Juno reveals the exquisite finesse of Jupiter's auroras
Two international research teams, including Denis Grodent, Jean-Claude Gérard and Bertrand Bonfond, researchers at the Laboratory for Planetary and Atmospheric Physicis (STAR Research Unit), analysed and compared images recorded by JIRAM and UVS instruments on board the JUNO probe. The results show unprecedented details on the interaction between the planet and its moons when auroras appear as well as fundamental discrepancies between the ultraviolet and he infrared aurora. These findings are the subject of two scientific publications in Science and Icarus (1-2).
rbiting Jupiter since July 2016, NASA's Juno mission offers us a unique view of the planet. The poles of Jupiter, like those of Earth, are crowned by polar auroras. At first glance, Jupiter's aurorae have a similar morphology to that of terrestrial aurorae. Most of the emissions take the form of rings enclosing each of the magnetic poles. On Earth as on Jupiter, the brightness of the auroras is due to the impact of charged and highly energetic particles with the upper atmosphere.
One of the particularities of the Jupiter auroras is the presence of luminous traces directly linked to the position of its four main moons : Io, Europe, Ganymede and Callisto. These moons form an obstacle that disrupts the flow of particles trapped in Jupiter's magnetic field. This disturbance propagates along the magnetic field and ends up in the atmosphere of the planet in the form of light traces. A complex set of reflections and electron projections from one hemisphere to the other sometimes multiplies these footprints.
Io and Ganymede trace whirls in Jupiter aurora
The JIRAM (Jovian InfraRed Auroral Mapper) instrument is an imager and spectrometer from the Italian Space Agency whose main purpose is to probe the upper layers of Jupiter's atmosphere. It has an extraordinary resolution, making close-up views possible and has uncovered unexpected features in these prints. JIRAM observed that Io's footprint consisted of a series of small swirling spots about 100 km apart. "No model predicted this kind of structure, but these shapes seem to indicate that turbulence plays a major role in the interaction between particles trapped in Jupiter's magnetic field and its moons," explains Bertrand Bonfond, FNRS scientific collaborator at the Laboratory for Planetary and Atmospheric Physics (LPAP).
Illustration of Jupiter's aurora and Io's footprint on the planet as seen by the Hubble Space Telescope. The insert on the right shows the details revealed by the Italian JIRAM camera on board the Juno probe (NASA). Appropriations: A. Mura-JIRAM/IAPS/NASA
After a few thousand kilometres, this series of spots gives way to two parallel arcs that still persist several hours after the passage of the moon Io. "Again, it's a surprise," says Denis Grodent, a researcher at LPAP and Director of the STAR Institute. We currently have no idea what is causing this duplication of Io's auroral trail. »
The spots of Ganymede's footprint are also full of surprises. Once seen up close, those that appeared to be simple spots turn out to be pairs of spots. Here, the explanation seems more obvious and these small spots seem to be the images of the beginning and the end of Ganymede's mini-magnetosphere. Indeed, Ganymede is the only satellite in the solar system with its own magnetic field and therefore has its own magnetosphere. Europe's and Callisto's footprints are much less bright and therefore more difficult to observe. They will be the subject of future studies, when more data become available.
Surprising comparisons between ultraviolet and infrared auroras
A second study, conducted by the same team of researchers, simultaneously observed Jupiter's auroras in the ultraviolet (UVS imager) and infrared (JIRAM). The images taken by the JIRAM instrument have a spatial resolution approximately 10 times higher than that in the ultraviolet. The first images of auroras captured in both domains simultaneously, published in the journal Icarus (2), made it possible to observe the structure of these auroras. The auroras of Jupiter have filamentous structures several tens of thousands of kilometres long and only 100 km wide. Although at first glance the morphology appears similar in both wavelengths, there are differences in the intensity distribution between the different components. LPAP researchers attribute these to the mechanism of production of the two emissions (one direct, the other resulting from a chemical reaction) and the role played by methane. Indeed, methane - a minority constituent of the Jovian atmosphere - is notably involved in controlling the quantity of electrically charged molecules that are at the origin of the infrared aurora.
Images of the northern polar aurora of Jupiter observed simultaneously by Juno in the ultraviolet (left) and infrared (right). The infrared view offers a better resolution of the auroral arc structure. Despite the similarity of the two images, there are differences in the intensity distribution between the emission zones. These make it possible to estimate the depth of the emission into the atmosphere.
Ultraviolet images can be used to estimate the heating of the atmosphere linked to the auroras, and images from JIRAM can be used to estimate its cooling by infrared emission. "However, the two terms are almost never equal, notes Jean-Claude Gérard, researcher at LPAP and member of the JIRAM team. Other processes, yet to be discovered, are therefore involved in the heat balance. »
Jupiter's poles therefore still have many surprises in store for us.
(1) A. Mura, A. Adriani, J. E. P. Connerney, S. Bolton, F. Altieri, F. Bagenal, B. Bonfond, B.M. Dinelli, J.-C. Gérard, T. Greathouse, D. Grodent, S. Levin, B. Mauk, M.L. Moriconi, J. Saur, J. H. Waite, Jr., M. Amoroso, A. Cicchetti, F. Fabiano, G. Filacchione, D. Grassi, A. Migliorini, R. Noschese, A. Olivieri, G. Piccioni, C. Plainaki, G. Sindoni, R. Sordini, F. Tosi, D. Turrini (2018). Juno observations of spot structures and a split tail in Io-induced aurorae on Jupiter, Science
(2) Gérard, J. C., Mura, A., Bonfond, B., Gladstone, G. R., Adriani, A., Hue, V., Dinelli, B.M. , Greathouse, T.K., Grodent, D., Altieri, F., Moriconi, M. L., (2018). Concurrent ultraviolet and infrared observations of the north Jovian aurora during Juno's first perijove, Icarus, 312, 145-156.
This article has been selected to illustrate the cover of Icarus magazine
Laboratory for Planetary and Atmospheric Physicis – LPAP I STAR Research Institute
Dr Bertrand BONFOND - b.bonfond@uliège.ac.be - +32 4 366-9772
Pr Jean-Claude GÉRARD - jc.gerard@uliège.ac.be - +32 4 366-9775
Pr Denis GRODENT - d.grodent@uliège.ac.be - +32 4 366-9773