Longitudinal modulation of the brightness of Io's auroral footprint emission: Comparison with models

The auroral emission close to the foot of Io's magnetic flux tube has been known for over a decade to be one of the key parameters characterizing the electrodynamic interaction between the satellite and Jupiter's magnetosphere. Ten years of observations of Io's magnetic footprint brightness have been conducted via far ultraviolet imaging by two instruments, the Space Telescope Imaging Spectrograph and the Advanced Camera for Surveys, on board the Hubble Space Telescope. The variation of Io's magnetic footprint brightness was found to have a strong correlation with the satellite's location in system III longitude. The persistent pattern of the variation of the brightness of the auroral emission at Io's magnetic footprint with longitude over 10 years of observations implies that the footprint emissions are primarily determined by processes other than temporal variations of the plasma torus. The changing location of Io in the plasma torus with longitude corresponds to changes in centrifugal latitude and to the plasma density in the vicinity of Io, likely affecting the electrodynamic interaction at the satellite. To test this, quantitative models of electron density and the generated power near Io are applied to simulate the observed footprint brightness variation pattern. We find, however, that the longitudinal variations in plasma conditions needed to produce changes in the electrodynamic interaction comparable to the observed modulation of the footprint emissions would require an unrealistically colder plasma torus than previously measured. We quantify the additional energy needed to produce the asymmetric emission peaks at 110 degrees and 270 degrees longitudes.