2D Kinetic Simulations of Whistler Wave Generation by Nonlinear Scattering of Lower-hybrid Waves in Turbulent Plasmas. *

Turbulent plasmas in space, laboratory experiments, and astrophysical domains can be described by weak turbulence theory which can be characterized as a broad spectrum of incoherent interacting waves. We investigate a fundamental nonlinear kinetic mechanism of weak turbulence that can explain the generation of whistler waves in homogeneous plasmas by nonlinear scattering of short wavelength electrostatic lower-hybrid (LH) waves. Two particle-in-cell (PIC) simulations with different mass ratios in two dimensions (2D) were performed to study the phenomenon of nonlinear scattering. We use a ring ion velocity distribution to excite broadband LH waves, which are converted to whistler waves and evolve in time, frequency, and wavenumber space. The simulations show the formation of quasi-modes, which are low frequency density perturbations driven by the ponderomotive force due to the beating of LH and whistler waves. These low frequency oscillations are damped due to resonant phase matching with thermal plasma particles. By comparing the phase and thermal speeds, we confirm the nonlinear scattering mechanism and the role of nonlinear scattering in the 2D evolution of the ring ion instability. Although the nonlinear scattering is theoretically slower in 2D than in 3D due to the absence of the vector nonlinearity, these simulations show that quasi-modes are an important diagnostic tool for PIC simulations that has not been utilized in the past. The fundamental nonlinear scattering mechanism described here plays an important role in the generation of whistler waves in active experiments such as the upcoming Space Measurement of a Rocket Release Turbulence (SMART) experiment.