C.8.3 Charge deposition

Internal dielectric charging occurs due to penetrating electrons, generally over 0,1 MeV. These are found in both the inner and outer radiation belt. However, the intensity of the outer belt reaches the highest levels, particularly at higher energies and, in practice, only the outer belt is associated with hazardous levels of internal charging. Although satellites in geostationary orbit are far from the peak of the outer belt, they are subject to continuous exposure and experience a significant risk of internal charging effects. The outer belt is highly dynamic and electron fluxes higher than 2 MeV can rise by two or three orders of magnitude over a period of hours. Such enhancements can persist for several days. There is a solar cycle effect, which means that peak fluxes are usually an order of magnitude higher during the declining phase of the solar cycle than at solar maximum.

Radiation belt electrons penetrate spacecraft surfaces and are deposited within internal materials. Their penetration depth is dependent on their energy and the material properties, particularly density. This can be calculated to good accuracy using Monte Carlo particle transport codes. There are also a number of empirically derived formulae relating range (i.e. penetration depth x material density) to incident electron energy, e.g. Feather (1938) (see [21]), Glendenin & Coryell (1948) (see [22]), Katz and Penfold (1952) (see [23]) and Weber (1964) (see [24]). Comparisons of these formulae (see [20]) showed that they gave very similar results apart from the Feather formula which diverged from the others below about 400 keV i.e. below the energies of main interest.

The formula of Weber giving range (R) as a function of initial electron energy (E) is shown below (E in MeV):

g cm^-2

Excellent agreement between this formula and Monte Carlo simulations using the ITS (see [26]) and GEANT (see [27]) codes has been reported (see [25]).