THE INFLUENCE OF IONOSPHERIC ABSORPTION ON MIDLATITUDE WHISTLER-MODE SIGNAL OCCURRENCE FROM VLF TRANSMITTERS

Whistler mode signals from VLF transmitters received at Faraday, Antar ctica (65-degrees-S, 64-degrees-W) during 1986-1991 show an annual var iation in the number of hours over which signals are observed, with a maximum in June and a minimum in December. The variation was larger at solar minimum than at maximum and can be understood in terms of chang es in absorption of VLF signals in the D-region, where the high geogra phic latitude of Faraday plays an important role in producing low atte nuation levels during the austral winter. In contrast, very little suc h variation was observed at Dunedin, New Zealand (46-degrees-S, 171-de grees-E) in 1991. Nighttime whistler mode signals have start and end t ime trends that are consistent with the influence of F-region absorpti on. Increases in whistler mode occurrence appear to be associated with periods of high geomagnetic activity at solar maximum but not during solar minimum. A possible mechanism involving decreased F-region absor ption is discussed.