Negative ions in comets

The primary negative ion sources in comets are shown to be: for the inner coma—both polar photodissociation of HCN, electron attachment of OH and collisions with alkalis; in the vicinity of the nucleus—plasma, excavated during interplanetary dust impacts on the nucleus; for both the contaminated solar wind region and sporadic discharges in the non-homogeneous inner coma plasma—dissociative electron attachment and charge inversion during keV positive ion scattering by cometary dust are also significant sources. Negative ion abundance for Halley's Comet has been estimated to be from 10^–6 to 10^–10 of electron densities. However, this ratio may be more due to the formation of clusters A^–(H₂O)_n. Some possible cometary plasma effects, caused by negative ions, have also been discussed.