Observation of a decrease in mid-latitude whistler mode signal occurrence prior to geomagnetic storms

VLF whistler mode signals received in 1986–1992 at Faraday, Antarctica (65° S, 64° W) and Dunedin, New Zealand (46° S, 171° E) show night-long decreases in occurrence which may be caused by changes in F-region absorption levels. The whistler mode occurrence normally decreases for one night and can only be detected during periods when whistler mode activity lasting several hours per night is usual. Decreases in occurrence are observed more often at Dunedin than at Faraday, probably due to long sub-ionospheric paths that result in weaker signals being received at Dunedin. The decreases in occurrence appear to be associated with solar disturbances and often occur a day before the onset of geomagnetic activity. Several of the events recur with a 27-day cycle that coincides with favourably placed solar coronal holes.