Scalings for the Alfvén-cyclotron instability: Linear dispersion theory and hybrid particle-in-cell simulations

Abstract

The Alfvén-cyclotron instability is driven by an ion temperature anisotropy such that T⊥/T|| >1 where ⊥ and || denote directions perpendicular and parallel to a uniform background magnetic field Bo, respectively. The computations presented here consider a model of a magnetized, homogeneous, collisionless plasma. Two representations of the proton velocity distribution are considered: a single bi-Maxwellian and a magnetospheric-like configuration of two components, a more dense, relatively cool, isotropic component and a less dense, relatively hot, bi-Maxwellian component which drives the instability. Only wave propagation parallel to Bo is considered. Using numerical solutions of the full kinetic linear dispersion equation, concise analytic expressions for the scaling of the dimensionless maximum instability growth rate and the corresponding dimensionless real frequency are derived as functions of three dimensionless variables: the hot proton temperature anisotropy, the relative hot proton density, and the hot proton β||. Furthermore, using one-dimensional hybrid particle-in-cell simulations of this same instability, a third relation for the scaling of the maximum amplitude of the dimensionless fluctuating magnetic field energy density is derived.