Small-scale bursts of the Langmuir oscillations in the polar cap

An analysis of the wave data obtained onboard the Interball-2 (Auroral Probe) satellite in the polar cap region at altitudes from 2 to 3 radii of the Earth is performed. The analysis shows that, in the conditions of a disturbed magnetosphere, when electron fluxes intruding from other magnetospheric regions are recorded, small-scale bursts of the Langmuir oscillations are observed in the region in question. The burst-like character, irregular structure, amplitude variations, and low-frequency modulations are typical features of these oscillations. Generation of the oscillations with such characteristic properties may be explained in the framework of the theory of stochastic wave growth. The comparison of the probability distributions for the measured values of the electric fields with the distributions predicted by the theory demonstrate a good agreement in the low-energy region when nonlinear wave processes are not important. As soon as the energy sufficient for nonlinear process development in the system is reached, the experimental curves decrease more steeply than the decrease predicted by the theory, which is true only in the linear approximation.