Simulation of radiation belt wave-particle interactions in an MHD-particle framework

Abstract

In this paper we describe K2, a comprehensive simulation model of Earth’s radiation belts that includes a wide range of relevant physical processes. Global MHD simulations are combined with guiding-center test-particle methods to model interactions with ultra low-frequency (ULF) waves, substorm injections, convective transport, drift-shell splitting, drift-orbit bifurcations, and magnetopause shadowing, all in self-consistent MHD fields. Simulation of local acceleration and pitch-angle scattering due to cyclotron-scale interactions is incorporated by including stochastic differential equation (SDE) methods in the MHD-particle framework. The SDEs are driven by event-specific bounce-averaged energy and pitch-angle diffusion coefficients. We present simulations of electron phase-space densities during a simplified particle acceleration event based on the 17 March 2013 event observed by the Van Allen Probes, with a focus on demonstrating the capabilities of the K2 model. The relative wave-particle effects of global scale ULF waves and very-low frequency (VLF) whistler-mode chorus waves are compared, and we show that the primary acceleration appears to be from the latter. We also show that the enhancement with both ULF and VLF processes included exceeds that of VLF waves alone, indicating a synergistic combination of energization and transport processes may be important.

The challenge to understand the zoo of particle transport regimes during resonant wave-particle interactions for given survey-mode wave spectra

“…The driftaveraging procedure then further averages these bounce-averaged diffusion coefficients over all magnetic local time. We also note other recently developed approaches to radiation belt modelling that involve the incorporation of test particles into different varieties of global-scale simulation (Desai et al, 2021;Chan et al, 2023;Lukin et al, 2024).…”

On the Two Approaches to Incorporate Wave‐Particle Resonant Effects Into Global Test Particle Simulations

“…A possible solution of this problem was proposed by Elkington et al (2018Elkington et al ( , 2019, who suggest that wave-particle interactions can be incorporated as stochastic perturbations of electron test orbits (see also Chan et al, 2023;Michael et al, 2023). The simplified version of this approach combines test particle equations of motion and continuous stochastic differential equations (SDEs), which are characteristics for the Fokker-Plank diffusion equation (Tao et al, 2008;Zheng et al, 2014).…”

Cross‐Scale Modeling of Storm‐Time Radiation Belt Variability

“…Pitch-angle scattering and energy diffusion were calculated using pre-computed, event-specific bounce-averaged diffusion coefficients from Ma et al (2018). Chan et al (2023) showed local acceleration can result in rapid changes in PSD and, combined with radial diffusion, can produce electron PSD enhancements in the outer radiation belts. However, the bounce-averaged diffusion coefficients used in Chan et al (2023), were computed using a dipolar magnetic field and a static density distribution.…”

“…Chan et al (2023) showed local acceleration can result in rapid changes in PSD and, combined with radial diffusion, can produce electron PSD enhancements in the outer radiation belts. However, the bounce-averaged diffusion coefficients used in Chan et al (2023), were computed using a dipolar magnetic field and a static density distribution. The diffusion coefficients, therefore, did not exhibit a realistic variability due to either the storm-time magnetic field or the cold plasma density, which can significantly affect both the estimated wave power (Longley et al, 2022) and the characteristics of the wave-particle interactions themselves (e.g., Kennel & Petschek, 1966).…”

“…Chan et al. (2023) incorporated the effects of cyclotron‐resonant wave‐particle interactions into their global magnetosphere and test‐particle simulation by using a stochastic differential equation (SDE) to solve the Fokker‐Planck equation (Tao et al., 2008; Zheng et al., 2014, 2021). Pitch‐angle scattering and energy diffusion were calculated using pre‐computed, event‐specific bounce‐averaged diffusion coefficients from Ma et al.…”