I got a chance to read the Io footprint paper that was published on Saturday and made the space news rounds yesterday. The article is titled, "UV Io footprint leading spot: A key feature for understanding the UV Io footprint multiplicity?", and was published in the journal Geophysical Research Letters (Geophys. Res. Lett. or GRL) by Bertrand Bonfond et al.
The Io footprint is an auroral glow at Jupiter's north and south polar regions that is generated by infalling ions traveling along Alfvén waves between Io and Jupiter. This connection between Io and Jupiter is an Alfvén wing known as the Io Flux Tube. This connection is bent with respect to the magnetic field lines depending on Io's position in the plasma torus as Alfvén waves travel slower through dense plasma (like in the torus). So if Io's is at its furthest point in its orbit above the plasma torus (the torus is tilted with respect to Io's orbit by 7 deg., as is Jupiter's magnetosphere), the waves travel essentially along the magnetic field lines to the north polar Io footprint since it travels through less plasma, while they are significantly bent as they travel to the south polar footprint. Also, the auroral emission at the north polar footprint is brighter than in the south. The reverse is true if Io is at its furtherest point below the torus in its orbit.
The authors looked at observations of the Io footprint in the UV from the Hubble telescope acquired between 1997 and 2007. They noted several spots in addition to the main Io footprint. In most cases, they resolved fainter spots trailing the main bright spot, and in some cases, a faint spot leading the main spot. They found that when Io is at its highest point above the torus in its orbit, the spots trailing the Io footprint in the north were at their brightest and there was a spot leading the Io footprint in the south. They found the reverse when Io was at its farthest point below the plasma torus; there were multiple bright spots trailing the main footprint in the south, and a leading spot in the north.
They interpret this data to indicate that the leading spot is the result of a connection along magnetic field lines between the brighter Io footprint and the other spot near the other pole (remember the fainter spot also trails the brighter footprint because the Alfvén waves that make it have to travel through more plasma in the plasma torus). The secondary spots trailing the brighter Io footprint are created by a connection along magnetic field lines from the fainter Io footprint to the brighter one. Since the fainter one trails, the waves will impact Jupiter's upper atmosphere behind the brighter spot.
A pretty interesting paper with an elegant model. Since I am not a plasma physicist, I can evaluate the model to see whether it passes the non-sense test, but as a non-specialist, it seemed to make sense to me.
ADDED: I replaced the original image in this post with a cartoon schematic illustrating the various connections that create the Io footprint. The blue connections are the Alfvén wings between Io and Jupiter. The connection to the south pole basically follows the magnetic field lines between Io and Jupiter because it doesn't encounter much plasma (in the cartoon, Io is near its southernmost point in its orbit so most of the Io plasma torus is above it). The connection to the north is bent as the Alfvén waves are slowed by the plasma torus. The red connections are the magnetic field lines that connection the Io footprint in one pole to the other pole.
Link: UV Io footprint leading spot [www.agu.org]