Small-scale bursts of Langmuir waves in the polar cap

Analysis of the VLF data obtained onboard the INTERBALL-2 (Auroral Probe) satellite in the polar cap region at altitudes from 2 to 3.5 Earth's radii shows small-scale bursts of electrostatic waves during the magnetic disturbances. These waves are detected in the vicinity of the local plasma frequency commonly in association with warm electron fluxes. The experimental waveform data have shown that Langmuir emissions appear as chains of wave packets with 1–40 mV/m amplitudes. The typical power spectrum width is about 10% of the local plasma frequency. General equations within a quasi-linear statistical approximation are calculated for a beam-plasma instability in the presence of low frequency turbulence. The numerical calculations of these equations show that the beat-type waveforms of Langmuir emissions may be explained by interference between waves excited by an electron beam and scattering due to the electron density fluctuations. Wave scattering may lead to such severe widening of the frequency width of the Langmuir waves that wave power gains access to phase space regions with small or even negative growth rates.