Electron resonant interactions with electromagnetic whistler-mode waves play an important role in electron flux dynamics in various space plasma systems: planetary radiation belts, bow shocks, solar wind and magnetic reconnection regions. Two key wave characteristics determining the regime of wave–particle interactions are the wave intensity and the wave coherency. The classical quasi-linear diffusion approach describes well electron diffusion by incoherent and low-amplitude waves, whereas the nonlinear resonant models describe electron phase bunching and trapping by highly coherent intense waves. This study is devoted to the investigation of the regime of electron resonant interactions with incoherent but intense waves. Although this regime is characterized by electron diffusion, we show that diffusion rates scale linearly with the wave amplitude, $D\propto B_w$ , in contrast to the quasi-linear diffusion scaling $D_{QL}\propto B_w^2$ . Using observed wave amplitude distributions, we demonstrate that the quasi-linear diffusion model significantly overestimates electron scattering by incoherent, but intense whistler-mode waves. We discuss the results obtained in the context of simulations of long-term electron flux dynamics in space plasma systems.

Whistler-mode waves are one of the most intense electromagnetic waves in the planetary magnetospheres. These waves are responsible for energetic electron losses into the atmosphere and for electron acceleration up to relativistic energies. Generation of whistler-mode waves is typically attributed to the thermal electron anisotropy. The anisotropy corresponding to the marginal stability for whistler-mode waves has been derived for a single-component Maxwellian plasma, but this criterion does not always work in the Earth's magnetosphere where whistler-mode waves are generated by an energy-confined, strongly anisotropic electron population. This study aims to generalize the marginal stability equation for multi-component plasma with a small, but strongly anisotropic, electron population. New analytical equations for the marginal stability have been derived from the linear analysis. We have also discussed applicability of the derived equations for different electron populations in the Earth's magnetosphere.