Investigating the role of Radial Diffusion on the Ultrarelativistic Electron Acceleration in the Outer Radiation Belt Using ULF Wave-Driven Flux Oscillation Observations

Abstract:

Since the discovery of the radiation belts in 1958, a fundamental unanswered question has been how radiation belt electrons can be accelerated to MeV energies. There are two main acceleration mechanisms that have been proposed: inward radial diffusion and local acceleration. More specifically, discerning the effect of inward radial diffusion on the ultrarelativistic electron acceleration, especially regarding its dependence on electron energy, is the focus of this presentation. We set out to quantify the role of radial diffusion on the energy-dependent acceleration of ultrarelativistic electrons by focusing on the periodic flux oscillations of these particles. These flux oscillations are typically driven by ultra-low frequency (ULF) waves and up until now radial diffusion has been characterized by directly measuring these waves. However, there are problems associated with this method: 1) There is limited spatiotemporal coverage of in-situ measurements of electric and magnetic fields, making it difficult to accurately estimate the ULF wave azimuthal wave number m, and 2) Observationally it is very difficult to differentiate between the azimuthal electrostatic field and any induced electric field resulting from the compressional perturbations of the magnetic field. An alternative way to characterize radial diffusion is to measure electron flux oscillations which are a unique identifier of this acceleration mechanism and are capable of being measured due to the Van Allen Probe’s superior energy resolution. In this study, using energetic particle and field data from the Van Allen Probes, we utilize this new method to investigate the role of radial diffusion in ultrarelativistic electron flux variations in the center of the outer belt. Event studies are carried out using multipoint observations, and the quantitative relation between electron flux oscillations and ULF wave power are derived. The results contribute to a novel approach to quantify the radiation belt electron radial diffusion and help to gain a more in-depth understanding of radiation belt electron dynamics.