Ma. Clilverd et al., THE INFLUENCE OF IONOSPHERIC ABSORPTION ON MIDLATITUDE WHISTLER-MODE SIGNAL OCCURRENCE FROM VLF TRANSMITTERS, Journal of atmospheric and terrestrial physics, 55(10), 1993, pp. 1469-1477
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.