Ionospheric beam-plasma interactions: production of quasi-mode in the ion density

New observations by the Poker Flat Incoherent Scatter Radar (PFISR) of the recently discovered coherent echoes [Akbari et al., 2012, 2013] suggest that the underlying beam-plasma instability occurs in a different parameter regime. These observations show simultaneous enhancements of the ion-acoustic (IA) spectrum, as well as the up- and down-shifted plasma-line spectra; the filled-in nature of these IA spectra is the subject of this study. High time-resolution measurements reveal that the IA spectrum consists of discrete peaks at frequencies less than the expected frequency of the ion-acoustic resonant mode. The superposition of these peaks will appear as a filled-in spectrum when the spectrum is averaged over many radar pulses. These peaks are the signature of non-resonant, low frequency wave modes, or quasi-modes of the plasma with subsonic speeds. Such quasi-modes can be produced by different regimes of the beam-plasma instability, as described by the Zakharov system of equations, namely the supersonic and subsonic modulational instabilities and the modified parametric decay instability. The subsonic modulational instability appears as a natural and well suited candidate due to its subsonic nature and the wavenumber range of the resulting turbulence. The drive responsible for the instability is generally assumed to be soft precipitations, i.e. electron beam of energy less than 500 eV. Such electron beam directly interacts with large-scale Langmuir waves, i.e. with small wavenumbers due to the matching resonance condition. According to Bragg scattering theory, IS radar only senses electrostatic waves at a single wavenumber. Therefore, observation of the turbulence by PFISR requires also a mechanism capable of transferring the energy to waves with larger wavenumber visible by the radar. Modulational instabilities and cavitating Langmuir turbulence are examples of such mechanisms. The results presented here are of great importance in the fundamental context of beam-plasma interactions.