A THEORETICAL-STUDY ON THE BROAD SYMMETRICAL STRUCTURE IN THE STIMULATED ELECTROMAGNETIC EMISSION-SPECTRUM

A modulational instability of the electron Bernstein wave is studied. The process involves decay of the pump electron Bernstein wave into both a Stokes electron Bernstein sideband and an anti-Stokes electron Bernstein sideband, together with a lower-hybrid decay mode. The threshold field of the proposed instability for third-harmonic resonance is found to be around 1 V/m in a height region sufficiently below the upper-hybrid resonance layer and increases with the harmonic number 1. It is then proposed that scattering of the anti-Stokes and Stokes sidebands of this modulational instability off field-aligned density irregularities produces frequency-upshifted and frequency-downshifted sidebands constituting the broad symmetric structure in the stimulated electromagnetic emission spectrum observed in ionospheric heating experiments.