Surprise discovery related to the polar auroras of Jupiter

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.

Jovian auroras can be broken down into three parts, each having its own mechanism of formation: a main oval area around the magnetic poles, polar emissions that are quite variable, and the footprints of the satellites Ganymede, Europe and Io, located closer to the equator than the main oval. The satellite footprint that is most clearly visible is that of Io. This is formed by a large spot followed by a streak, and sometimes one or more smaller spots appear in front of the streak. The physics of the origin of the main oval is relatively well known, as Bertrand Bonfond explains: “Because of centrifugal force, charged particles within the plasma torus migrate outside the torus and get progressively further away from the planet, which makes necessary a regular increase in their speed in order for them to follow the magnetic field as it rotates every 10 hours. In the beginning they manage it, but at some point they aren’t able to accelerate enough. Electric currents thus are established between the atmosphere of Jupiter and these particles. These currents cause electrons to accelerate along the lines of the magnetic field. Then they strike the upper atmosphere of Jupiter with great speed. The resulting shock causes light to be emitted in the form of an oval aurora appearing at the foot of these currents.”

A scenario was also proposed to model the auroral imprint of Io. The relative movement of Io within the plasma torus also generates a current that circulates in Io and in each of the poles of Jupiter, along the lines of force of the magnetic field. “An analogy will help us understand what is happening, “ Bonfond suggests. “Imagine a rock in the middle of a current of water. The rock will cause waves that will propagate over the surface of the water, and touch the banks downstream from the rock. If the rock is closer to the right bank than the left, the waves that are produced that travel toward the left bank will reach that bank later and further downstream than the waves that were heading toward the right bank (which is nearer). Similarly, since Io is not gravitating around Jupiter at the same speed as the Jovian magnetic field, the satellite is an obstacle for part of the particle flux in the plasma torus, and this creates a perturbation that gets propagated the length of the lines of force of the magnetic field in the form of plasma waves (Alfvén waves), all the way to the poles. Once they get close to Jupiter, these waves accelerate electrons toward the planet, which create the main spot in Io’s footprint. As with the rock in the stream, if Io is closer to the northern part of the plasma torus, the southern spot will form in a leading position, further downstream than the spot in the north.” The leading position of the largest spot can be explained by the fact that the gas that has just been ionized close to Io is still turning at the same speed as Io, and not yet at the speed of the magnetic field. Streaks then form, as in the case of the main oval.

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