Role of auroral and photoelectrons on the abundances of methane and ammonia in the coma of Comet Halley

Abstract

A comparative study of ionization rates due to (i) solar EUV radiation, (ii) photoelectrons, and (iii) auroral electrons, is carried out on the ionosphere of Comet Halley for various ionic species. It is found that, in the vicinity of the ionization peak, the photoelectron impact ionization rate is higher than the photoionization rate. However, at all radial distances auroral electron impact ionization is always greater than either of the other two. Coupled continuity equations for chemical steady state conditions are solved to compute the densities of water group ions (H₃O⁺, H₂O⁺, OH⁺, H⁺), ammonia group ions (NH⁺₄, NH⁺₃, NH⁺₂, NH⁺), and methane group ions (CH⁺₅, CH⁺₄, CH⁺₃, CH⁺₂). Ratios of masses (19/18, 17/18, 15/18) are determined and compared with Giotto ion mass spectrometer (IMS) data to derive the relative abundances of NH₃ and CH₄ in the coma of Comet Halley. The effects of different ionization sources, with varying NH₃ and CH₄ abundances, on the ratio profiles and on the ion-electron densities are studied in detail. To find a reasonable fit to all of the Giotto measured ratio data, about 1.5 ± 0.1% NH₃ and 0.5 ± 0.1% CH₄ (relative to H₂O) are required in "normal" ionization conditions. In "enhanced" ionization conditions (when the auroral electron source is added to the solar EUV and photoelectron sources of ionization) a smaller concentration of NH₃ is required. However, the Giotto IMS data (because of large error bars) do not provide sufficient resolution to distinguish the effects of enhanced ionization in terms of the required abundances of NH₃ and CH₄. The photoelectron ionization source is found to play a significant role in determining the ion densities in cometary ionospheres. We have also examined the effect of using different sets of dissociative recombination rates, taking a constant electron temperature profile (T_e = 340.0 K), including an additional loss mechanism for H₃O ions, and using different dissociative ionization cross sections in e-H₂O collisions.