Probing Jupiter’s Inner Radiation Belts: Multi-Timescale Variability Revealed by Juno

Jupiter hosts the most intense electron radiation belts in our solar system—so energetic that relativistic electrons emit synchrotron radiation detectable at radio wavelengths. However, direct in-situ measurements of these belts are exceptionally challenging: the high fluxes of instrument-penetrating (ultra)relativistic electrons interfere with particle detectors, masking key information on energy spectra and angular distributions. As a result, remote sensing of synchrotron radiation has long been the primary method for probing the inner belts, with temporal variations in the observed radio emissions offering critical insight into the physical mechanisms shaping this extreme environment. Such variations have been observed on timescales ranging from days to years and have been linked to both external drivers—such as solar wind pressure changes, solar EUV control of Jovian upper atmosphere dynamics, and cometary impacts—and internal processes, including pitch-angle scattering by moons like Amalthea.

A fundamental limitation of synchrotron observations, however, is their integrated nature: even when spatial resolution is achieved, the signal combines emissions from a wide range of magnetic distances and electron energies along the line of sight. This complicates efforts to localize variability and disentangle overlapping physical processes. The Juno mission, with more than 65 close passes through Jupiter’s radiation environment between 2016 and 2025, presents a unique opportunity to complement remote observations with in-situ measurements of the same system. Although Juno's instruments (JADE, JEDI) cannot resolve spectra of >1 MeV electrons in the synchrotron-emitting region, the intensity of those instrument-penetrating electrons serves as a reliable proxy for tracking their content at different distances.

By subtracting a long-term average belt model from individual Juno passes, we extract the residual signal and track its temporal variations as a function of L-shell. This analysis reveals two dynamically distinct regions, separated near the orbit of Amalthea (L ~2.3), with largely decoupled variability. Both regions exhibit recurring quasi-periodic variations on ~100-day timescales, though often out of phase. In the outer belts (L > 2.3), this mid-term modulation is superimposed on a longer-term trend that develops into a flux minimum around the time of the solar minimum. The long-term perturbation propagates inward from Io’s orbit (L ~6) toward Amalthea over roughly one year. Surprisingly, a comparison with synchrotron belt variations observed by the Goldstone-Apple Valley Radio Telescope (GAVRT) shows a stronger correlation with the outer belt region, despite the inner belts dominating the synchrotron signal. These findings highlight the value of long-term, multi-instrument monitoring and provide a framework for interpreting remotely observed radiation belts in extrasolar systems, such as brown dwarfs.