Nonlinear Interaction of Langmuir and Whistler Waves Observed with Incoherent Scatter Radar

High-latitude ionosphere is characterized by particle precipitations of different origins. Among these are electron precipitation caused by quasi-static parallel electric fields and Alfven wave-particle interactions. In-situ measurements of fields and particles have commonly detected various plasma modes, such as Langmuir and whistler, enhanced by these precipitating electrons. The waves have been shown to undergo various nonlinear wave-wave and wave-particle interaction including parametric type instabilities. Detecting such processes with in-situ instruments however is not always straightforward and certain processes may remain undetected. We present new incoherent scatter radar data from the auroral F-region where strong echoes simultaneously appear in the ion- and both up- and down-shifted plasma lines channels. While aspects of these observations have been previously discussed in detail in terms of electron beam-generated Langmuir turbulence, some new aspects, namely the presence of two peaks separated by 300 kHz in both the up- and down-shifted plasma line channels are discussed in this paper. The unique and asymmetric displacement of the peaks with respect to the radar transmitting frequency suggests that the anomalous spectra are produced as a result of the existence of non-resonant waves generated by nonlinear beating between intense Langmuir and whistler modes. The results suggest that such nonlinear interactions contribute to the appearance of wave activities close to the plasma frequency as observed by in-situ electric field spectral measurements and that not all these wave activities are directly generated by the initial electron beam. The anomalous plasma lines spectra are often observed just above the altitude where Langmuir turbulence is observed. This altitudinal morphology and its implications are also discussed is this paper.