Laboratory investigation of nonlinear whistler wave processes

Abstract:

Nonlinear interactions involving whistler wave turbulence can result from wave-particle interactions and instabilities in sharp boundary layers. Given sufficient whistler energy density, nonlinear scattering off thermal electrons substantially changes the wave vector direction and energy flux, while inducing a small frequency shift (see Crabtree, Phys. Plasmas 19, 032903 (2012)). In the magnetosphere, boundary layers containing highly sheared plasma flows drive lower hybrid waves, leading to the formation of quasi-static structures in the nonlinearly saturated state. Such processes are being investigated in the NRL Space Physics Simulation Chamber (SPSC) in conditions scaled to match the respective environments. The specific nonlinear process being examined is the scattering of a transversely propagating, primarily electrostatic, lower hybrid wave into a more parallel propagating electromagnetic whistler mode. Sufficiently large amplitude lower hybrid waves have been observed to scatter into whistler modes by scattering from thermal electrons. The plasma response as a function of transmitted lower hybrid wave amplitude is monitored with magnetic antennas. The experiments have demonstrated large changes in wave propagation angle and small frequency downshifts consistent with nonlinear Landau damping when pump wave amplitudes exceed the small threshold value (dB/B₀ ~ 4×10^-7). *This work supported by the NRL Base Program.