Auroral evidence of a localized magnetic anomaly in Jupiter's northern hemisphere

We analyze more than 1000 HST/Advanced Camera for Survey images of the ultraviolet auroral emissions appearing in the northern hemisphere of Jupiter. The auroral footprints of Io, Europa, and Ganymede form individual footpaths, which are fitted with three reference contours. The satellite footprints provide a convenient mapping between the northern Jovian ionosphere and the equatorial plane in the middle magnetosphere, independent of any magnetic field model. The VIP4 magnetic field model is in relatively good agreement with the observed footprint of Io. However, in the auroral kink sector, between the 80 degrees and 150 degrees System III meridians, the model significantly departs from the observation. One possible way to improve the agreement between the VIP4 model and the observed footprints is to include a magnetic anomaly. We suggest that this anomaly is characterized by a weakening of the surface magnetic field in the kink sector and by an added localized tilted dipole field. This dipole rotates with the planet at a depth of 0.245 R-J below the surface, and its magnitude is set to similar to 1% of Jupiter's dipole moment. The anomaly has a very limited influence on the magnetic field intensity in the equatorial plane between the orbits of Io and Ganymede. However, it is sufficient to bend the field lines near the high-latitude atmosphere and to reproduce the observed satellite ultraviolet footpaths. JUNO's in situ measurements will determine the structure of Jupiter's magnetic field in detail to expand on these results.