Relativistic electron pitch‐angle scattering by electromagnetic ion cyclotron waves during geomagnetic storms

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Abstract
During magnetic storms, relativistic electrons execute nearly circular orbits about the Earth and traverse a spatially confined zone within the duskside plasmapause where electromagnetic ion cyclotron (EMIC) waves are preferentially excited. We examine the mechanism of electron pitch‐angle diffusion by gyroresonant interaction with EMIC waves as a cause of relativistic electron precipitation loss from the outer radiation belt. Detailed calculations are carried out of electron cyclotron resonant pitch‐angle diffusion coefficients Dαα for EMIC waves in a multi‐ion (H+, He+, O+) plasma. A simple functional form for Dαα is used, based on quasi‐linear theory that is valid for parallel‐propagating, small‐amplitude electromagnetic waves of general spectral density. For typical observed EMIC wave amplitudes (1–10nT), the rates of resonant pitch‐angle diffusion are close to the limit of “strong” diffusion, leading to intense electron precipitation. In order for gyroresonance to take place, electrons must possess a minimum kinetic energy Emin which depends on the value of the ratio (electron plasma frequency/electron gyrofrequency); Emin also depends on the properties of the EMIC wave spectrum and the ion composition. Geophysically interesting scattering, with Emin comparable to 1 MeV, can only occur in regions where (electron plasma frequency/electron gyrofrequency) ≥10, which typically occurs within the duskside plasmapause. Under such conditions, electrons with energy ≥1 MeV can be removed from the outer radiation belt by EMIC wave scattering during a magnetic storm over a time‐scale of several hours to a day.