Whistler modes guided by a nonuniform cylindrical duct in a magnetoplasma below the lower hybrid frequency

Abstract

Guided propagation of whistler waves and their excitation by a loop antenna in the presence of a field-aligned cylindrical duct with enhanced density in a magnetoplasma are studied below the lower hybrid resonance frequency. The emphasis is placed on the case where the duct is transversely nonuniform such that the plasma density in it decreases monotonically from a constant value in the near-axis inner region to the lower background value in the outer medium. Conditions are determined under which the dispersion characteristics and field structures of the modes guided by such a duct allow an approximate description within the framework of perturbation theory. The approximate results are shown to be in good agreement with the numerical results obtained using a full-wave approach. It is also found that the radiation resistance of a loop antenna located in the smooth-walled enhanced density duct can be notably greater than that of the same antenna immersed in the background magnetoplasma due to the excitation of the modes guided by the duct. The reported results extend previous analyses performed for sharp-walled uniform ducts to the case of a more realistic plasma density profile in the duct and can be useful in understanding the basic features of whistler wave guidance and excitation as well as in planning the related experiments.