C.5.4 Surface emission processes
C.5.4.1. General
Impacting particles or photons with energy of the order of 10 eV or above can induce a significant amount of electron emission.
C.5.4.2. Photo-electron emission
For average effects involving the bulk of the distribution function of photo-electron speed, the distribution can be represented by a Maxwellian of equivalent temperature Tph. [8]. The current leaving the surface A of a spacecraft perpendicular to the sun direction is given by
•
for 
: 
•
for 
:
¾
for a planar source, 
¾
for a point source, 
where jph is the photo-electron density current at saturation.
The source can be considered as planar if the sheath thickness is much less than the curvature radius of the surface and as punctual if the sheath thickness is much larger than the curvature radius of the surface. The saturation photo-electron current, Jph, depends on the material.
In some cases a better approximation is obtained by using a more sophisticated description of the photo-electrons, as for example a bi-Maxwellian distribution [9].
C.5.4.3. Secondary-electron emission
Secondary electrons are also emitted as a result of primary electron and ion radiation in the energy range 10 eV to a few 10s of keV. For spacecraft charging computations it is common practice to use the approach of Katz et al. 1977 (see [6]) to calculate secondary yield.
Secondary emission plays a crucial role in determining whether hazardous levels of charging arise. An example of the dependence of secondary yield on incident particle energy is given in 0. At high energy where yield is below 1, an incident electron causes charging to build up. At lower energies where the secondary yield is greater than 1, the incident electron actually reduces charging. Hence the charging produced depends on the ratio of electrons above and below the point where the yield equals 1. This point is typically in the range of a few keV for most dielectrics.
NOTE: Curve calculated according to the formula of Katz et al. 1977 [6]
Figure C-2: Example secondary yield curve
C.5.4.4. Other surface emission processes
It is important to note that there are other processes that can increase the number of charged particles leaving surfaces, including, charge exchange, backscattering and sputtering and this can significantly affect surface potential levels. All these surface processes can vary strongly from one material to another.