Electron Acceleration to MeV Energies at Jupiter and Saturn. Issue 11 (9th November 2018)

• Record Type: Journal Article
• Authors: Kollmann, P.; Roussos, E.; Paranicas, C.; Woodfield, E. E.; Mauk, B. H.; Clark, G.; Smith, D. C.; Vandegriff, J.
• Is Part Of: Journal of geophysical research. Volume 123:Issue 11(2018)
• Page Start: 9110
• Page End: 9129
• Publication Date: 2018-11-09
• Subjects: electron radiation belt -- adiabatic heating -- wave acceleration -- radial diffusion -- Galileo/EPD -- Cassini/MIMI; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1029/2018JA025665
• Languages: English
• ISSNs: 2169-9380

Abstract

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. Plain Language Summary: Space is filled with a radiation of protons and electrons moving with almost light speed. While in free space this is cosmic radiation of extreme energies, magnetized planets trap radiation particles of lower energies, typically in the megaelectronvolt range. These are the so called radiation belts that are found, for example, around Jupiter, Saturn, and Earth. All radiation particles initially start out from being almost at rest and are over time accelerated to almost light speed. The physical mechanisms responsible for this are the subject of ongoing research. Here we focus on the processes accelerating electrons around Jupiter and Saturn based on data from the Galileo and Cassini orbiters. During their life, electrons change their orbits around a planet, get closer to the planetary surface, and are exposed to stronger magnetic fields that these planets are producing. It is known that such changes in magnetic field exposure are in principle able to accelerate particles. Here we find that this exposure is indeed the main reason for acceleration of electrons at relatively large distances from Jupiter and Saturn, not other candidate processes that could in principle also have been responsible. Key Point: Electrons in Saturn's and Jupiter's magnetosphere, outside the most intense radiation belts, are accelerated through adiabatic transport