Formation of Whistler-Mode Cavity in the Magnetosphere by Nonlinear Induced Scattering

In the Earth's dipole magnetic field whistler-mode waves originating in the ionosphere with frequencies relevant to pitch angle scattering of relativistic electrons quickly propagate toward the hydrogen lower-hybrid resonant surface in the magnetosphere. The perpendicular wave-vector increases such that wave packets become quasi-electrostatic, experience large Landau and collisional damping, and quickly become less effective at pitch angle scattering. Recently Ganguli et al. [1] showed that through nonlinear (NL) induced scattering by thermal electrons in low $\beta$ plasmas the direction of the wave-vector of whistler-mode waves can change substantially with only a small change in the frequency. Here we apply this mechanism to demonstrate that when the turbulent whistler-mode energy density is large enough, NL scattering allows a portion of whistler-mode waves to return toward the ionosphere and reduces the perpendicular wave-vector such that the corresponding linear damping is reduced and the wave's ability to pitch angle scatter relativistic electrons is revived. Through multiple NL scatterings and ionospheric reflections a long-lived wave cavity in the Earth's magnetosphere may be formed with the appropriate properties to efficiently pitch-angle scatter trapped relativistic electrons.\\[4pt] [1] Ganguli et al. PoP (2010)