Gyrokinetic electron and fully kinetic ion simulations of fast magnetosonic waves in the magnetosphere

Abstract

Two-dimensional simulations using a gyrokinetic electron and fully kinetic ion (GeFi) scheme are preformed to study the excitation of fast magnetosonic waves in the terrestrial magnetosphere, which arise from the ion Bernstein instability driven by proton velocity distributions with a positive slope with respect to the perpendicular velocity. Since both ion and electron kinetics are relevant, particle-in-cell (PIC) simulations have often been employed to study the wave excitation. However, the full particle-in-cell scheme is computationally expensive for simulating waves in the ion scale because the electron scale must be fully resolved. Therefore, such simulations are limited to reduced proton-to-electron mass ratio (mp/me) and light-to-Alfvén speed ratio (c/vA). The present study exploits the GeFi scheme that can break through these limitations to some extent, so larger mp/me and c/vA can be used. In the simulations presented, the ion Bernstein instability is driven by a proton velocity distribution composed of 10% energetic protons with a shell distribution and 90% relatively cool, background protons with a Maxwellian distribution. The capability of the GeFi code in simulating the ion Bernstein instability is first demonstrated by comparing a GeFi simulation using reduced mass ratio (mp/me=100) and speed ratio (c/vA=15) to a corresponding PIC simulation as well as linear dispersion analysis. A realistic speed ratio (c/vA=400) and a larger mass ratio (mp/me=400) are then adopted in the GeFi code to explore how the results vary. It is shown that, as the increased mp/me and c/vA lead to a larger lower hybrid frequency, ion Bernstein waves are excited at more ion cyclotron harmonics, consistent with the general prediction of linear dispersion theory. On the other hand, the GeFi simulations also revealed some interesting features after the instability saturation, which are likely related to nonlinear wave-wave interactions.