On the parameter dependence of the whistler anisotropy instability

Abstract

The evolution of the whistler anisotropy instability relevant to whistler-mode chorus waves in the Earth's inner magnetosphere is studied using kinetic simulations and is compared with satellite observations. The electron distribution is constrained by the whistler anisotropy instability to a marginal stability state and presents an upper bound of electron anisotropy, which agrees with satellite observations. The electron beta β∥e separates whistler waves into two groups: (i) quasi-parallel whistler waves for 𝛽 ∥e ≳ 0.02 and (ii) oblique whistler waves close to the resonance cone for 𝛽 ∥e ≲ 0.02. Landau damping is important in the saturation and relaxation stage of the oblique whistler wave growth. The saturated magnetic field energy of whistler waves roughly scales with the electron beta 𝛽2∥e, shown in both simulations and satellite observations. These results suggest the critical role of electron beta β∥e in determining the whistler wave properties in the inner magnetosphere.