Tromsø Heating Experiments: Stimulated emission at HF pump harmonic and subharmonic frequencies

Observations of electromagnetic emissions on the HF heater subharmonic and several harmonic frequencies are presented. Often the second harmonic emissions show characteristic amplitude variation, frequency shift, and spectral broadening that clearly have their origin in ionospheric plasma processes. The strongest candidate is a combined parametric decay and Raman up‐conversion process. The subharmonic emission is believed to be evidence of absolute stimulated Raman scattering and/or two plasmon decay processes which have not been observed before in ionospheric heating experiments.