Modelling the Io Plasma Torus and Application to the Variability of the Io Footprint Position observed by Juno-JIRAM

Jupiter hosts very intense auroral emissions, which originates from various magnetospheric processes. One of these emissions is associated with the orbital motion of the innermost Galilean satellite Io, which orbits at ~5.9 R_J from Jupiter’s centre (1 R_J = 71492km). At that distance, the magnetospheric plasma is forced to corotation by the strong planetary magnetic field. Therefore Io, which orbits at a slower speed than the corotating plasma, is continuously swept by both the plasma and the Jovian magnetic field. The relative velocity between Io and the plasma triggers a perturbation that propagates along the magnetic field lines and towards the ionosphere as Alfvén waves. Along their way, the Alfvén waves can accelerate electrons into the planetary atmosphere, where they ultimately generate an auroral emission called the Io footprint. The position of the Io footprint depends on the speed of the Alfvén waves, which in turn depends on the magnetic field geometry and magnitude as well as on the plasma mass distribution around Io, whose sulfur-dioxide-rich atmosphere constantly supply a dense cloud of plasma around Jupiter, called the Io Plasma Torus.

In 2016, Juno reached the Jupiter system and, since then, the Jovian InfraRed Auroral Mapper (JIRAM) has been observing the infrared emission associated with the Io footprint with a spatial resolution of ~ few tens of km/pixel. Thanks to the high resolutions of JIRAM, we report evidences of variability in the Io footprint position that are not related to the System III (i.e: the frame corotating with Jupiter) longitude of Io. Using a model for the plasma distribution of the Io Plasma Torus and the magnetic field, we quantitatively determine the state of the plasma distribution corresponding the JIRAM observations. This is the first attempt to retrieve quantitative information on the torus variability by using the Io footprint position. The best-fit plasma density and temperature are consistent with previous observations and analysis of the Io Plasma Torus from the Voyager 1, Voyager 2, Cassini, Galileo and Hisaki spacecrafts. Besides, we found that both density and temperature can exhibit remarkable non-System III variability, which can be ascribed either to local time asymmetry of the plasma in the Io torus or to temporal variation in the torus mass loading.