Parametric decay of beam-driven Langmuir wave and enhanced ion-acoustic fluctuations in the ionosphere: a weak turbulence approach

We present a model that describes the decay of beam generated Langmuir wave s into ion-acoustic waves in the topside ionosphere. This calculation is do ne within the frame of the weak turbulence approximation. We study the spec tral signature of such a process as seen by a VHF incoherent scatter radar. An incoherent scatter (IS) spectrum is characterized by two maxima at k(ra dar) and -k(radar), the right and left ion lines respectively. It is shown that, for reasonable beam parameters, the parametric decay of beam-generate d Langmuir waves can enhance either the right, the left or both ion lines s imultaneously. The shape of the spectrum can change drastically on time sca le of about 0.1 to 1 s. The role of the beam parameter as well as the ionos pheric parameters is also investigated. For a given beam number density, th e beam energy or the background density are important to trigger either the left or the right ion line. A large energy spread of the beam or low elect ron collision frequencies can explain the simultaneous observations of the left and the right ion line. The importance of the electron collision frequ ency can explain the altitude distribution of the coherent echoes observed by incoherent scatter radars.