Vol 59, No 10 (2017)

By using numerical simulations we generalize certain relationships between the parameters of quasimonochromatic whistler-mode waves generated at the linear and nonlinear stages of the cyclotron instability in the backward-wave oscillator regime. One of these relationships is between the wave amplitude at the nonlinear stage and the linear growth rate of the cyclotron instability. It was obtained analytically by V.Yu.Trakhtengerts (1984) for a uniform medium under the assumption of constant frequency and amplitude of the generated wave. We show that a similar relationship also holds for the signals generated in a nonuniform magnetic field and having a discrete structure in the form of short wave packets (elements) with fast frequency drift inside each element. We also generalize the formula for the linear growth rate of absolute cyclotron instability in a nonuniform medium and analyze the relationship between the frequency drift rate in the discrete elements and the wave amplitude. These relationships are important for analyzing the links between the parameters of chorus emissions in the Earth’s and planetary magnetospheres and the characteristics of the energetic charged particles generating these signals.

We consider degenerate parametric interaction in intense acoustic beams. It is shown that the field at large times has universal structure which is determined only by the wave period. At the same time, the distance in which this universal waveform is formed strongly depends on the fine structure of the initial field. The main part of this work is devoted to an experimental study of degenerate interaction in the intense acoustic beams. The employed experimental tools allow one to change the phase relationships among the radiated signals and study the processes of formation of the universal signal waveform at large distances, depending on the initial conditions.

We present the results of measuring the coefficient of specific attenuation in supercooled water in the pores of silica gel at a frequency of 34 GHz in the temperature range from +20°C to −190°C. It is shown that in moistened silica gel with nanosized pores, noticeable radiation losses occur down to a temperature of about −100°C. This is due to the presence of a certain amount of liquid water in the pore space. Part of the water, which is strongly bound with the surface of the pores, does not freeze at temperatures down to −136°C, at which vitrification takes place. The properties of the other fraction, which is weakly bound with the surface are close to that for the volumetric water, i.e., it crystallizes completely on being cooled down to the critical temperature −(38–45)°C, which is determined by the properties of the surrounding medium. Absorption of the electromagnetic radiation in supercooled water captures in pores should be allowed for, e.g., in the problems of radiation transfer in an atmospheric moistened aerosol, especially in the millimeter

wavelength band.

We describe the process of adaptive self-inversion of an image (nonlinear switching) of smallscale opaque object, when the amplitude-modulated laser beam, which illuminates it, is focused in a weakly absorbing medium. It is shown that, despite the nonlocal character of the process, which is due to thermal nonlinearity, the brightness-inverse image is characterized by acceptable quality and a high conversion coefficient. It is shown that the coefficient of conversion of the original image to the inverse one depends on the ratio of the object dimensions and the size of the illuminating beam, and decreases sharply for relatively large objects. The obtained experimental data agree with the numerical calculations. Inversion of the images of several model objects and microdefects in a nonlinear KDP crystal is demonstrated.

We consider an adaptive antenna array, which forms a great number of beams for the parallel or successive observation of space and ensures the maximum ratio of the signal power to the mean noise power at the output. An algorithm for adaptive signal processing in the basis of the power vectors is proposed and the regularized estimates of the weight vectors are obtained using the limited number of the input-process samples, which can be both greater and smaller (the case of a short sample) than the number of the elements. The results of simulation of the adaptive signal processing are given, which show the high signal-processing efficiency in the adaptive antenna array.