Electron Acceleration to MeV Energies at Jupiter and Saturn

Abstract

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.

Spectral Signatures of Adiabatic Electron Acceleration at Saturn Through Corotation Drift Cancelation

“…However, the nonconstant μ c shown in Figures and does not imply that the acceleration process is nonadiabatic. The spectral cutoff across different L shells does not evolve from a single part of the electron distribution function, as initially assumed by Paranicas et al () and Kollmann et al ().…”

“…shows E c increases with decreasing L shell, agreeing withParanicas et al (2010) andKollmann et al (2018). Median values of E c (black solid line) follow E CDR (shown for two different pitch angles) both in absolute value and trend.…”

“…They presented that Z mode and chorus waves could accelerate electrons to megaelectron volt energies inside 7 R S through cyclotron resonant interactions. Up to now, which acceleration mechanism dominates is debated, just like the discussions on Earth (Green & Kivelson, 2004; 10.1029/2019GL084113 Horne et al, 2005; Thorne et al, 2013;Zong et al, 2009) and Jupiter (Horne et al, 2008;Kollmann et al, 2018;Woodfield et al, 2014).…”

Evidence of Europa Neutral Gas Torii From Energetic Sulfur Ion Measurements

“…In this paper, we use the unidirectional count rates obtained by EPD-CMS, that we convert to unidirectional differential fluxes with the geometric factors and energy passbands provided by Kollmann et al (2018). Following Kollmann et al (2016), we focus in this paper on particles coming from directions that have an angle with the plasma flow of at least 60°.…”

Jovian electrons in the inner heliosphere

“…Because the existence of the Jovian source seems to be linked to the planet's extended and strong magnetic field, the question as to whether these processes also apply close to Saturn is also relevant (see e.g Lange & Fichtner 2008). Recently, Cassini measurements (as reported by Palmaerts et al 2016;Roussos et al 2016) revived this discussion and support, together with Galileo data, a dominant role of adiabatic processes within both planets' magnetospheres in order to accelerate MeV electrons (see Kollmann et al 2018, and references therein).…”