Evolution of the Io footprint brightness I: Far-UV observations

• The ratio of the brightness of the different spots of the Io UV footprint varies with System III.
• Precipitated energy fluxes as high as 2000 mW/m2 have been measured.
• The location of Io in the plasma torus is not the only parameter controlling the spots' brightness; the magnetic field asymmetries also play a key role.
• The brightness variations of the Io footprint's spots on timescales of minutes are ubiquitous.

The Io footprint (IFP) is a set of auroral spots and an extended tail resulting from the strong interaction between Io and the Jovian magnetosphere. For the first time, we present measurements of the brightness and precipitated power for each individual spot, using the image database gathered from 1997 to 2009 with the Hubble Space Telescope in the Far-UV domain. We show that the relative brightness of the spots varies with the System III longitude of Io. Moreover, our novel measurement method based on 3D simulations of the auroral features allows to derive the precipitated energy fluxes from images on which the emission region is observed at a slant angle. Peak values as high as 2 W/m2 are observed for the main spot, probably triggering a localized and sudden heating of the atmosphere. Additionally, strong brightness differences are observed from one hemisphere to another. This result indicates that the location of Io in the plasma torus is not the only parameter to control the brightness, but that the magnetic field asymmetries also play a key role. Finally, we present new data confirming that significant variations of the spots' brightness on timescales of 2–4 min are ubiquitous, which suggests a relationship with intermittent double layers close to Jovian surface.