Alfvénic Acceleration Sustains Ganymede's Footprint Tail Aurora

Integrating simultaneous in situ measurements of magnetic field fluctuations, precipitating electrons, and ultraviolet auroral emissions, we find that Alfvénic acceleration mechanisms are responsible for Ganymede's auroral footprint tail. Magnetic field perturbations exhibit enhanced Alfvénic activity with Poynting fluxes of ~100 mW/m². These perturbations are capable of accelerating the observed broadband electrons with precipitating fluxes of ~11 mW/m², such that Alfvénic power is transferred to electron acceleration with ~10% efficiency. The ultraviolet emissions are consistent with in situ electron measurements, indicating 13 ± 3 mW/m² of precipitating electron flux. Juno crosses flux tubes with both upward and downward currents connected to the auroral tail exhibiting small-scale structure. We identify an upward electron conic in the downward current region, possibly due to acceleration by inertial Alfvén waves near the Jovian ionosphere. In concert with in situ observations at Io's footprint tail, these results suggest that Alfvénic acceleration processes are broadly applicable to magnetosphere-satellite interactions.