Hybrid fluid‐particle simulation of whistler‐mode waves in a compressed dipole magnetic field: Implications for dayside high‐latitude chorus

Abstract

In this work we present a methodology for simulating whistler‐mode waves self‐consistently generated by electron temperature anisotropy in the inner magnetosphere. We present simulation results using a hybrid fluid/particle‐in‐cell code that treats the hot, anisotropic (i.e., ring current) electron population as particles and the background (i.e., the cold and inertialess) electrons as fluid. Since the hot electrons are only a small fraction of the total population, warm (and isotropic) particle electrons are added to the simulation to increase the fraction of particles with mass, providing a more accurate characterization of the wave dispersion relation. Ions are treated as a fixed background of positive charge density. The plasma transport equations are coupled to Maxwell's equations and solved in a meridional plane (a 2‐D simulation with 3‐D fields). We use a curvilinear coordinate system that follows the topological curvature of Earth's geomagnetic field lines, based on an analytic expression for a compressed dipole magnetic field. Hence, we are able to simulate whistler wave generation at dawn (pure dipole field lines) and dayside (compressed dipole) by simply adjusting one scalar quantity. We demonstrate how, on the dayside, whistler‐mode waves can be locally generated at a range of high latitudes, within pockets of minimum magnetic field, and propagate equatorward. The obtained dayside waves (in a compressed dipole field) have similar amplitude and frequency content to their dawn sector counterparts (in a pure dipole field) but tend to propagate more field aligned.

Particle Dynamics in the Earth's Radiation Belts: Review of Current Research and Open Questions

“…For instance, Omura et al (2009) provide the comparison between a hybrid and a full computation in which the energetic and cold components of electrons are treated as particles. Hybrid codes are used to investigate the self‐consistent generation of whistler waves in the inner magnetosphere, such as the nonlinear generation and growth mechanisms of chorus waves (e.g., Katoh & Omura, 2004, 2006, , 2013; Wu et al, ; da Silva et al, ) and EMIC waves (e.g., Hu & Denton, ; Hu et al, ; Denton et al, , in this collection). These methods have significant potential.…”

Particle‐in‐Cell Experiments Examine Electron Diffusion by Whistler‐Mode Waves: 2. Quasi‐Linear and Nonlinear Dynamics

“…Most of the works that consider numerical experiments of nonlinear wave-particle interactions consider (i) the (in some respects) more physically realistic scenario of background magnetic fields with curvature (or at least spatial gradients) and hence bounce motion and bounce averaging (e.g., see Gan et al, 2020;Silva et al, 2017;; (ii) test particle codes for which the wave spectrum is exactly specified (e.g., see Mourenas et al, 2018;Silva et al, 2018;Tao, Bortnik, et al 2011); and (iii) interactions with chorus wave packets (e.g., see Artemyev et al, 2018;Vainchtein et al, 2018). In this work we intentionally study a scenario that may be considered more basic in some respects.…”

Test‐Particle Simulations of Linear and Nonlinear Interactions Between a 2‐D Whistler‐Mode Wave Packet and Radiation Belt Electrons

“…In our hybrid model the electromagnetic fields are evaluated from electron MHD equations (da Silva et al, , equations (1)–(3)). The electric field is obtained from a simplified Ohm's law involving the cold electron population velocity , which predicts that the parallel electric field E w, q is very small, even when waves are propagating with a finite wave‐normal angle.…”

“…These input parameters are representative of observations (see, e.g., Fu et al, ). The detailed initial conditions of the simulations were presented in a previous publication (da Silva et al, , Table 1 and Figure 4).…”