Formation of Butterfly Pitch Angle Distributions of Relativistic Electrons in the Outer Radiation Belt With a Monochromatic Pc5 Wave

Abstract

AbstractRadial transport of relativistic electrons in the inner magnetosphere has been considered one of the acceleration mechanisms of the outer radiation belt electrons and can be driven by the drift resonance with Pc5 ultralow‐frequency waves. In this study, we focus on the pitch angle dependences of the radial transport and investigate the pitch angle distributions (PADs) of the relativistic electrons after interaction with a monochromatic Pc5 wave, using two simulation models of the inner magnetosphere: Geomagnetic Environment Modeling System for Integrated Studies (GEMSIS)‐Ring Current and GEMSIS‐Radiation Belt models. The results show that butterfly‐like PADs (two peaks in small pitch angles) are produced at a fixed radial distance (L‐shell) and energy, where the electrons with small pitch angles satisfy the resonance condition. The radial range and timescale of the butterfly PAD appearance correspond to the maximum radial transport of the electron (i.e., resonant width) and its oscillation period in the radial direction, respectively. We analytically derive the resonant width and oscillation period of relativistic electrons interacting with a monochromatic Pc5 wave, assuming conservation of first and second adiabatic invariants, and compare them with the simulation result. The results show that electrons with small equatorial pitch angles at a fixed magnetic moment can interact with a monochromatic Pc5 wave in a wider L‐shell range than perpendicular electrons due to the pitch angle changes, in the course of the radial transport.