Energetic heavy ions in Jupiter’s innermost radiation belts: revisiting Heavy Ion Counter data from Galileo

Abstract:

Jupiter’s radiation belts constitute a complex, multi-component system, trapping high intensities of electrons, protons and heavier ions. Observations and discoveries by the Juno/JEDI instrument and recent re-analysis of Galileo’s Energetic Particle Detector (EPD) data have put back the spotlight on heavy ions as an important medium to understand the interplay between charged particle acceleration and losses as well as for probing the properties of different elements of the jovian system (planet, moons, neutral tori, rings). That has motivated us to revisit measurements from Galileo’s Heavy Ion Counter (HIC) instrument, a high-quality dataset that, on balance, has received much less attention compared to past observations by Galileo/EPD. HIC measurements partly overlap with the highest energies covered by Galileo/EPD and Juno/JEDI (~<10 MeV/n) but extend up ~100 MeV/n, thus providing key complementary observations for those two instruments. Thanks to its large geometry factor and event-based measurement capabilities, HIC is also able to clearly resolve 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 presentation 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 We concentrate on previously unpublished observations from Galileo’s last few orbits inward of Io’s L-shell. A major finding is that above 40 MeV/n, Jupiter’s heavy ion radiation belts appear very stable and are highly structured by strong losses at the orbits of Thebe and Amalthea, a structure reminiscent of that observed in Saturn’s proton radiation belts. Furthermore, we observe that at the highest energies the HIC signal remains at background levels at Io’s orbit, raising questions about the process which supplies and sustains the large ion intensities closer to the planet. We brainstorm about different possibilities to explain these observations using spectral, composition and pitch angle information provided by HIC.