Non-ducted whistler-mode signals at low latitudes

In an earlier paper (Andrews, 1974) it was shown that a component of VLF signal from station NLK, near Seattle, received on a trans-equatorial path at Wellington appeared to have left the Earth-ionosphere waveguide at low latitudes and crossed the equator in the whistler mode. Results from a VLF recorder at Rarotonga (lat. 21°S) show that similar signals are indeed common at low latitudes. Recent calculations by Singh (1976) have established that a non-ducted VLF mode could exist at a latitude of about 20° in the presence of horizontal gradients of ionisation. It is pointed out that a second non-ducted path, first implied by Scarabucci (1970), appears possible at still lower latitudes when the O⁺ to H⁺ transition height in the ionosphere is high. While firm conclusions cannot be drawn, the low Doppler shift on the observed signals and the terrdency for their occurrence to maximise before dawn and at solar maximum are features which tend to favour the very low latitude path.