The propagation of whistler waves in a magnetized plasma containing multiple small-scale (100 m to 1 km) field-aligned irregularities of enhanced electron density is considered analytically and by means of numerical simulations. Such systems of irregularities can develop in the upper ionosphere during the generation of density ducts by high-frequency heating facilities and other types of active experiments. The simulation parameters are close to those of an active experiment where a whistler wave of 18 kHz emitted by a ground-based very low frequency (VLF) transmitter was received onboard the DEMETER satellite at 700 km above the SURA heater. The study reveals a number of remarkable properties of the VLF waves' propagation, including the existence of specific waveguide modes of the small-scale density structures and of a characteristic transverse size d 0 of the irregularities. Irregularities with small density enhancements around 10-20% and transverse sizes larger than d 0 ∼ 1 km can serve as separate waveguides for VLF waves. In their turn, single irregularities narrower than d 0 cannot be considered as individual ducting structures. Numerical simulations show that, for the analysis of the electromagnetic whistlers' propagation, a system of closely spaced irregularities with scales narrower than d 0 can be modeled by an equivalent ducting structure with a smoothed density profile. Such equivalent structure has the same ducting properties for whistlers and can be produced by averaging with a sliding window of a scale about d 0 the original density distribution.

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The dynamics of a group of narrow field-aligned plasma density irregularities is studied using the large-scale KROT plasma device. The chosen experimental conditions are similar to the conditions in the ionosphere of the Earth, where such irregularities develop under the impact of high-power, high-frequency radio waves. In our laboratory experiment, the irregularities are created by a set of rf antennas arranged in a row across the ambient magnetic field due to the heating of electrons and subsequent redistribution of the plasma. Near the heating sources, the irregularities have the form of a set of channels with reduced plasma density, which develop almost independently. At the same time, the relaxation time of a group of neighboring irregularities differs from that of each single irregularity. Evolution of density perturbations at the periphery of the heating region differs for a single irregularity and for a group of irregularities significantly. These effects are probably due to eddy currents excited by each of the irregularities in the background plasma, since narrow irregularities can develop in the “unipolar” thermal diffusion regime. The preliminary experimental results are consistent with the theoretical predictions of unipolar diffusion and excitation of eddy currents.

The dynamics of narrow, field-aligned magnetoplasma irregularities is studied, which develop under the action of a short rf pulse. The laboratory experiment is aimed at demonstrating the rapid, so-called “unipolar” plasma transport mode, which is accompanied by excitation of eddy currents, in the case of localized rf heating of plasma electrons. The experimental parameters are chosen in a special way. The size of the heating spot, determined by the diameter of the loop antenna, exceeds the electron gyroradius significantly but is smaller than the ion gyroradius. The rf pulse duration encompasses several electron collision times but is shorter than the gyroperiod of ions. As a result, the electrons, which are strongly magnetized, acquire energy in rf antenna vicinity and can escape the heating region only along the magnetic field B0. In turn, collisionless ions can travel across B0 under the action of space-charge electric fields. For these conditions, redistribution of the plasma occurs with “unipolar” transport coefficients and is accompanied by excitation of electric currents. Weak plasma density disturbances, which are less than 5% of the background, are measured precisely with a microwave resonator probe. Parallel electron currents are obtained from magnetic probe measurements; the ion current across B0 is restored from the density profile modifications in their dynamics. It is shown that the ions traveling across B0 with a velocity about one third of the ion-acoustic velocity can easily close the current loop, which is driven by the parallel motion of heated electrons. This regime of plasma irregularities evolution is discussed in application to previous laboratory measurements, as well as to active ionospheric experiments.