Abstract
The dipole configuration of the Earth's magnetic field allows for the trapping of highly energetic particles, which form the radiation belts. Although significant advances have been made in understanding the acceleration mechanisms in the radiation belts, the loss processes remain poorly understood. Unique observations on 17 January 2013 provide detailed information throughout the belts on the energy spectrum and pitch angle (angle between the velocity of a particle and the magnetic field) distribution of electrons up to ultra-relativistic energies. Here we show that although relativistic electrons are enhanced, ultra-relativistic electrons become depleted and distributions of particles show very clear telltale signatures of electromagnetic ion cyclotron wave-induced loss. Comparisons between observations and modelling of the evolution of the electron flux and pitch angle show that electromagnetic ion cyclotron waves provide the dominant loss mechanism at ultra-relativistic energies and produce a profound dropout of the ultra-relativistic radiation belt fluxes.
Highlights
The dipole configuration of the Earth’s magnetic field allows for the trapping of highly energetic particles, which form the radiation belts
We report observations of electron flux at different energies and observations of electron pitch angle distributions during 17 January 2013 storm, which are consistent with electromagnetic ion cyclotron (EMIC) wave-induced loss for ultra-relativistic electrons, whereas electrons at relativistic energies are not affected by EMIC wave scattering
After the October storm, the ultra-relativistic remnant belt slowly diffused inward to L between B3 and 4 (L is the distance from the centre of the Earth to a given magnetic field line in the equatorial plane measured in RE), where the ultra-relativistic electrons cannot be significantly affected by the loss to the magnetopause and outward radial diffusion, and scattering by hiss inside the plasmasphere is weak
Summary
Observations during the 17 January 2013 storm. The environment close to the Earth (radial distances o3–6 Earth radii (RE)) is usually occupied by the cold plasma bubble co-rotating with the Earth, which is referred to as the plasmasphere. This loss of ultra-relativistic electrons is not produced by the loss to the magnetopause, as the multi-MeV electron belt is located deep inside the outer zone, below 4.5 RE, whereas the magnetopause for this event was compressed down to 7.1 RE according to an empirical model17( see Supplementary Note 2 and Supplementary Figs 2 and 3) and the variation of the global magnetic field in the inner magnetosphere was not significantly large (see Supplementary Note 3 and Supplementary Fig. 4) Such difference in the evolution of fluxes between 1.02 and 4.2 MeV can be explained by the presence of scattering by EMIC waves that affects only electrons above certain minimum threshold energy, which is the minimum energy for which electrons can resonantly interact with EMIC waves. The modelled pitch angle distributions at energies of 0.46 and 3.4 MeV agree with observations and are presented in Supplementary Fig. 6
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