Jupiter's Enigmatic Ionosphere: Electron Density Profiles From the Pioneer, Voyager, and Galileo Radio Occultation Experiments
M. Mendillo, C. Narvaez, Luke Moore, Paul Withers
Abstract
Abstract Radio occultation experiments on the Pioneer and Voyager missions obtained the first seven electron density profiles N e ( h ) of Jupiter's ionosphere. We use the five complete N e ( h ) observations to assess patterns and processes linked to photo‐chemical‐equilibrium (PCE) theory, modeled previously to be the domain below ∼1,000 km. We find that the N e ( h ) profiles are highly structured and identification of the maximum electron density and its height do not follow PCE expectations for layers produced by the Sun's soft X‐rays and extreme ultraviolet. Pre‐dawn profiles often show larger electron densities than dusk‐side profiles, inconsistent with simple chemical decay throughout nighttime. We examined total electron content (TEC) values, defined as N e ( h ) integrated up to a 3,500 km height, and found statistically significant TEC correlations (correlation coefficient ∼0.85) with solar fluxes over solar cycle time scales. The subsequent set of 25 N e ( h ) profiles obtained during the Galileo mission confirmed all of the variability patterns found by Pioneer and Voyager. Most notable was a weaker solar cycle pattern for TEC. Yet, different solar cycle characteristics during the three missions cannot explain their different values for TEC. Average N e ( h ) profiles from the early missions (P10‐11; V1‐2) revealed a three‐layer system that was confirmed by average Galileo results. Models using faster electron‐ion recombination caused by vibrationally excited H 2 converting atomic ions to molecular ions could lead to enhanced removal of plasma near ∼1,000 km, and thus the topside layer formation that often appears at ∼1,500 km, while XUV radiation likely produces the two lower layers in the PCE domain.