### 9. IONISATION AND EXCITATION BY NON-THERMAL ELECTRONS

In most cases, astrophysical plasmas are modelled using Maxwellian
velocity distributions, even under NEI or PIE conditions. Often this is
justified, but there are situations where deviations from a Maxwellian
distribution occur. Examples are accelerated electrons penetrating
cooler regions, as may occur in or near shocks or flares. We refrain
here from a full discussion, but only mention here that spectral codes
such as SPEX (Sect. 10.1) allow the
calculation of the ionisation balance and the emanating spectrum under
such circumstances, provided the true electron distribution can be
approximated by the sum of non-relativistic Maxwellians. When there are
strong relativistic electron contributions, this approach cannot be used
anymore as here relativistic corrections to the cross sections are
needed, which are usually not taken into account in most plasma codes.