Electron Densities in Jupiter's Upper Ionosphere Inferred From Juno Plasma Wave Observations

Abstract

Juno's highly eccentric polar orbit takes it to perijove distances of $\sim$1.06 $R_J$ on each orbit. For the first perijove, this occurred just north of the jovigraphic equator, but has precessed north by about a degree per orbit over the mission. Minimum altitudes vary from $\sim$3,200–8,000 km through the mission. The Waves instrument observes a number of plasma wave modes in and near the non-auroral ionosphere that provide information on the local electron number density, including electron plasma oscillations that occur at the electron plasma frequency $f_{pe}$ and whistler-mode hiss which has an upper frequency limit of $f_{pe}$ in Jupiter's strongly magnetized inner magnetosphere. The electron plasma frequency provides the electron number density. We present electron densities in the topside ionosphere, similar to Earth's F2 layer, from the $\sim$59 perijoves analyzed to date. Peak densities range from $\sim$100 to 80,000 ${\text{cm}}^{-3}$ at latitudes up to $\sim 40°$. The density profiles can be highly variable from one perijove to the next. And, there can be deviations from simple smooth variations with altitude within individual ionospheric passes. Spatial variations may be responsible for some of the variability, perhaps related to Jupiter's complex magnetic field. We show the variation in ionospheric density profiles and the distribution of peak densities as a function of latitude and longitude as well as other geometric parameters. In addition to the complex magnetic field, possible factors affecting ionospheric density variations investigated here are ionospheric dynamos analogous to those at Earth and precipitation of energetic particles.