Comparative ionospheres: Terrestrial and giant planets

Abstract

a b s t r a c tThe study of planetary ionospheres within our solar system offers a variety of settings to probe mechanisms of photo-ionization, chemical loss, and plasma transport. Ionospheres are a minor component of upper atmospheres, and thus their mix of ions observed depends on the neutral gas composition of their parent atmospheres. The same solar irradiance (x-rays and extreme-ultra-violet vs. wavelength) impinges upon each of these atmospheres, with solar flux magnitudes changed only by the inverse square of distance from the Sun. If all planets had the same neutral atmosphere-with ionospheres governed by photochemical equilibrium (production = loss)-their peak electron densities would decrease as the inverse of distance from the Sun, and any changes in solar output would exhibit coherent effects throughout the solar system.Here we examine the outer planet with the most observations of its ionosphere (Saturn) and compare its patterns of electron density with those at Earth under the same-day solar conditions. We show that, while the average magnitudes of the major layers of molecular ions at Earth and Saturn are approximately in accord with distance effects, only minor correlations exist between solar effects and day-to-day electron densities. This is in marked contrast to the strong correlations found between the ionospheres of Earth and Mars. Moreover, the variability observed for Saturn's ionosphere (maximum electron density and total electron content) is much larger than found at Earth and Mars. With solar irradiance changes far too small to cause such effects, we use model results to explore the roles of other agents. We find that water sources from Enceladus at low latitudes, and 'ring rain' at middle latitudes, contribute substantially to variability via water ion chemistry. Thermospheric winds and electrodynamics generated at auroral latitudes are suggested causes of high latitude ionospheric variability, but remain inconclusive due to the lack of relevant observations.

Jupiter's Enigmatic Ionosphere: Electron Density Profiles From the Pioneer, Voyager, and Galileo Radio Occultation Experiments

“…The radar on the Mars Reconnaissance Orbiter also provided global information about TEC at Mars (Campbell & Watters, 2016; Mendillo et al., 2017). TEC has also been studied at Saturn (Mendillo et al., 2018) and Venus (Mendillo et al., 2020) using height‐integrated ROX profiles.…”

“…This approach addresses the question if the few days of observations at Jupiter happened to be unusual periods of solar behavior—a question easily answered using terrestrial observations. This “comparative ionospheres” validation exercise was used successfully in our earlier study of Saturn's ionosphere (Mendillo et al., 2018).…”

“…TEC has also been studied at Saturn (Mendillo et al, 2018) and Venus (Mendillo et al, 2020) using height-integrated ROX profiles.…”

“…For Saturn, the situation is very different. At best, the application of Equation yields a CC ∼ 0.50 (Mendillo et al., 2018). The reason for “poorly behaved” layers at Saturn is that the PCE conditions are affected by non‐local processes, in particular, large influxes of water and other external contaminants that modify atmospheric chemistry (Moore et al., 2018).…”

Mars’ plasma system. Scientific potential of coordinated multipoint missions: “The next generation”

“…1 for the Mars case; e.g. [98,101]). At Mars, although still at a preliminary stage, this is a growing topic thanks to missions such as Mars Express and MAVEN, but it requires a longer and more exhaustive global coverage of observations.…”