charged due to the accumulation of charge on the surface by colliding plasma and the outflow of photoelectrons from the surface. Such surface charging is the main factor that determines the electrostatic environment near celestial surfaces. In general, space plasma has the ability to negatively charge solid surfaces, and it has been believed that electron emission processes such as the photoelectric effect are essential to maintain the lunar surface at a positive floating potential. In fact, orbital observations by lunar explorers have suggested that the lunar nocturnal surface is positively charged. On the other hand, the lunar surface has various spatial-scale irregularities ranging from topographic features such as craters, longitudinal pores, and boulders to small regolith particles. Several simulation studies have shown that these irregularities limit free plasma motion in space and create a unique electrostatic environment due to the surface topography.
We have conducted simulations of the variation of the charging characteristics of the lunar surface due to its surface topography, using a cavity with an aperture as a simplified model, and assuming a situation where solar wind plasma pours down from the sky. The simulation results show that the charging characteristics depend on both the cavity shape and the cavity size. In particular, it was found that for cavities with apertures less than the Debye length, the solar wind plasma flow forms a positive potential in a simple rectangular cavity and can positively charge the cavity to several 100 V, which is equivalent to the kinetic energy of solar wind ion particles, as the width-depth ratio of the cavity increases [1]. This result suggests that very strong electric fields can be formed in small voids, such as between rocks and regolith, which may drive the electrostatic flight of regolith, reflect protons due to the electric field formed, and break down insulation in the voids.
In this study, we focus on the contribution of photoelectrons generated by the photoelectric effect to these surface topography-dependent charging properties. In general, photoelectrons have two effects: one is their contribution to positive charging due to the outflow of negative charge from the surface, and the other is potential difference relaxation due to the migration of the outflowing electrons along the electric field between surfaces. The aforementioned intracavity charging due to ion currents creates an electric field inside the cavity, which affects the relaxation effect due to photoelectrons. This study provides important insights into the prediction of charging effects on the actual day-side lunar surface.
[1] Nakazono, J., and Y. Miyake. 2023. "Unconventional Surface Charging within Deep Cavities on Airless Planetary Bodies: Particle-in-cell Plasma Simulations" Journal of Geophysical Research. Planets 128 (2). https://doi.org/10.1029/2022je007589.
We have conducted simulations of the variation of the charging characteristics of the lunar surface due to its surface topography, using a cavity with an aperture as a simplified model, and assuming a situation where solar wind plasma pours down from the sky. The simulation results show that the charging characteristics depend on both the cavity shape and the cavity size. In particular, it was found that for cavities with apertures less than the Debye length, the solar wind plasma flow forms a positive potential in a simple rectangular cavity and can positively charge the cavity to several 100 V, which is equivalent to the kinetic energy of solar wind ion particles, as the width-depth ratio of the cavity increases [1]. This result suggests that very strong electric fields can be formed in small voids, such as between rocks and regolith, which may drive the electrostatic flight of regolith, reflect protons due to the electric field formed, and break down insulation in the voids.
In this study, we focus on the contribution of photoelectrons generated by the photoelectric effect to these surface topography-dependent charging properties. In general, photoelectrons have two effects: one is their contribution to positive charging due to the outflow of negative charge from the surface, and the other is potential difference relaxation due to the migration of the outflowing electrons along the electric field between surfaces. The aforementioned intracavity charging due to ion currents creates an electric field inside the cavity, which affects the relaxation effect due to photoelectrons. This study provides important insights into the prediction of charging effects on the actual day-side lunar surface.
[1] Nakazono, J., and Y. Miyake. 2023. "Unconventional Surface Charging within Deep Cavities on Airless Planetary Bodies: Particle-in-cell Plasma Simulations" Journal of Geophysical Research. Planets 128 (2). https://doi.org/10.1029/2022je007589.