Whistler instability: Electron anisotropy upper bound

If the electron distribution function is approximately bi-Maxwellian with T-perpendicular to/T-parallel to > 1, where the subscript symbols denote directions perpendicular and parallel > 1, to the background magnetic field, and if this temperature anisotropy is sufficiently large, the whistler anisotropy instability is excited. This mode is studied using two-dimensional particle-in-cell simulations in a spatially homogeneous plasma model. Theory predicts a threshold electron anisotropy for this instability which depends inversely on the electron parallel beta. The simulations show that wave-particle scattering by enhanced whistler fluctuations maintain the initially bi-Maxwellian character of the electron distribution, and that this scattering imposes an upper bound on the electron T-perpendicular to/T-parallel to commensurate with that predicted by linear theory.