The Io torus as observed by the Juno ASC

The Juno spacecraft has been orbiting Jupiter since 2016. The evolution of Juno's orbit allows the later orbits to provide unprecedented insights into the inner regions of the Jovian environment. The Advanced Star Compass (ASC) primarily serves to determine the orientation of the magnetometer. However, the ASC detector is also sensitive to high-energy particles, enabling it to measure the Jovian radiation environment. Specifically, the ASC can detect electrons with energies greater than 15 MeV and protons with energies exceeding 120 MeV.

Jupiter’s moon Io orbits at a distance of approximately 5.9 Jupiter radii. Due to its intense volcanic activity, Io ejects large amounts of ionized gases and dust into space, which form a dense, donut-shaped plasma region around Jupiter known as the Io torus, located along Io’s orbit. Jupiter’s strong magnetic field traps highly energetic radiation environment with the most intense region near Io.

Since its arrival at Jupiter, Juno has completed numerous orbits, traversing multiple longitudinal regions of the Jovian system and with the orbit evolution drift of the line of apsides south, effectively scanning the entire Jovian radiation belts. Specifically, the ASC has consistently recorded variations in radiation levels when Juno crosses magnetic field lines connected to the Io torus, which interacts with it. These systematic variations provide valuable data for understanding the structure and dynamics of this unique plasma environment.

We present a detailed map of the locations where these radiation variations are observed, enabling us to quantify the geometry and spatial distribution of the Io torus relative to Jupiter’s magnetic field. Additionally, we explore possible physical mechanisms driving these observed variations, such as the acceleration and trapping of particles within the torus or their interactions with Io’s volcanic emissions and Jupiter’s magnetospheric processes.