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

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 B o , respectively. Kinetic linear dispersion theory predicts that, if the ratio of the electron plasma frequency ω e to the electron cyclotron frequency Ω e 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 B o 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 δ E ∥ , which heats the electrons leading to the formation of a substantial suprathermal component in the electron parallel velocity distribution.