Whistler anisotropy instability with a cold electron component: Linear theory

The whistler anisotropy instability is driven by an electron temperature anisotropy T-perpendicular to/T-parallel to > 1 where perpendicular to and parallel to denote directions perpendicular and parallel, respectively, to the background magnetic field B-o. Here kinetic linear theory in a magnetized, homogeneous, collisionless plasma model is used to study this instability when the electron velocity distribution may be represented as the sum of a hot, anisotropic bi-Maxwellian and a cold, isotropic component. The critical beta(parallel to e), the value at which the maximum growth rate of the instability changes from propagation parallel to B-o to oblique propagation, decreases with increasing n(c)/n(e), where n(c) is the cold electron density and n(e) is the total electron density. At parallel propagation the maximum growth rate increases with n(c)/n(e) up to n(c)/n(e) similar or equal to 0.8, but then diminishes with further increases of the relative cold electron density. Introduction of a cold electron component can reduce the hot electron anisotropy necessary to excite this instability by up to a factor of 2.