Hybrid Fluid-Particle Simulation of Whistler-Mode Waves in a Compressed Dipole Magnetic Field: Implications for Dayside High-Latitude Chorus

In this work we present a methodology for simulating self-consistent generation of whistler-mode waves due to electron temperature anisotropy in the inner magnetosphere. We present simulations results using a hybrid fluid/particle-in-cell code [Hu and Denton, JGR, 114, A12217, 2009; Wu et al., JGR, 120, 1908-1923, 2015], that treats the hot, anisotropic (i.e., ring current) electron population as particles and the background (i.e., the cold, 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 and improve the dispersion relation of generated waves. 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. We design a curvilinear coordinate system that follows the topological curvature of Earth's geomagnetic field lines, based on an analytic expression for the compressed dipole magnetic field [e.g., McCollough et al., JGR, 117, A01208, 2012, equations (1)-(5)]. Hence, we are able to simulate whistler wave generation at dusk (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 high latitudes, within pockets of minimum magnetic field [see e.g., Tsurutani and Smith, JGR, 82, 32, 1977], and propagate equatorward. We finally use our simulation results to discuss the differences between dayside and nightside chorus.