Energetic neutral atom imaging reveals nearly 11-year cycle of the ring current of Saturn

The drift motion of energetic charged particles can generate an azimuthal electric current around the planet known as the ring current, which regulates the field configuration of the magnetosphere. However, limited coverage of in-situ measurements makes it challenging to investigate the long-term variations of the global ring current. Taking advantage of the energetic neutral atom (ENA) imaging onboard the Cassini mission, we present a nearly 11-year cycle of the suprathermal ring current populations in Saturn's magnetosphere. We find that the peak location of the suprathermal ring current in local time oscillates between post-midnight and pre-midnight sectors, and its intensity minimizes during the solar maximum. These results indicate that the modulation of the suprathermal ring current is closely related to the solar cycle. Our analysis also offers a preview of the ring current at other giant planets, such as Jupiter, which will be imaged by ENA cameras onboard the JUICE mission.

Electron Acceleration to MeV Energies at Jupiter and Saturn

The radiation belts and magnetospheres of Jupiter and Saturn show significant intensities of relativistic electrons with energies up to tens of megaelectronvolts (MeV). To date, the question on how the electrons reach such high energies is not fully answered. This is largely due to the lack of high-quality electron spectra in the MeV energy range that models could be fit to. We reprocess data throughout the Galileo orbiter mission in order to derive Jupiter's electron spectra up to tens of MeV. In the case of Saturn, the spectra from the Cassini orbiter are readily available and we provide a systematic analysis aiming to study their acceleration mechanisms. Our analysis focuses on the magnetospheres of these planets, at distances of L > 20 and L > 4 for Jupiter and Saturn, respectively, where electron intensities are not yet at radiation belt levels. We find no support that MeV electrons are dominantly accelerated by wave-particle interactions in the magnetospheres of both planets at these distances. Instead, electron acceleration is consistent with adiabatic transport. While this is a common assumption, confirmation of this fact is important since many studies on sources, losses, and transport of energetic particles rely on it. Adiabatic heating can be driven through various radial transport mechanisms, for example, injections driven by the interchange instability or radial diffusion. We cannot distinguish these processes at Saturn with our technique. For Jupiter, we suggest that the dominating acceleration process is radial diffusion because injections are never observed at MeV energies.