Observation and numerical modelling of ionospheric beat-wave Brillouin scattering at EISCAT

We report new results of a beat-wave driven Brillouin scattering experiment at EISCAT, Northern Norway, in which large amplitude radio waves with ordinary (O) and extraordinary (X) mode polarization are injected into the overhead ionosphere. The descending and ascending electromagnetic (EM) waves interact resonantly at altitudes where the difference frequency and wave vector of the probe and pump locally match those of the ion-acoustic (IA) wave. Full-wave simulations confirm that the observed frequency sidebands are due to nonlinear wave mixing in the ionosphere between the EM pump and probe waves. The dispersive properties for O and X mode polarization lead to differences in the turbulent absorption processes in the ionosphere and in the received signals escaping the ionosphere, where the O mode is significantly absorbed by electrostatic ionospheric turbulence and the X mode is reflected before reaching the turbulent layers. The results have relevance for laser-driven inertial confinement fusion, heating and current drive in magnetic fusion devices, and for fundamental plasma physics experiments in laboratory and space.