Propagation characteristics of VLF hiss observed at low latitude ground stations

Propagation characteristics of VLF hiss observed at the low latitude ground stations of Moshiri (geomagnetic latitude 34.3 deg N) and Hiraiso (geomagnetic latitude 26.2 deg N) in Japan during quiet and disturbed days are discussed in the light of both the ducted and nonducted modes of propagation. The ducted propagation is studied by computing the enhancement factors at different altitudes along the field lines where the hiss is likely to be generated, whereas the nonducted propagation is studied by making use of ray tracing computations. It is shown that the storm-time hiss, generated at small wave normal angles (wave normal below the field line) can propagate to low latitude ground stations in the nonducted mode of propagation under the influence of negative latitudinal gradients of ionization, whereas the quiet-time hiss, generated at relatively large wave normal angles, can propagate in the normal nonducted mode of propagation.