Evidence for local acceleration of heavy >10 MeV/n oxygen and sulphur in Jupiter's innermost radiation belts

Jupiter's radiation belts constitute a multi-component system, trapping high intensities of electrons, protons and heavier ions. We revisit measurements from Galileo's Heavy Ion Counter (HIC) instrument, a high-quality dataset that extends considerably the energy range covered by Galileo/EPD and Juno/JEDI (<10 MeV/n) up to ~100 MeV/n, providing key complementary observations for those two instruments in the equatorial radiation belts. Thanks to HIC's large geometry factor and event-based measurement capabilities, the instrument clearly resolves trace ions of both heliospheric and magnetospheric origin, such as Carbon, Nitrogen, Sodium, Magnesium, Iron and others, besides the much more abundant Oxygen and Sulfur. In this work we re-evaluate aspects of HIC's calibration, particularly for the analysis of measurements obtained at the innermost, intense radiation belts of Jupiter, which are currently monitored by Juno. We concentrate on previously unpublished observations from Galileo's last two orbits, reaching inward of Amalthea's orbit, including a close flyby of this moon. We show that the structure and composition of the heavy ion belts depends strongly on energy, L-shell and pitch angle. We find that above 50 MeV/n, Jupiter's heavy ion radiation belts are dominated by oxygen, appearing stable and are highly structured by strong losses at the orbits of Io, Thebe and Amalthea, a structure reminiscent of that observed in Saturn's proton radiation belts. In addition, heavy ion spectra and the corresponding phase space density profiles indicate that a local source of energy exists at least inward of Amalthea, accelerating oxygen above 100 MeV/n and sulphur above ˜50 MeV/n. Between the orbits of Io and Amalthea, PSD profiles indicate contributions from local and adiabatic acceleration for both ion species, with the former dominating at the highest energies resolved in that region (˜50 MeV/n). In conclusion, unlike Earth's radiation belts, where the highest energy protons or ions observed reach the terrestrial magnetosphere pre-accelerated to the MeV range in the form of solar, anomalous or galactic cosmic rays, Jupiter can efficiently accelerate oxygen and sulphur, which originate at at eV energies at Io and its torus, by 7-8 decades in energy.