Linear growth and nonlinear saturation of proton ring-driven instabilities in the inner magnetosphere: Linear theory and PIC simulations

Abstract:

In the inner magnetosphere, the energy-dependent convection of ring current ions can lead to the ring-type proton velocity distributions with ∂f_p(v_perp)/∂v_perp > 0 and ring speeds around the Alfvén speed. This ring-type velocity distribution is known to drive fast magnetosonic waves at propagation quasi-perpendicular to the background magnetic field B₀ and, with sufficient temperature anisotropy, electromagnetic ion cyclotron (EMIC) waves at propagation parallel to B₀. While there is an abundant literature on linear theory and computer simulations of EMIC waves driven by bi-Maxwellian ion distributions, the literature on the instabilities associated with ring-type proton velocity distributions in the inner magnetosphere is less substantial. Even less studied is the interplay of the two instabilities which lead to the growth of EMIC and fast magnetosonic waves, respectively. The goal of this paper is to provide a comprehensive picture of the instabilities responsible for the two types of waves and their interplay in the conditions of the inner magnetosphere, using linear dispersion theory and self-consistent particle-in-cell (PIC) simulations. For systematic analyses, two-component proton distributions f_p = f_r + f_b are used, where f_r represents a tenuous energetic proton velocity distribution with ∂f_r(v_perp)/∂v_perp > 0 providing free energy and f_b represents a dense Maxwellian background with sufficiently small beta corresponding to the inner magnetospheric condition. Both an ideal velocity ring and a partial shell with sinⁿ-type pitch angle dependence will be considered for the f_r component.