Jovian High-Latitude Ionospheric Ions: Juno In Situ Observations

The low-altitude, high-velocity trajectory of the Juno spacecraft enables the Jovian Auroral Distributions Experiment to make the first in situ observations of the high-latitude ionospheric plasma. Ions are observed to energies below 1 eV. The high-latitude ionospheric ions are observed simultaneously with a loss cone in the magnetospheric ions, suggesting precipitating magnetospheric ions contribute to the heating of the upper ionosphere, raising the scale height, and pushing ionospheric ions to altitudes of 0.5 R-J above the planet where they are observed by Jovian Auroral Distributions Experiment. The source of the magnetospheric ions is tied to the Io torus and plasma sheet, indicated by the cutoff seen in both the magnetospheric and ionospheric plasma at the Io M-shells. Equatorward of the Io M-shell boundary, the ionospheric ions are not observed, indicating a drop in the scale height of the ionospheric ions at those latitudes. Plain Language Summary The Jovian Auroral Distributions Experiment (JADE) ion sensor has made the first in situ observations of the upper, high-latitude ionosphere of Jupiter. Flown on the Juno spacecraft, JADE observes the ionosphere at altitudes of approximately half a Jovian radii, with the spacecraft traveling at the high speed of similar to 50 km/s. For comparison, a proton traveling at 50 km/s has an energy of approximately 10 eV. The combination of the low-altitude and high ram velocity enables JADE to measure ionospheric ions to energies below 1 eV. These observations reveal a cold ionospheric population of protons at high latitudes, seen coincident with precipitating magnetospheric ions. This indicates that the precipitating magnetospheric ions heat the upper ionosphere, raising the height where these protons can be observed. The ionospheric protons are seen in bands in the northern and southern latitudes, bounded on the equator edge by the field lines that connect to Io, and inside the auroral oval to the poleward side.