Jupiter presents the brightest auroras in the solar system. One of the unique features of this planet, which it shares with Saturn, is also the presence of auroral emissions caused by three of its largest moons: Io, Europa, and Ganymede.
Artist's representation of Jovian auroras. Height elongation created by artificial intelligence.
© The Watchers - M. Flouriot
These distinct emissions, called "auroral footprints," are locally visible in multiple wavelength ranges. They are created by charged particles, mainly electrons, that propagate along magnetic field lines connecting the moons to Jupiter. As these electrons precipitate into the giant planet's atmosphere, they induce characteristic auroras, which have been studied since the 2000s, notably thanks to observations from the Hubble Space Telescope in the ultraviolet spectrum.
Since July 2016, the Juno probe has been flying over Jupiter's poles at only a few thousand miles (a few thousand kilometers) of altitude, allowing for a detailed characterization of the auroral footprints from the moons. The combined analysis of data obtained on board Juno by the UVS spectrograph and the JADE spectrometer, for which IRAP contributed to the electrostatic optical system, allows for probing both the properties of these emissions and the charged particles inducing them.
Focusing their study on the auroral footprint of Ganymede, the largest moon in the solar system and the only one generating its own magnetic field, a team including scientists from CNRS Earth & Universe, in close collaboration with the Juno mission teams (SwRI, Princeton University), highlighted, among other things, the influence of Ganymede's mini-magnetosphere on its auroral footprint.
They confirmed that the size of flux tubes, these tube-shaped magnetic field lines connecting the moons to Jupiter's atmosphere and through which electromagnetic waves and charged particles propagate, is significantly larger than those reported for Io and Europa by previous studies.
The observations of the auroral footprint by Juno thus provide a new method to study Ganymede's mini-magnetosphere, which will be explored in situ in an unprecedented manner by the ESA's JUICE mission currently en route to Jupiter.
Juno (white orbit on the left) crosses a flux tube from Ganymede. On Jupiter's south pole, the auroral footprints of the three moons are simultaneously observed in ultraviolet by the Juno/UVS instrument and represented here in false colors. These feature two bright spots appearing white, followed by a diffuse emission called the auroral tail.
© CDPP-Inetum / NASA / SwRI / Juno-UVS / ESA / STScI / Jonas Rabia / Vincent Hue
Rabia, J., Hue, V., André, N., Nénon, Q., Szalay, J. R., Allegrini, F., et al. (2024).
Properties of electrons accelerated by the Ganymede-magnetosphere interaction: Survey of Juno high-latitude observations.
Journal of Geophysical Research: Space Physics, 129, e2024JA032604.
https://doi.org/10.1029/2024JA032604.