A study is made of the excitation and guided propagation of whistler waves along magnetic-field-aligned cylindrical ducts with enhanced plasma density. The ducts have been created in the large plasma device as a result of the thermal-diffusion-driven redistribution of plasma due to electron heating in the quasistatic field of a current loop having a radius commensurate with the electron heat-conduction length in the radial direction. The whistler waves are excited by a comparatively small magnetic loop antenna immersed in the duct. Detailed measurements of the excited field and the density distribution are reported. It is concluded that thermally generated ducts observed in the experiments can guide whistler-mode waves launched from the magnetic antenna. With the use of the full wave formulation, the total source-excited field is calculated and compared with the experimental data. Excellent agreement is found between the measured and calculated wave patterns. The results are relevant to both the basic properties of whistlers and to applications such as transmitting systems using artificial near-antenna ducts in space plasmas.

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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Abstract-A study is made of the characteristics of a perfectly conducting cylindrical antenna insulated from the surrounding cold collisionless magnetoplasma by an isotropic coaxial cylindrical sheath for the case where the antenna is aligned with an external magnetic field and is excited by means of a delta-function voltage generator. A rigorous representation is obtained for the current distribution on an infinitely long antenna. It is shown that in the whistler frequency range, the current distribution of a sufficiently thin antenna is determined mainly by the eigenmode whose guided propagation is found to be supported along the antenna. Based on the results obtained for an infinitely long antenna, a generalized transmission-line theory is developed for determining the current distribution and the input impedance of an insulated antenna of finite length located in a resonant magnetoplasma. The influence of the sheath parameters on the antenna characteristics is analyzed.

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With the use of a full wave formulation, a study is made of radiation and propagation of electromagnetic waves in nonuniform cylindrical ducts excited by given azimuthally symmetric sources. For an insnitely long duct, the eigenfwction set involving both localized and non-localized waves is obtained for arbitrary values of driving frequency, external magnetic-field strength, plasma density, and its variation transverse to the duct axis. Using this set, the radiation characteristics of an annular current immersed in the duct are found for the whistler range. The subsequent propagation of excited waves is then analyzed in a duct whose parameters vary slowly along the external magnetic field. The applications of the results obtained are discussed.

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