Abstract
In order to investigate the propagation mechanism of tweek atmospherics near mode cut-off frequencies, the attenuation coefficient and excitation factor of the Earth-ionosphere waveguide are calculated for both quasitransverse magnetic (QTM) and quasitransverse electric (QTE) modes. Computations are presented for propagation under a homogeneous anisotropic ionosphere, assuming the QL approximation, in the frequency range 1.5 to 10 kHz. It is shown that the QTE modes (here numbered 1, 3, 5, …) are efficiently excited by a vertical dipole source, especially just above the mode cut-off frequency where they exhibit very low attenuation constants (typically < 1 dB Mm ), and it is suggested that these modes play an important role in the propagation of tweek atmospherics.
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