A Parametric Study of VLF Modes Below Anisotropic Ionospheres


  • Floyd P. Snyder,

  • Richard A. Pappert


Weakly attenuated components of the VLF mode spectrum associated with propagation below highly anisotropic ionospheres are presented as a function of frequency and azimuth. In particular, results for the phase velocity, attenuation, polarization mixing ratio, and excitation factor for vertical dipole excitation are presented in the frequency range of 10–30 kHz for midlatitude paths, whereas azimuthal dependencies are presented for a frequency of 19.8 kHz and dip angles of 0° and 60°. The ionospheres are described by exponential electron density and collision frequency profiles, and the primary region of wave-plasma interaction falls in the highly anisotropic region of the ionosphere. It is shown that polarization mixing is much more pronounced for a westerly propagation path than for an easterly path at midlatitudes, and, as a result, principally TE modes can be expected to influence the mode sum for frequencies at least as low as 20 kHz for propagation to the west. On the other hand, as has been shown previously, contribution to the mode sum by principally TE modes for propagation to the east becomes significant only near 30 kHz.

Azimuthal anomalies include drastic polarization changes in going from easterly to westerly paths. For example, in the case of transverse propagation at the magnetic equator, it is shown that modes that are pure TM for propagation to the east may be pure TE for propagation to the west. Tantamount to this is the statement that modes that have dominant excitation for propagation to the east may have vanishing excitation for propagation to the west. Azimuthal dependencies are shown to be very often characterized by rapid variation of the mode constants in the neighborhood of north-south or south-north propagation. These variations manifest themselves in marked differences of the mode sum for azimuthal changes at least as small as 10°. Unfortunately, the azimuthal dependencies of the mode constants do not appear to lend themselves to any simple analytical approximations.