Spotlight on Jupiter’s aurorae

Observations conducted with the help of the Hubble and Hisaki satellites has made it possible to understand the process at work behind the appearance of some aurorae on Jupiter. These aurorae are caused by a redistribution of plasma in a region of the planet's magnetosphere. Researchers have also been able to completely rule out the solar wind as being the cause of these particular aurorae. The dynamics of the aurorae on Jupiter depend on a number of different processes which are often very different to those we can observe on Earth. The mechanisms involved have not yet revealed all their secrets. The singular nature of these mechanisms within the solar system could hide the fact that they are quite common in the wider universe given that the gas giant Jupiter is very similar to the majority of exoplanets that have been discovered to date. Understanding these physical phenomena and differentiating them from what we already know should enable us to develop tools in order to better understand different mechanisms spread throughout the cosmos.

“The isn’t such a thing as “The” aurora at Jupiter, but there are many aurorae.", affirms Bertrand Bonfond, a post-doctoral researcher at the FNRS at the Laboratory for Planetary and Atmospheric Physics of the University of Liege (LPAP), to illustrate the multiplicity of physical mechanisms behind the Jovian aurorae. By way of a reminder, an aurora is a light phenomenon linked to the excitation of molecules or atoms in the upper atmosphere following impacts by charged particles (usually electrons) from the magnetosphere. On Jupiter, this neutral atmospheric gas is Hydrogen (nitrogen and oxygen on Earth). On Earth, the aurora is dominated by a phenomenon of reconnection between the magnetic field of the planet and that of the solar wind. (See articles “A transpolar arc discovered on Saturn” and “Surprise around Jupiter’s aurorae”). The aurorae of the gas giant are mostly linked to an internal process. “The Jovian system also has another very important ingredient, the moon Io which is the most volcanic body in the solar system. This satellite which is constantly experiencing eruptions, releases a large quantity of gas which essentially remains along its orbital path at first and rotates at the same speed as Io around Jupiter. It is then transformed into plasma. Captured by Jupiter’s magnetic field, it is then accelerated until it reaches the rotation speed of the planet.  But this plasma cannot accumulate indefinitely. It will migrate towards the exterior and will finally be released by the magnetosphere." It is the circulation of this plasma in the magnetosphere which causes the electric currents necessary for the formation of a large amount of the Jovian aurorae. But this is not the full story.  

Difficult to know exactly what is happening

The influence of the solar wind remains a great unknown. We know the role the solar wind plays on Earth but it is difficult to assess its role on Jupiter. The gas giant is much further away from the sun so the impression of the solar wind is much weaker there. In addition, Jupiter’s magnetic field is incomparably more intense than Earth’s and the internal phenomena linked to the circulation of plasma dominate. “Having said that, explains the planetologist, “it is not correct to say that the solar wind has no influence even if it is only of secondary importance because when the wind is stronger, it can considerably compress the Jovian magnetosphere and influence how the plasma circulates in its interior”. Based on this observation, the challenge is to differentiate between aurorae linked to the solar wind and those that are not. This is the real puzzle. It is certainly true that there are models which make it possible to estimate the variations in the solar wind on Jupiter and their probable evolutions based on measurements taken on Earth. But the precision of these models is limited and does not provide an understanding of the time delay observed between the estimated time of arrival of an enhancement of the solar wind and the level of intensity of the aurora linked to this enhancement.

The interest of such measurements pertains to compared planetology. The size of Jupiter makes it possible to better distinguish the different mechanisms than on Earth, where everything is mixed together. We can then verify whether the theories that work on our planet also apply elsewhere. With regard to the processes that are to be found only on Jupiter and to a lesser extent on Saturn, it is not just a question of understanding unusual physical phenomena. “Jupiter has a unique profile in our solar system. On the other hand, most of the known exoplanets are gas giants. We are beginning to wonder if, in systems with even more powerful magnetic fields and composed of even more volcanic moons, it might be possible to directly detect auroral emissions in the infrared or ultraviolet. If these processes reveal themselves to be common in the universe, Jupiter becomes a precious tool for understanding the way this works. Given the dimensions and the magnetic fields involved, it is the closest analogy we have at our disposal”. 

The different aurorae of Jupiter

Distinguishing the influence of the solar wind from that of internal processes is not the only challenge. This is because many internal processes are involved in the circulation of plasma in the magnetosphere of Jupiter and each one possesses its own auroral signature. Among the different structures which form the aurorae of Jupiter, the easiest one to identify is in the form of an almost continuous contour known as main emission or main oval. This depends on one of these mechanisms. “Once the gas from Io has been ionized while escaping from the moon, it is captured by Jupiter’s magnetic field and begins to rotate around Jupiter at the same speed as the planet rotates on its axis. This ionized gas rotates around Jupiter 4 times faster than Io. The magnetic tension prevents these particles from being ejected immediately but the centrifugal force still allows the plasma to progressively migrate toward the exterior. The further it migrates, the greater the distance it has to travel in order to complete a full circle. If it stays at the same speed, it does not rotate as fast as Jupiter. In order to maintain what is called the corotation, that is to say, an angular velocity that is equal everywhere, the plasma has to be accelerated”. On Jupiter, the further the plasma moves away, the more it loses its angular velocity. This loss generates a torsion of the magnetic field and the associated electric current, as is always the case in electromagnetism, circulates in a direction such that its effects will be opposite to the cause which gave rise to it in the first place. In simple terms, the current will accelerate the plasma so that it reaches a speed close to that of the corotation again. But another consequence of this intense electric current is the acceleration of charged particles in another place: along the magnetic field lines. When the particles strike Jupiter’s atmosphere, they give rise to the main oval.

It is also possible to detect “small” round spots which are the auroral footprint of Io on Jupiter. Although they do not seem to be very large, the Io footprint reaches the sizeable length of 1000 kilometers. It is also possible to observe bigger structures on the exterior of the main oval whose contours are less clear. This is the region that was studied during this campaign. These aurorae are linked to the redistributions of plasma, to the injection of hot plasma and the ejection of cold plasma (see below). They are therefore also linked to the circulation of plasma from Io but it is not the same mechanism as the one that gives rise to the main oval nor is the same part of the magnetosphere involved. “Moreover, in the center of the oval there are all these extremely dynamic regions. We are trying to establish whether they are linked to the solar wind but we have just begun to study them. Jupiter’s aurorae have not yet revealed all their secrets”.

Two satellites are better than one

At the beginning of 2014, Bertrand Bonfond joined a campaign led by Sarah Badman, of the Department of Physics of the University of Lancaster in England, and Tomoki Kimura, of the Japan Aerospace Exploration Agency (Jaxa). The analyses, recently published in Geophysical Research Letters (1), have made it possible to identify the phenomena of a precise zone of the Jovian aurorae. “The observations were jointly conducted by means of telescopes from two different satellites. We were planning the observations of Hubble and the Japanese team was planning the observations of the Hisaki satellite”, he explains.

Hubble (HST), which is a space telescope developed by NASA and the ESA, is one of the major tools in our quest to conquer the cosmos. Orbiting the Earth for 25 years, it has contributed to a better understanding of the expansion of the universe, the confirmation of the presence of supermassive black holes at the center of certain galaxies or even the existence of dark matter. It also makes it possible to observe the planets of the solar system in high definition. But it is in great demand and is typically used for the study of Jovian aurorae for a few periods of 90 minutes every year (or rather 45 minutes, because the rest of the time the Earth is between Hubble and Jupiter!). Hisaki is much smaller and can only observe in the extreme ultraviolet (where Hubble can observe at wavelengths from infrared to the far ultraviolet), with a more limited spatial resolution. “But it is dedicated to the observation of the interaction between the solar wind and the atmospheres and magnetospheres of the planets of the solar system. In accordance with its position and the alignment of the Earth and Jupiter, it can observe the Jovian aurorae continuously for long periods. For this campaign, Hisaki was able to gather data for the first two months of 2014. ” For two weeks during this period, the European researchers were able to observe Jupiter for 45 minutes every day by means of Hubble. This is a relatively long period for a telescope that is in such high demand.  

The Japanese telescope therefore observed Jupiter almost without interruption, which is unprecedented, but its resolution did not enable a very precise level of analysis. The aurora studied was integrated in a single pixel. It was possible to detect variations in the auroral brightness, but not to determine the most affected regions. This was an advantage of using Hubble but for a much shorter observation time. Combining the data gathered by the two tools, i.e. gathering continuity and high special resolution, led to an understanding of these different phenomena.

The solar wind is ruled out

Up to now, the origin of aurorae located at the exterior of the principal oval was unknown. “We had already observed variations in their brightness and I put forward the theory of an internal origin, that is to say, an intensification of the volcanic activity of Io, which increased the quantity of gas emitted”, recalls Bertrand Bonfond. “This was bound to create more events involving the redistribution of plasma in the Jovian system but this was only one interpretation. We lacked the data which made it possible to verify this theory”. During the two weeks of observation at the beginning of 2014, Earth and Jupiter were aligned in relation to the sun. This was the optimal situation for extrapolating the intensity of the solar wind on Jupiter according to its characteristics as seen from Earth. “For an entire week, the solar wind was very constant on Earth. This was a Godsend. While the variations are difficult to extrapolate with precision we can conclude that if nothing occurred on Earth then nothing occurred on Jupiter either. ” During this period, the researchers located peaks of brightness which were as intense as they were sudden. “ We therefore concluded from this that the solar wind had nothing to do with these auroral intensifications. The process involved was internally driven. In-depth analyses enabled us to confirm that there was a redistribution of plasma. “

The next few years look promising

If analyses of repeated images and the considerable improvement of observation tools have made it possible to understand the previously unpublished physical processes on Earth, there are still a lot of grey areas in relation to Jupiter. “For example, for the events linked to the solar wind, we know that it has a role to play, notably by the compression and release of Jupiter’s magnetosphere. At the same time, the Jovian system is enormous. On Earth, everything happens very quickly. On Jupiter, we do not know the response time of the system in relation to the influence of the solar wind. In theory, the brightness of the principal oval should diminish when the solar wind intensifies. And yet what we observe is the opposite. We think that the chronological order of the different events when the solar wind is enhanced is crucial. Unfortunately, it is exactly this precise timing which eludes us for the moment”, explains Bertrand Bonfond.

Always on the lookout for discoveries and new analyses, the astrophysicist impatiently awaits the year 2016 and the arrival of the space probe Juno around Jupiter. “Juno possesses a magnetometer and will be capable of observing the variations in the solar wind in real time. We have requested more time on the Hubble telescope. We do not yet know if this will be accepted. But for the first time, we will be capable of observing the aurorae of Jupiter while taking account of the solar wind. Observing what happened when the solar wind was stable was one thing. The next stage will be to observe what happens when its speed and pressure vary in a known way”.

(1) Kimura, T., et al. (2015), Transient internally driven aurora at Jupiter discovered by Hisaki and the Hubble Space Telescope, Geophys. Res. Lett., 42, 1662–1668, doi:10.1002/2015GL063272.