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.