Landau resonance and cyclotron resonance of coherent whistler mode waves and energetic electrons are explored for magnetoplasmas with appreciable gradients in the plasma density and magnetic field strength. It is shown that in the topside ionosphere of the earth near the ion transition height the gradients in plasma density and magnetic field strength along a magnetic field line may match in a way which enhances both Landau and cyclotron interactions between waves and electrons at the loss cone pitch angle. The pitch angle scattering induced by a signal from a ground-based VLF transmitter in the ionosphere above the transmitter has been estimated and compared to the pitch angle scattering induced by naturally occurring ELF hiss through cyclotron resonance. It is found that the expected scattering due to plasmaspheric hiss is an order of magnitude larger than that due to Landau resonance in the topside ionosphere. Pitch angle scattering due to cyclotron resonance in the topside ionosphere, however, may be larger by a factor of 2. We suggest that the “fast Trimpi” effect may be caused by a cyclotron resonance interaction in the topside ionosphere.
Neubert, T., Bell, T. F., & Storey, L. R. O. (1987). Resonance between coherent whistler mode waves and electrons in the topside ionosphere. Journal of Geophysical Research-space Physics, 92(A1), 255-265. https://doi.org/10.1029/JA092iA01p00255