Langmuir wave structures registered by FREJA: analysis and modeling Original Research Article

A large set of observations of Langmuir emissions generated by super-thermal electron beams in the topside polar ionosphere has been obtained during the recent FREJA mission. Langmuir emissions appeared as a chain of strongly modulated wave packets; in many cases the packet envelopes were modulated periodically. In present report we suggest the theory which explains these observations. We suggest that packet-like waveforms are the beatings of several Langmuir modes. One of modes (the primary mode) appears as a result of beam-plasma instability, other modes — as a result of parametric decay of primary mode to the secondary Langmuir wave and lower-hybrid wave (or as a result of decay cascade). We present the quantitative theory of three-wave interaction L→L+LH which explain many details of FREJA observations.