No abstract

Abstract. Langmuir emissions generated by electron beams in space plasmas usually appear as chains of strongly modulated wave packets. In this paper, we present a quantitative analysis of three-wave interaction between Langmuir and lower hybrid waves, L1 q-> L2+LH, which explains many details of recent Freja observations in the topside ionosphere. The packet-like waveforms are generated as the beating of several Langmuir modes. The primary Langmuir mode (L1) is produced by beam-plasma instability, and the other modes are produced as a result of parametric decay to secondary Langmuir waves (L2) and lower hybrid modes (LH). We show that the decay instability has a very low threshold and high growth rate. The limited transverse dimensions of electron beams in the polar ionosphere cause radiation losses from the beam region which influence spectra of the beam-plasma and parametric instabilities.

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 LoL+LH which explain many details of FREJA observations.

No abstract

No abstract