The Io, Europa and Ganymede auroral footprints at Jupiter in the ultraviolet: Positions and equatorial lead angles

Jupiter's satellite auroral footprints are a consequence of the interaction between the Jovian magnetic field with co-rotating iogenic plasma and the Galilean moons. The disturbances created near the moons propagate as Alfvén waves along the magnetic field lines. The position of the moons is therefore “Alfvénically” connected to their respective auroral footprint. The angular separation from the instantaneous magnetic footprint can be estimated by the so-called lead angle. That lead angle varies periodically as a function of orbital longitude, since the time for the Alfvén waves to reach the Jovian ionosphere varies accordingly. Using spectral images of the Main Alfvén Wing auroral spots collected by Juno-UVS during the first 43 orbits, this work provides the first empirical model of the Io, Europa, and Ganymede equatorial lead angles for the northern and southern hemispheres. Alfvén travel times between the three innermost Galilean moons to Jupiter's northern and southern hemispheres are estimated from the lead angle measurements. We also demonstrate the accuracy of the mapping from the Juno magnetic field reference model (JRM33) at the completion of the prime mission for M-shells extending to at least 15 R_J. Finally, we shows how the added knowledge of the lead angle can improve the interpretation of the moon-induced decametric emissions.