A smaller spot was often visible in front of this streak in previous observations from Hubble. To explain its physical origin, the researchers referred to the partial reflection of waves that were responsible for the principal spot at the edges of the torus because of the difference in density between the interior and the exterior of the torus. Since reflected waves travel a greater distance within the torus, they reach the pole later than the direct wave. This explains why the smaller spot is always observed leading the larger spot, and thus in front of the streak.

The extended program for the observation of Jupiter in 2007 also showed that a faint spot could also appear upstream from the main spot. In the preceding scenario, that was impossible, because the reflected plasma wave could not arrive on Jupiter before the direct wave. Bertrand Bonfond devoted his research to the understanding of Io’s auroral footprint on Jupiter. He imagined a new scenario that could account for the presence of spots sometimes upstream, sometimes downstream from the main spot. This scenario was the cover story for the March 16 edition of Geophysical Research Letters***.

 “At first I noticed that the downstream spot appeared in one hemisphere precisely when an upstream spot was present in the opposite hemisphere, “ Bonfond explains. “That suggested the existence of a direct magnetic connection between the auroras at the north pole and those at the south pole. Then I remembered some observations made by the Galileo probe at the end of the 1990s, that had been forgotten since then: Galileo had taken very low-altitude photos of Io that showed not only very impressive views of volcanoes, but also the existence of electrons that went back and forth from one hemisphere of Jupiter to the other, without necessarily reaching the poles. The explanation given at the time was that the waves generated by Io accelerated electrons not only toward Jupiter, which caused the main spot, but also in the other direction, causing the observed bundles of electrons. Now the bundles of electrons are not perturbed when they cross the plasma torus, in contrast to the plasma waves that are slowed by the density of the torus. So I supposed that a part of these electrons could in fact reach the other hemisphere, creating a faint spot. That way, if Io happens to be in the upper (northern) part of the torus, the plasma waves will reach the north pole quickly and form the main auroral spot, while the waves that started off toward the south will be slowed down by having to cross the torus. Result: the bundle of electrons that set off North will arrive in the South before the plasma waves. The faint spot created by the bundle of electrons will thus form upstream from the main spot. This scenario explains the downstream spot as well, which we see in the north. Since the main spot in the south forms further downstream than the north spot, the bundle of electrons that was headed south creates a smaller spot downstream from the main spot in the north.” The relative position of the main and secondary spots depends on the position of Io within the plasma torus. This scenario will be tested when Hubble makes new observations, focusing on configurations that have not yet been observed.

Bertrand Bonfond can give three good reasons for our being interested in studying the electromagnetic interaction between Jupiter and its satellite Io: “At the fundamental level first of all, we are trying to understand the system of Jupiter, and its moons, as surprising as they are fascinating. Next, the interaction between Io and Jupiter is the best example in astronomy of interaction between a body that is a conductor and a body that has a powerful magnetic field. Thus this is a typical case, in which the physics involved can later be applied to other astronomical systems of the same type, but which may be more difficult to observe. This could be the case with an exoplanet around its star, or with binary systems of white dwarf stars. Finally there is certainly an obvious interest in understanding the precise behavior of the Earth’s magnetosphere, because the phenomena that take place there influence everyday life (telecommunication and navigation satellites, networks of electrical current, etc.). Comparing the mechanisms of formation of auroras on other planets allows us to model a phenomenon that is marginal on Earth, and thus difficult to measure; we are able to observe the phenomenon under better conditions somewhere else.”

*** Bonfond B., J.-C. Gérard , D. Grodent, and J. Saur (2007), "Ultraviolet Io footprint short timescale dynamics", Geophysical Research Letters, 34, doi:10.1029/2006GL28765 Read an abstract