Observation of two preferred propagation paths for whistler mode VLF signals received at a non-conjugate location
Whistler mode signals from NLK (24.8 kHz) were received at Dunedin, New Zealand during 1989-90. The arrival bearing of the signals shows a bimodal distribution which is consistent with the results of ALLCOCK and MCNEILL (1966), who suggested the existence of two paths with less total transmission loss than other paths. PATH 1 signals (which have a duct entry region just south of Dunedin’s conjugate) are observed at all times of the year, while PATH 2 signals (which have a duct entry region just south of the NLK transmitter) show an annual variation in occurrence with a maximum in May and a minimum in November which may be a consequence of horizontal ionospheric electron density gradients near the duct entry region. Similarly preferred paths have been observed at Faraday, Antarctica from the NSS transmitter.