The wave distribution function in a hot magnetospheric plasma: The direct problem

Studying electromagnetic waves in a magnetospheric plasma, it is often important to find their directions of propagation. When the assumption of a plane wave is not valid, the determination of the wave distribution function (WDF) is required. The WDF specifies the distribution of wave‐energy density with respect to the frequency and the wave‐normal direction. An alternative approach to the estimation of the experimentally accessible data for a given WDF is suggested with the hot‐plasma and/or Doppler effects taken into account. The method is based on an explicit frequency dependence of the WDF. Examples of solutions, including the determination of the wave‐refractive index, the wave‐growth rate, the group velocity, and theoretical predictions of experimental data at predefined frequencies, are given in different cases. First, whistler‐mode waves in a hot plasma near the geostationary orbit were studied. Second, the low‐frequency waves observed by the low‐altitude satellite Freja in the auroral zone were subjected to theoretical analysis. A complex, multimodal structure of the wave characteristics was found, mainly around the resonance angle. In the case of extremely short‐wavelength waves, the influence of the Doppler effect produced qualitative changes of the wave‐normal dependence of wave parameters.