Jupiter may possess polar auroras similar to those on Earth, but the emissions coming from the auroras on Jupiter have characteristics that continue to surprise scientists. One of the special characteristics of the Jovian auroras is that some of Jupiter’s satellites are able to cause auroral spots on the planet surface. The 2007 observation program using the Hubble telescope revealed the existence of a faint secondary spot not far from the primary spot connected with the satellite Io, but located in a place no model had predicted. Bertrand Bonfond has imagined a different scenario that is capable of accounting for the presence of the small spot that is sometimes in front of and sometimes behind the main spot.

We could certainly say that 2007 was a good year for the observation of auroras on Jupiter from the Hubble Space Telescope. In fact the American space probe New Horizons, en route to Pluto, passed close to Jupiter in 2007. Astronomers wanted to take advantage of this opportunity to observe Jupiter from two places at once, from the passing space probe and from the vicinity of Earth through the Hubble telescope. Researchers from the Laboratory for Atmospheric and Planetary Physics of the University of Liège were also able to benefit from the expanded program of observations by Hubble: “…a normal program of observations consists of a half-dozen Hubble orbits, but in 2007 we got 80, all for the observation of auroras on Jupiter,” said Bertrand Bonfond, researcher and doctoral candidate at the Laboratory (LPAP).

The phenomenon of auroras is well known on Earth, where it appears as magnificent curtains in the sky, blue, red, and green, visible in high latitudes. On Earth polar auroras are caused by the collision between solar winds and Earth’s atmosphere. Auroras are observed on other planets, but the mechanisms involved in their formation may differ.

Under the direction of Professor Gérard, the planetologists of LPAP have been studying the phenomenon of auroras on Jupiter and Saturn using Hubble-produced images for the last 15 years. They have been able to show, for example, that the role of solar winds in the formation of auroras on Jupiter is very small. Their origin is principally internal within the Jovian system, and connected to the Jupiter-Io system in particular.

The system formed by the planet Jupiter and one of its satellites, Io, is extremely unusual. Jupiter is the largest planet in the solar system, and it has the most powerful magnetic field. Io has 200 volcanoes in permanent eruption, and is the most volcanic satellite in the solar system. These volcanoes project a ton of matter every second into space, and when this material becomes ionized, it gets trapped by the magnetosphere of Jupiter. Next, it is fed into a plasma torus (ring) that is concentrated in the vicinity of Io’s orbit around Jupiter (think of a large doughnut lying around the planet). This torus is spun around by Jupiter’s magnetic field at a speed four times that of Io itself: the magnetic field itself completes a revolution in ten hours, while it takes Io 42 hours to revolve around the planet. In addition, since Jupiter’s magnetic axis is tilted relative to its axis of rotation, the plane of the plasma torus is tilted in relation to the orbital plane of Io. Thus Io is not always situated at the center of the torus, but is always drifting upward and downward within the torus. The interaction between the magnetosphere of Jupiter and the plasma thrown off by Io is responsible for the auroras observed in Jupiter’s upper atmosphere, whereas on Earth this is the result of interaction between the magnetosphere and the solar wind. As on Earth, auroral emissions appear simultaneously at both poles.