Unconventional Surface Charging Within Deep Cavities on Airless Planetary Bodies: Particle-In-Cell Plasma Simulations

Surface charging properties of a nonconducting surface that has a deep cavity and is in contact with the solar wind plasma are investigated by means of the particle-in-cell plasma simulations. The modeled topography is an abstract representation of deep depressions found on the irregular surfaces of solid planetary bodies. The simulations have revealed unconventional charging features in that the cavity bottom is charged up to positive values even without any electron emission processes such as photoemission, provided that the surface location is accessible to a portion of incoming solar wind ions. The major driver of the positive charging is identified as drifting ions of the solar wind plasma, and an uncommon current ordering where ion currents exceed electron currents is established at the innermost part of the deep cavity. This also implies that the deep cavity bottom may have a positive potential of several hundred volts, which is similar in magnitude to the kinetic energy of an ion drift motion. The present study also clarifies the role of photoelectrons in developing the distinctive charging environment inside the cavity. The photoemitted electrons can no longer trigger positive charging at the cavity bottom, but rather exhibit the effect of relaxing positive potentials caused by the solar wind ions. The identified charging process, which is primarily due to the solar wind ions, is localized at the depths of the cavity and may be one possible scenario for generating intense electric fields inside the cavity.