The electron component of the cometary plasma is not a simple Maxwellian due to the several electron sources including the shocked and unshocked solar wind, secondary electrons produced by the impact of solar wind electrons and photoelectrons. In each case, collisions with the neutral gas of the cometary coma modifies the electrons as they travel along magnetic field lines.
We model the electron population using methods described by Gan et al. (1990). Gan constructed a model of the spatial and energy distributions of electrons in the vicinity of comet Halley. The method involves the calculation of suprathermal electron fluxes using a two-stream transport technique which included electron impact cross sections for rotational, vibrational, and electronic excitation of water molecules. Their model also calculated electron temperatures for the thermal electron population and found that cold, "ionospheric" electrons should indeed be present for cometocentric distances less than a few thousand km.
The Rosetta target comet, 67P/Churyumov-Gerasimenko, even near perihelion, is a considerably weaker comet than Halley was in 1986. Nonetheless, we have undertaken electron modeling for this comet near perihelion using the methods described in Gan et al. (1990). In our model, the electron transport is constrained by the magnetic field topology which we have extracted from the perihelion results of the MHD model discussed above.
For the results presented here, the field line shape is approximately parabolic over the region of the coma where the calculation occurs. In principle, the model can be used for any field line which threads the coma, however below we present results from a representative field line which crosses the Sun-comet line at 50km upstream of the nucleus.
Gan, L. and Cravens, T.E. 1990, J. Geophys. Res., 95, 6285