Nonlinear whistler wave generation, in low beta plasmas, by induced scattering: 2D PIC simulations.*

We present recent results on whistler wave generation by nonlinear induced scattering. Nonlinear induced scattering (NLIS) is a process that allows transfer of energy from one unstable mode ω₁ into another mode ω₂ and the particles that can satisfy the nonlinear Landau resonant condition. In our particular example, the generation of whistler waves through NLIS is achieved via lower-hybrid (LH) beat-wave coupling with thermal particles. The LH waves are first generated by a cold but energetic ring ion distribution that is unstable to these waves [1]. The nonlinear induced scattering acts as a saturation mechanism for the linearly unstable LH modes. We show for the first time, in a PIC simulation, how the damped nonlinear density perturbations associated with the beat-wave coupling, a.k.a. quasi-modes, arise in the nonlinear induced scattering of LH/whistler waves. This fundamental, and interesting, nonlinear mechanism is at the heart of the upcoming SMART experiment where LH electrostatic waves will be converted to whistler waves via NLIS [2]. Although NLIS is fastest in a realistic three-dimensional (3D) setup, a slower version of the phenomenon survives in 2D and thus it can be investigated with a careful choice of simulation parameters. We present 2D PIC simulations with parameters close to those found in the low beta plasma β<10^-3 environment of Earth’s ionosphere at 500 km of altitude --where the SMART experiment will be performed. However, we point out that nonlinear induced scattering is a universal phenomenon in turbulent plasmas.