Photochemistry in Jupiter’s Ionosphere: Insights from Simultaneous H 3 + and Electron Density Observations during Juno Perijove 54

Abstract Observations of continuous vertical electron and ion distributions at Jupiter were obtained simultaneously for the first time on 2023 September 7 UTC. On that date, Juno performed a radio occultation experiment and, in parallel, JWST and Keck scanned Jupiter’s limb, focused on characterizing H 3 + . Here, using Keck infrared spectra, we derive constraints on H 3 + densities and temperatures. While on-disk column densities are roughly constant with latitude, with a mean value of (5.7 ± 2.1) × 10 15 m −2 , temperatures decrease smoothly from 774 ± 50 K at high latitude to 683 ± 28 K at low latitude. Vertical H 3 + temperatures are comparable to their on-disk values and mostly isothermal. Keck spectra sampled Jupiter’s dawn limb, finding H 3 + volumetric number densities of (1–40) × 10 8 m −3 with peak altitudes near 1500 km. Comparison with the Juno electron altitude density profile confirms Jupiter’s ionosphere is proton dominated. Ionospheric model simulations that can broadly replicate observed plasma densities indicate that field-aligned vertical transport driven by neutral winds is primarily responsible for the observed high-altitude peak. We use model outputs, validated by comparison with plasma observations, to demonstrate that H 3 + is in photochemical equilibrium (PCE) below ∼1600 km altitude in Jupiter’s ionosphere. PCE analysis further reveals that the diurnal peak in N H 3 + can occur before noon, that peak electron densities should generally be