EVIDENCE OF MORE EFFICIENT WHISTLER-MODE TRANSMISSION DURING PERIODS OF INCREASED MAGNETIC ACTIVITY

In a previous study it was reported that whistler-mode signals receive d at Faraday, Antarctica (65 degrees S, 64 degrees W) and Dunedin, New Zealand (46 degrees S, 171 degrees E) with entry regions in Pacific l ongitudes (typically from the VLF transmitter NLK, Seattle, USA) showe d an increase in transmission of wave energy as magnetic activity incr eased. However, signals with entry regions in Atlantic longitudes (typ ically from the NSS transmitter, Annapolis, USA) did not appear to sho w such a relationship. This paper reports the results of a study of th e same two longitude ranges but with the opposite transmitter providin g additional whistler-mode signal information, with L-values in the ra nge 1.8-2.6. Transmissions from NLK once again indicate a relationship between the transmission of wave energy and magnetic activity even th ough the signals were propagating in Atlantic longitudes, not Pacific. Any trend in NSS events observed at Dunedin was obscured by a limited range of magnetic activity, and duct exit regions so close to the rec eiver that small-scale excitation effects appeared to be occurring. Ho wever, by combining data from both longitudes, i.e Pacific and Atlanti c, and using only ducts with exit regions that were > 500 km from the receiver, NSS events were found to show the same trend as NLK events. No significant longitude-dependent or transmitter-dependent variations in duct efficiency could be detected. Duct efficiency increases by a factor of about 30 with K-p = 2-8 and this result is discussed in term s of changes in wave-particle interactions and duct size.