C.8.2 Relationship to surface charging

The distinction between surface and deep-dielectric charging can, on occasions, be unclear since thick dielectrics on the spacecraft surface are subject to charge deposition over a range of depths. In a general analysis of the problem, the charge build-up near the surface and deeper within the material are both considered together. However, there are practical reasons for treating internal and surface charging separately:

• Surface charging is associated with large currents of a low-energy (~ 10 keV and below) plasma population which usually varies on time-scales of minutes. Secondary and photo-emission are major considerations and are often the dominant currents. The time-scale for charging to occur is usually seconds for absolute charging and minutes for differential charging. These time-scales are associated with the time taken for equilibrium between primary and secondary currents to be achieved and the inter-surface capacitative time-constant. These timescales are generally too short for significant charge to pass through the insulator by internal conduction in dielectric materials. Surface charging typically involves conduction in the outermost micron of the material.

• Internal charging is associated with small currents of a higher-energy (> 0,1 MeV) plasma population which usually varies on timescales of hours to days. Photo-emission is not a consideration because the region of interest is not exposed to sunlight. Secondary emission is generally unimportant since secondary emission yields are low at the primary energies concerned. The timescale for charging is often days or longer and is usually determined by the capacitative time-constant across the material. Internally conducted current is a significant contributor to the overall current balance.

Between the energies of surface charging electrons and trapped radiation belt electrons, are electrons that can deposit charges at high rate just below the surface of surface dielectric materials. This can lead to delayed discharges a few hours later.

Surface collected current can, in principle, contribute to the currents flowing within dielectrics and hence to the internal charge state. However, in practice, on the long time-scales associated with internal charging, surface collected current tends to be neutralized by the other surface currents and hence appears only as an effectively grounded surface, from the point of view of internal charging.