Formation of electron radiation belts at Saturn by Z-mode wave acceleration

E E Woodfield,J D Menietti,S A Glauert,Y Y Shprits,W S Kurth,R B Horne

doi:10.1038/s41467-018-07549-4

Abstract

At Saturn electrons are trapped in the planet’s magnetic field and accelerated to relativistic energies to form the radiation belts, but how this dramatic increase in electron energy occurs is still unknown. Until now the mechanism of radial diffusion has been assumed but we show here that in-situ acceleration through wave particle interactions, which initial studies dismissed as ineffectual at Saturn, is in fact a vital part of the energetic particle dynamics there. We present evidence from numerical simulations based on Cassini spacecraft data that a particular plasma wave, known as Z-mode, accelerates electrons to MeV energies inside 4 RS (1 RS = 60,330 km) through a Doppler shifted cyclotron resonant interaction. Our results show that the Z-mode waves observed are not oblique as previously assumed and are much better accelerators than O-mode waves, resulting in an electron energy spectrum that closely approaches observed values without any transport effects included.

Highlights

At Saturn electrons are trapped in the planet’s magnetic field and accelerated to relativistic energies to form the radiation belts, but how this dramatic increase in electron energy occurs is still unknown
Whilst radial diffusion can result in transport of charged particles towards the high magnetic field strength nearer the planet increasing their energy, wave particle interactions are a local process acting in situ
Z-mode waves are frequently observed inside 4 RS where the combination of low plasma density and higher magnetic field strength resulting from proximity to the planet allows an abundance of Z-mode waves to propagate[14]

Summary

Introduction

At Saturn electrons are trapped in the planet’s magnetic field and accelerated to relativistic energies to form the radiation belts, but how this dramatic increase in electron energy occurs is still unknown. Radial diffusion is usually assumed to be the dominant mechanism accelerating electrons in planetary radiation belts[4]. Whilst radial diffusion can result in transport of charged particles towards the high magnetic field strength nearer the planet increasing their energy (assuming the first two adiabatic invariants are conserved), wave particle interactions are a local process acting in situ. Waves can diffuse electrons in pitch angle (a change in the particle’s velocity vector with respect to the local magnetic field) and in energy resulting in particle acceleration[5] and an increase in the electron flux at higher energies. We show that Z-mode waves propagate much closer to the magnetic field than previously assumed[16] and that O-mode waves are much less effective at accelerating electrons than the Z-mode

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Conclusion