Whistler anisotropy instability at low electron β: Particle-in-cell simulations

Abstract

The whistler anisotropy instability is studied in a magnetized, homogeneous, collisionless plasma model. The electrons (denoted by subscript e) are represented initially with a single bi-Maxwellian velocity distribution with a temperature anisotropy T⊥e/T∥e>1, where ⊥ and ∥ denote directions perpendicular and parallel to the background magnetic field Bo, respectively. Kinetic linear dispersion theory predicts that, if the ratio of the electron plasma frequency ωe to the electron cyclotron frequency Ωe is greater than unity and β∥e≥0.025, the maximum growth rate of this instability is at parallel propagation, where the fluctuating fields are strictly electromagnetic. At smaller values of β∥e, however, the maximum growth rate shifts to propagation oblique to Bo and the fluctuating electric fields become predominantly electrostatic. Linear theory and two-dimensional particle-in-cell simulations are used to examine the consequences of this transition. Three simulations are carried out, with initial β∥e=0.10, 0.03, and 0.01. The fluctuating fields of the β∥e=0.10 run are predominantly electromagnetic, with nonlinear consequences similar to those of simulations already described in the literature. In contrast, the growth of fluctuations at oblique propagation in the low electron β runs leads to a significant δE∥, which heats the electrons leading to the formation of a substantial suprathermal component in the electron parallel velocity distribution.