Alfvenic 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 Alfvenic acceleration mechanisms are responsible for Ganymede's auroral footprint tail. Magnetic field perturbations exhibit enhanced Alfvenic activity with Poynting fluxes of similar to 100 mW/m(2). These perturbations are capable of accelerating the observed broadband electrons with precipitating fluxes of similar to 11 mW/m(2), such that Alfvenic power is transferred to electron acceleration with similar to 10% efficiency. The ultraviolet emissions are consistent with in situ electron measurements, indicating 13 +/- 3 mW/m(2) 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 Alfven waves near the Jovian ionosphere. In concert with in situ observations at Io's footprint tail, these results suggest that Alfvenic acceleration processes are broadly applicable to magnetosphere-satellite interactions. Plain Language Summary Jupiter's moon Ganymede interacts with the planet's rapidly rotating magnetic field, which generates an aurora in the Jovian upper atmosphere. The Juno spacecraft crossed magnetic field lines connected to this aurora. We found that a specific type of wave, similar to a wave produced when a string is plucked, is responsible for accelerating the electrons sustaining this aurora. This type of interaction between a moon and the planet it orbits is likely a common process occurring at other exoplanetary systems.