A GENERATION MECHANISM FOR THE DOWNSHIFTED PEAK IN STIMULATED ELECTROMAGNETIC EMISSION-SPECTRUM

Parametric decay of an electron Bernstein pump wave into an electron Bernstein sideband wave and a nearly perpendicularly propagating ion acoustic (or electrostatic ion cyclotron) decay wave is studied. When incorporated with appropriate nonlinear scattering processes, this instability process along with its cascading is proposed to be the generation mechanism for the downshifted peak (DP) and 2DP features as well as the upshifted peak (UP) feature in the stimulated electromagnetic emission spectrum observed in ionospheric heating experiments. It is found that the instability can be excited most favorably when the heater wave frequency f(o) is in a very narrow frequency range near the third harmonic of the electron cyclotron frequency f(c) of the ambiet ionosphere. Moreover, the frequencies associated with the low-frequency decay mode decrease as f(o) is increased toward 3 f(c), consistent with experimental observations.