OBSERVATION OF A DECREASE IN MIDLATITUDE WHISTLER-MODE SIGNAL OCCURRENCE PRIOR TO GEOMAGNETIC STORMS

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.