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 nonlinearly to drive ion-acoustic fluctuations, resulting in down-shifted and upshifted frequency sidebands. 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 Langmuir 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.