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-perpendicular to e/T-parallel to e>1, where perpendicular to and parallel to 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 omega(e) to the electron cyclotron frequency Omega(e) is greater than unity and beta(parallel to 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 beta(parallel to 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 beta(parallel to e) = 0.10, 0.03, and 0.01. The fluctuating fields of the beta(parallel to 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 beta runs leads to a significant delta E-parallel to, which heats the electrons leading to the formation of a substantial suprathermal component in the electron parallel velocity distribution. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3610378]