Ma. Clilverd et al., OBSERVATION OF A DECREASE IN MIDLATITUDE WHISTLER-MODE SIGNAL OCCURRENCE PRIOR TO GEOMAGNETIC STORMS, Journal of atmospheric and terrestrial physics, 55(10), 1993, pp. 1479-1485
VLF whistler mode signals received in 1986-1992 at Faraday, Antarctica
(65-degrees-S, 64-degrees-W) and Dunedin, New Zealand (46-degrees-S,
171-degrees-E) show night-long decreases in occurrence which may be ca
used by changes in F-region absorption levels. The whistler mode occur
rence normally decreases for one night and can only be detected during
periods when whistler mode activity lasting several hours per night i
s usual. Decreases in occurrence are observed more often at Dunedin th
an at Faraday, probably due to long sub-ionospheric paths that result
in weaker signals being received at Dunedin. The decreases in occurren
ce appear to be associated with solar disturbances and often occur a d
ay before the onset of geomagnetic activity. Several of the events rec
ur with a 27-day cycle that coincides with favourably placed solar cor
onal holes.