Charges can appear on spacecraft surfaces via various processes including: conduction, irradiation, ionization and polarization. Even ambient charged particles with very low energy can a priori deposit charge on material surfaces. Therefore, surface charging usually involves the ambient space plasma which has typical ion and electron temperature ranging from 0,1 eV (in the ionosphere) to several keV in the aurora or the plasma sheet (see ECSS‑E‑ST‑10‑04).
The charge deposition rate on the surface due to the low energy ambient charged particles has been explained in the clause C.3.
Ionization is due to the higher energy part of the space radiation spectrum (UV, soft X-ray and charged particles with energy of the order of 100 eV or above).
Charging levels have been observed in a range between tens of volts positive to tens of kilovolts negative.