Evidence of more efficient whistler-mode transmission during periods of increased magnetic activity

In a previous study it was reported that whistler- mode signals received at Faraday, Antarctica (65°S,64°W) and Dunedin, New Zealand (46°S, 171°E) with entry regions in Pacific longitudes (typically from the VLF transmitter NLK, Seattle, USA) showed an increase in transmission of wave energy as magnetic activity increased. However, signals with entry regions in Atlantic longitudes (typically from the NSS transmitter, Annapolis, USA) did not appear to show such a relationship. This paper reports the results of a study of the same two longitude ranges but with the opposite transmitter providing additional whistler-mode signal information, with L-values in the range 1.8–2.6. Transmissions from NLK once again indicate a relationship between the transmission of wave energy and magnetic activity even though 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 receiver that small-scale excitation effects appeared to be occurring. However, by combining data from both longitudes, i.e Pacific and Atlantic, 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 terms of changes in wave-particle interactions and duct size.