Magnetosphere-ionosphere-thermosphere at the outer planets

Abstract

Jupiter and Saturn host some of the most brilliant aurorae in the Solar System. Omnipresent, yet variable in intensity and planetographic extent, these emissions are just one sign of magnetosphere-ionosphere-thermosphere (MIT) coupling between giant planets and their surroundings. These rapidly rotating planets are coupled to their surrounding plasma discs through their planetary magnetic fields, which mediate the exchange of angular momentum and energy through a complex system of currents. Lorentz forces, Joule heating, and precipitating particles associated with MIT coupling modify the underlying atmosphere and affect magnetospheric flows. In the magnetosphere, angular momentum drawn from each planet accelerates plasma as it is transported away from its source location, predominately Io at Jupiter and Enceladus at Saturn. Alfvén waves and quasi-static electric fields associated with MIT coupling accelerate particles into the planetary atmospheres, generating auroral emission. Energy is also deposited into the thermosphere through Joule heating associated with corotation enforcement currents. However, questions remain as to how this energy is redistributed across the planets to produce the observed thermospheric temperatures, which exceed predictions by 100s of Kelvin. This tutorial talk reviews the current understanding of MIT coupling at the outer planets, focusing primarily on the Jovian system in light of recent advances in understanding from Juno.