Dissociative Recombination in Cometary Ionospheres

Abstract

Ionospheres are formed throughout the solar system by the ionization of the neutral constituents of the upper atmospheres of the planets, satellites, and comets. Ionization processes include photoionization by solar extreme ultraviolet (EUV) radiation and impact ionization by superthermal particles such as photoelectrons and energetic auroral electrons. Ion-neutral reactions are important for altering the chemical composition of an ionosphere, and often transform atomic ions into molecular ions. Dissociative recombination (DR) of molecular ions with thermal electrons is the main loss process for ionospheric plasma (see the review by Fox1) and is a key process in determining ion and electron densities. The dynamics and energetics of the ionospheric plasma are also strongly affected by DR, mainly due to the density effect but secondarily due to its role in the collisional source terms associated with the momentum and energy conservation equations for the plasma. The role of DR in cometary ionospheres will be emphasized in this paper.