A theoretical model for the broad upshifted maximum in the stimulated electromagnetic emission spectrum

Abstract

A second‐order four‐wave interaction process including two pump photons, an upper hybrid plasmon, and an electron Bernstein plasmon is studied. The pump is the second harmonic of the HF heater in the plasma. It is found that, when the heater wave frequency f0 is above a harmonic of the electron cyclotron frequency fc, frequency‐upshifted upper hybrid waves and frequency‐downshifted electron Bernstein waves can be excited above the upper hybrid resonance layer via the considered process. The process occurs in a local region where the heater wave frequency is about the mean of the upper hybrid wave frequency and the electron Bernstein wave frequency. Moreover, in this interaction process, a low‐frequency electrostatic oscillation in the frequency range of the lower hybrid wave is generated through nonlinear coupling of the HF heater wave with the excited high‐frequency electrostatic waves. However, this wave does not satisfy the linear dispersion relation of the lower hybrid wave and is thus a driven wave. The excited frequency‐upshifted upper hybrid waves can then scatter off field‐aligned density irregularities to generate O ‐ mode emissions with frequencies around 2f0 − nfc This is consistent with observations of the broad upshifted maximum (BUM) feature in the stimulated electromagnetic emission (SEE) spectrum. The concomitantly excited frequency‐downshifted electron Bernstein waves are found to have much smaller amplitudes; hence their scattering products are also relatively weak. This explains why only BUM lines are detected. Furthermore, the driven low‐frequency fluctuations can also be the scatterers to convert the upper hybrid waves into emissions with frequencies aroundy fBUM + fLH and fBUM − fLH, where fLH is the lower hybrid wave frequency. This is suggested to be the generation mechanism of the second BUM feature which appears when the shoulder of the BUM feature is not very high.