Figure 1: Jupiter’s magnetosphere, its moons, and the plasma ring around Io.
Credits: John Spencer (Southwest Research Institute).
Io, a natural satellite of Jupiter, is the most volcanic object in the Solar System. Its intense activity releases large quantities of gas that escape into space. Once ionized, these gases form a dense plasma ring along its orbit, known as the « plasma torus ».
This torus plays a central role in many physical processes within Jupiter’s magnetic environment, called the magnetosphere. It also influences the environment of nearby moons, particularly Europa and Ganymede. Understanding its spatial structure is therefore a key challenge for studying the Jovian system as a whole.
Due to Jupiter’s rapid rotation, the torus should theoretically have a circular shape. However, the presence of an electric field in Jupiter’s near environment exerts forces that distort it. Until now, Earth-based observations had made it possible to estimate the amplitude of this deformation, but not to determine its true direction, since the Earth–Jupiter geometry only provides access to part of the ring.
A new approach using the Juno spacecraft
To overcome these limitations, the team analyzed ultraviolet images acquired between 2016 and 2022 by the UVS instrument aboard the Juno mission, currently orbiting Jupiter (see Figure 2).
The researchers precisely measured a specific Jovian auroral emission directly linked to the interaction between Io and the gas giant. This auroral footprint is produced when electromagnetic waves, generated by the interaction between Io and the plasma torus, propagate along Jupiter’s magnetic field lines, creating luminous signatures in its atmosphere.
The position of this footprint depends on the physical properties of the plasma ring. Scientists therefore performed numerical simulations of these signatures using Juno data to determine the torus structure most consistent with the observations.
A distortion characterized for the first time
Thanks to Juno’s orbital trajectory, the observations covered the plasma ring over 360°, making it possible to unambiguously determine the true orientation of the distortion—something that could not be achieved from Earth. The results confirm that the electric field responsible for the deformation acts in this direction and suggest that it varies over time.
The plasma torus influences many physical phenomena within the Jovian environment. Having a reliable method to measure its structure therefore represents a major scientific advance. The newly developed technique now allows detailed characterization of plasma–satellite interactions.
Beyond Io, this approach can be applied to the study of the plasma environments of Europa and Ganymede, to better understand how their tenuous atmospheres and icy surfaces are shaped by electromagnetic interactions.
Combining these results with observations from the Japanese satellite HISAKI should eventually reveal the detailed dynamic variations of the plasma torus. It will also help prepare the scientific exploitation phase of the European JUICE mission (Jupiter Icy Moons Explorer), which is scheduled to arrive in the Jovian system in 2031.
| Publication |
| These results were published on February 9, 2026, inThe Planetary Science Journal.
Satoh, Hue, Tsuchiya, et al. (2026). Dawn–dusk asymmetry of the Io plasma torus derived from Io’s auroral footprints observed by Juno-UVS. The Planetary Science Journal, 7(2), 34. https://doi.org/10.3847/PSJ/ae3678 |
Figure 2: Left: View of Jupiter’s south pole and its ultraviolet auroras (credits: Bonfond / University of Liège / SwRI / NASA) / Right: Ultraviolet observations made by the Juno mission, showing Io’s auroral footprint (credits: NASA / SwRI / Satoh).
Contact : Hue Vincent, vincent.hue@lam.fr
