Local heating of radiation belt electrons to ultra-relativistic energies

Hayley J Allison,Yuri Y Shprits

doi:10.1038/s41467-020-18053-z

Abstract

Electrically charged particles are trapped by the Earth’s magnetic field, forming the Van Allen radiation belts. Observations show that electrons in this region can have energies in excess of 7 MeV. However, whether electrons at these ultra-relativistic energies are locally accelerated, arise from betatron and Fermi acceleration due to transport across the magnetic field, or if a combination of both mechanisms is required, has remained an unanswered question in radiation belt physics. Here, we present a unique way of analyzing satellite observations which demonstrates that local acceleration is capable of heating electrons up to 7 MeV. By considering the evolution of phase space density peaks in magnetic coordinate space, we observe distinct signatures of local acceleration and the subsequent outward radial diffusion of ultra-relativistic electron populations. The results have important implications for understanding the origin of ultra-relativistic electrons in Earth’s radiation belts, as well as in magnetized plasmas throughout the solar system.

Highlights

Charged particles are trapped by the Earth’s magnetic field, forming the Van Allen radiation belts
Distinguishing the relative importance of local acceleration and inward radial diffusion in energizing radiation belt electrons is possible by considering the radial profile of the electron differential flux divided by the particle momentum squared[8], a quantity known as phase space density
An intense geomagnetic storm occurred on the 9 October 2012, during which the Relativistic Electron–Proton Telescope (REPT)[17] on the NASA Van Allen probes recorded flux enhancements across a range of energies, including at 7.7 MeV (Fig. 1)

Summary

Introduction

Charged particles are trapped by the Earth’s magnetic field, forming the Van Allen radiation belts. By considering the evolution of phase space density peaks in magnetic coordinate space, we observe distinct signatures of local acceleration and the subsequent outward radial diffusion of ultra-relativistic electron populations. By moving into regions of stronger magnetic field, the kinetic energies of the particles are increased This mechanism alone was shown to be insufficient to fully explain observed enhancements in the relativistic electron flux[5]. Distinguishing the relative importance of local acceleration and inward radial diffusion in energizing radiation belt electrons is possible by considering the radial profile of the electron differential flux divided by the particle momentum squared[8] (given in the magnetic coordinates that constrain electron motion: μ, K, and L*—see Supplementary Note 1), a quantity known as phase space density. To date, there have been no observations of local acceleration acting for these ultra-relativistic energies and analysis of phase space density profiles across a range of μ values is required to determine whether local acceleration can generate these populations

Methods

Results

Conclusion