Surface charging phenomena on the lunar surface are significantly influenced by topographical features such as craters, boulders, and cavities. This study employs Particle-in-Cell (PIC) simulations to explore how the size of surface cavities affects charging under typical solar wind conditions. Our results show that cavities smaller than the lunar sheath thickness develop strong positive potentials at the depths due to ion currents. However, as cavity size increases beyond the sheath thickness, the influence of ion currents is reduced, resulting in a more moderate potential change inside the cavities. This transition is driven by a shift from surface-charge-dominated ($\sim\!size^2$) to space-charge-dominated ($\sim size^3$) electrostatic structures, as larger cavities allow for greater electron inflow and contribution of the space charge. These findings suggest that both macroscopic and microscopic surface irregularities need to be evaluated according to their spatial scale when considering a global charging environment, which would be significant for understanding dust transport and potential breakdown processes.

Surface charging properties of a non-conducting 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 intended with a portion of irregular surfaces found on 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 cavity bottom surface may have a positive potential of several hundred volts, corresponding to the kinetic energy of the ions. 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 are primarily due to the solar wind ions, are localized at the depths of the cavity and may be one possible scenario for generating intense electric fields inside the cavity.