Kinetic Description of a Whistler Wave Propagating in Plasma Along the Magnetic Field

Resonant wave–particle interaction represents one of the most important phenomena that determines the spectra of waves and dynamics of high-energy particles in space plasma. This interaction was studied most comprehensively for the case in which, with high degree of accuracy, the plasma can be divided into two components: a cold component that determines the dispersion properties of waves but does not participate in resonance interaction and a high-energy component characterized by low density relative to that of the cold component, so that it has no impact on wave dispersion. On the contrary, the high-energy particles participate in resonance interaction with a wave thereby determining its kinetic collisionless damping (or amplification in the case of unstable plasma). To calculate the damping or growth rate of the wave, distribution function of high-energy particles is usually assumed to be specified analytically. It is also assumed that the damping or growth rate is much smaller than the wave frequency. In the present work, we develop an approach to analysis of linear resonance interaction of whistler waves propagating along an external magnetic field with high-energy electrons that allows one to lift the limitations mentioned above and find the real and imaginary parts of frequency at arbitrary relation between them for a wave with a given wave vector. Moreover, the electron distribution function that is not divided into cold and high-energy components can be specified numerically, e.g., based on satellite measurements of differential particles fluxes. The developed approach is illustrated using electron fluxes measured by Van Allen Probe-A and MMS satellites as examples.