A preliminary consideration of the electron impact ionization effect in cometary comas

From an analysis of the electron number densities and velocity distributions as observed at comet Giacobini-Zinner and comet Halley, it appears that in the so-called "transition region" of the coma electron impact ionization can exceed the nominal photoionization rate of H₂O molecules but not by an order of magnitude. It is possible in localized regions where electron heating by ion acoustic waves and lower hybrid waves occurs, electron impact would become the dominant ionization (and dissociation) mechanism. The overall effect as limited by in-situ measurements of the neutral gas density distribution could be shown to be small, however.