H3+ in the Jovian ionosphere

Abstract

Recent measurements and theoretical calculations suggest that H3+ ions in their ground vibrational level recombine much more slowly than in the vibrational levels υ ≥ 3. Using these results as a guide, we have investigated the impact of modified H3+ chemistry on the structure of the Jovian ionosphere for both a solar EUV ionization source and a high‐altitude ionization source, associated with the electroglow. The results are sensitive to the value of the H3+ recombination coefficient, k10, adopted for υ = 0. For the solar EUV source of ionization, and a value of k10 = 2 × 10−8 cm³ s−1 equal to the published experimental upper limit, we find that H+ is the major ion at the peak of the ionosphere, similar to previous results. However, given the current uncertainty in k10, an ionosphere dominated by H3+ cannot be ruled out. A value of k10 ∼ 10−10 cm³ s−1 will produce an ionosphere that consists mainly of H3+. By including a high‐altitude source of ionization we find that H3+ is the major component of the upper ionosphere, for all values of k10 less than or equal to the experimental upper limit. Thus, our calculations suggest that the topside plasma scale height measured by the radio science experiments may be indicative of electron temperatures ∼3000–5000 K or greater, consistent with current models for calculation of electron temperature. Our results may also help shed some light on the Voyager low‐energy charged particle observations of H3+ ions in the magnetosphere. We also reemphasize the fact that H2 molecules in the Jovian upper atmosphere must be vibrationally excited.