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 ∼
1.06 RJ 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 ∼3200 to 8000 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 fpe and
whistler-mode hiss which has an upper frequency limit of fpe in
Jupiter’s strongly magnetized inner magnetosphere. The electron plasma
frequency provides the electron number density. Over the ∼59 perijoves
analyzed to date, peak densities range from ∼100 to 80,000 cm-3. More
recent perijoves reveal topside ionospheric peaks at latitudes greater
than about 40°. The density profiles can be highly variable from one
perijove to the next. And, there can be deviations from simple smooth
increases and decreases with altitude within individual ionospheric
passes. Spatial variations may be responsible for some of the
variability, perhaps related to Jupiter’s complex, higher order magnetic
field. We show the variation in ionospheric density profiles and the
distribution of peak densities as a function of latitude and System III
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.