Determining the composition of Jupiter’s energetic ion environment: some preliminary results based on the analysis of Galileo/HIC observations

Jupiter’s radiation belts represent one of the most extreme particle environments in the solar system—not only due to their intense fluxes of energetic electrons and protons, but also because they host a diverse, high-flux population of heavy ions with energies extending well above 5 MeV/nucleon - a critical energy range, not accessible by Juno or future missions like JUICE. Understanding the composition and distribution of these ions is essential for constraining the sources, acceleration, and loss processes of charged particles across a mass and energy range not typically accessible in other planetary magnetospheres.
Different ion species act as powerful tracers of magnetospheric dynamics. For instance, ions of solar origin—such as carbon, neon, or silicon—can indicate periods of enhanced solar wind penetration, which may challenge the paradigm of Jupiter being dominated by internal processes. Conversely, ions generated internally, such as those from Io’s volcanic output, may trace moon-magnetosphere interactions processes and internal acceleration mechanisms. Beyond studying particle processes, high-energy heavy ions could also play a role in altering the surfaces and forming exospheres of Jupiter’s moons through implantation and sputtering and are therefore critical to understand the evolution of these materials. They also pose risks to spacecraft via single event upsets—making their detailed mapping important for mission planning and radiation shielding design.
The first and so far only comprehensive investigation of Jupiter’s energetic heavy ion composition above 5 MeV/nucleon was performed by NASA’s Galileo mission (1995–2003), using the Heavy Ion Counter (HIC). HIC consisted of two particle telescopes, LET-B and LET-E. LET-B was designed to measure ions with Z ≥ 6 in the ~5–25 MeV/nucleon range using a stack of four solid-state detectors within a 25° conical aperture. LET-E had a similar configuration, but with five detectors and a broader angular acceptance (25°–45°), optimized for ions between 15-50 MeV/nucleon. LET-E also included a channel for instrument-penetrating >50 MeV/nucleon ions, with a broader 75° effective aperture.
Previous HIC studies primarily focused on the dominant energetic ion species—carbon (C), oxygen (O), and sulfur (S). However, HIC was capable of resolving additional elements, including nitrogen (N), neon (Ne), sodium (Na), magnesium (Mg), silicon (Si), and trace species with Z > 16, which have received limited attention to date. While earlier work presented small samples of such detections, their occurrence, variability, and significance have not been systematically explored.
In our ongoing investigation, we are analyzing the full HIC dataset from the Galileo mission to characterize Jupiter’s energetic ion composition, with an extra focus on these lesser-studied species. We present preliminary results on their energy spectra and relative abundances, along with their spatial (radial) distributions. Our goal is to resolve variations that may reflect different source regions or mechanisms, essentially assessing whether these species are of external (e.g., solar) or internal (e.g., Iogenic) origin.
 
These findings offer new constraints on the nature and variability of Jupiter’s heavy ion environment, supporting the development of improved radiation belt models and informing the interpretation of current in-situ observations from missions such as Juno. They are particularly complementary for future missions like ESA’s JUICE, which lacks an instrument equivalent to HIC. Our results may help refine expectations of the energetic heavy ion environment that drives surface modification processes on Jupiter’s Galilean moons.