A New Approach to Constructing Models of Electron Diffusion by EMIC Waves in the Radiation Belts

J P J Ross,E E Woodfield,C E Watt,S A Glauert,R B Horne,N P Meredith

doi:10.1029/2020gl088976

Abstract

AbstractElectromagnetic ion cyclotron (EMIC) waves play an important role in relativistic electron losses in the radiation belts through diffusion via resonant wave‐particle interactions. We present a new approach for calculating bounce and drift‐averaged EMIC electron diffusion coefficients. We calculate bounce‐averaged diffusion coefficients, using quasi‐linear theory, for each individual Combined Release and Radiation Effects Satellite (CRRES) EMIC wave observation using fitted wave properties, the plasma density and the background magnetic field. These calculations are then combined into bounce‐averaged diffusion coefficients. The resulting coefficients therefore capture the combined effects of individual spectra and plasma properties as opposed to previous approaches that use average spectral and plasma properties, resulting in diffusion over a wider range of energies and pitch angles. These calculations, and their role in radiation belt simulations, are then compared against existing diffusion models. The new diffusion coefficients are found to significantly improve the agreement between the calculated decay of relativistic electrons and Van Allen Probes data.

Highlights

The electrons that make up the outer electron radiation belt exist over a wide range of energies, extending up to well over 10 MeV, where curvature and gradient drift dominates and they complete closed paths around the Earth
In this paper we have presented a new approach for calculating, using quasi-linear theory, average diffusion coefficients representing the diffusion of electrons by Electromagnetic ion cyclotron (EMIC) waves by averaging over observation-specific diffusion coefficients
In our EMIC diffusion model, we are assuming that electrons interact with EMIC waves sufficiently frequently on the diffusion timescale for an average representation to be appropriate

Summary

Introduction

The electrons that make up the outer electron radiation belt exist over a wide range of energies, extending up to well over 10 MeV, where curvature and gradient drift dominates and they complete closed paths around the Earth. Studies using helium band EMIC waves with Gaussian spectra with central frequencies of 0.9f cHe, where f cHe is the helium ion gyrofrequency, have been found to improve the agreement between radiation belt models of the ≳1 MeV electron flux and observations (Drozdov et al, 2017; Ma et al, 2015). These new diffusion coefficients are compared to existing models using average wave and plasma properties.

EMIC Wave Spectra

Diffusion Coefficients

Effects of EMIC Waves on Electron Fluxes

Discussion

Data Availability Statement