The observed and simulated saturation characteristics of whistler-mode chorus waves*

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 remarkably well with satellite observations. The electron beta $\beta_{\parallel e}$ separates whistler waves into two groups: (i) quasi-parallel whistler waves for $\beta_{\parallel e} \gtrsim 0.02$ and (ii) oblique whistler waves close to the resonance cone for $\beta_{\parallel e} \lesssim 0.02$. Landau damping is important in the saturation and relaxation stage of the oblique whistler wave growth. The magnetic amplitude of whistler waves roughly scales with the electron beta $\beta_{\parallel e}$, shown in both simulations and satellite observations. These results suggest the critical role of electron beta $\beta_{\parallel e}$ in determining the whistler wave properties in the inner magnetosphere.