The thesis is devoted to the study of wave processes in the vicinity of Earth's bow shock and interplanetary shock. It mostly deals with the Langmuir waves intensity statistics in the Earth's electron foreshock region. The possibility of the transition radiation of relativistic electrons from the interplanetary shock front is discussed. A numerical model for wave propagation in unstable plasma with inhomogeneities was developed. This model described the linear interaction of Langmuir wave packets with an electron beam. It took into account the angular diffusion of the wave vector due to wave scattering on small-amplitude density fluctuations, as well as resonant interaction of waves with beam particles inside the density perturbations of relatively large amplitude. Using this model, the evolution of the wave packets in inhomogeneous plasmas with an electron beam was studied. Statistical study of Langmuir waves observed by the WHISPER and WBD instruments of CLUSTER spacecraft in the Earth's electron foreshock and solar wind was performed. Pearson technique was used to classify the probability density distributions of the logarithms of the wave energies for data obtained in space experiments and numerical modeling. It was shown that both experimental distributions obtained within the Earth's electron foreshock aboard the CLUSTER spacecraft and model distributions for the logarithm of wave intensity can be better, in a statistical sense, approximated by ?-distribution or Pearson Type IV distribution rather than by a normal distribution predicted by the Stochastic Growth Theory. The main reason for deviations of empirical distributions from the normal one is that the effective number of regions where the waves grow is not very large and, as a consequence, the central limit theorem fails to be true under the typical conditions for the Earth's electron foreshock. The nonstationarity of the Earth bow shock front was studied by applying a new method for remote sensing of the quasiperpendicular part of the bow shock surface. This method is based on analysis of HF electric field fluctuations corresponding to Langmuir, upshifted, and downshifted oscillations in the electron foreshock. Langmuir waves usually have maximum intensity at the upstream boundary of this region. All these waves are generated by energetic electrons accelerated by quasiperpendicular zone of the shock front. Nonstationary behavior of the shock, in particular due to rippling, should result in modulation of energetic electron fluxes, thereby giving rise to variations of Langmuir waves intensity. For upshifted and downshifted oscillations, the variations of both intensity and central frequency can be observed. Analysis of hidden periodicities in plasma wave energy reveals shock front nonstationarity in the frequency range 0.33 fBi<f <fBi , where fBi is the proton gyrofrequency upstream of the shock, and shows that the probability to observe such a nonstationarity increases with Mach number. The profiles observed aboard different spacecraft and the dominating frequencies of the periodicities are usually different. Hence nonstationarity and/or rippling seem to be irregular in space and time rather than resembling a quasiregular wave propagating on the shock surface. The model of the transition radiation from an electron drifting through the interplanetary shock region is suggested to explain multisatellite observations. CLUSTER and WIND satellites performed in situ measurements of strong electromagnetic emission at the frequency ~1.2-1.6fpe in the vicinity of the interplanetary quasi-perpendicular shock crossing on 22 January, 2004. Simultaneously in the same region the increased density of relativistic electrons detected. The transition radiation was suggested to be a possible mechanism of the electromagnetic waves' generation. Taking into account the actual parameters of the event, shock and relativistic electrons, it was concluded that the role of magnetic field consisted only in the formation of the curvilinear trajectories of electrons and the magnetic field influence on dielectric permittivity was neglected. The model of relativistic electron rotating around motionless centre inside inhomogeneity region was considered. The maximum of the transition radiation spectrum obtained for one electron rotating in the interplanetary shock front lied in the same frequency range as observed electromagnetic emission. This result justified the assumption of the possible contribution of relativistic electrons' transition radiation to the observed radiation. The spectrum of wave vectors for given frequency obtained by expanding the electron current into plane waves by applying Fourier transform turned out to be quite wide. This is possible in the considered model because the electron rotates around the magnetic field lines perpendicular to the plasma density gradient. As a result, the resonant scattering of the current waves on the Fourier components of dielectric permittivity into electromagnetic emission becomes possible, on the contrary to the case of straight motion of the electron along the density gradient. This effect results into considerable growth of the transition radiation intensity for the electron performing cyclotron rotation in comparison with the case of straight motion. Key words: collisionless shocks, electron foreshock, Langmuir waves, transition radiation.
