The nature of the solar wind interaction with CO2/CO-dominated comets

Abstract

The varying overall nature of the solar wind interaction with the ionospheres of CO and CO2-dominated comets is investigated and compared with previous results for H2O-dominated comets. It is shown that as a comet approaches the sun, it may exhibit one of two types of ionospheric transitions. (In rare circumstances, the cometary ionosphere may display a third type of transition in addition to one of the first two). For both transitions, the ionosphere turns from being hard (in other words, the ionosphere is not susceptible to compression under sudden solar wind pressure increases) to soft. However, for one type of transition, the bow shock changes from being weak (M≈2) to being strong (M≈10), whereas for the other type of transition, the bow shock remains weak. The heliocentric distance at which these transitions may occur is found to be a function of the cometary nuclear radius, the latent heat of sublimation of the surface volatiles, the surface bolometric albedo and the following ionospheric properties: the optical depth, the average ionization time scale and the amount of heat addition. Two important consequences of the strong shocks are the large solar wind velocity modulation of the energization of electrons at the bow-shock and the relatively quick formation of cometary plasma tails.