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

In a previous study it was reported that whistlermode signals received at Faraday, Antarctica …65 S; 64 W† and Dunedin, New Zealand …46 S; 171 E† with entry regions in Paci®c 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 Paci®c. 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 e€ects appeared to be occurring. However, by combining data from both longitudes, i.e Paci®c 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 signi®cant longitude-dependent or transmitter-dependent variations in duct eciency could be detected. Duct eciency increases by a factor of about 30 with Kp ˆ 2ÿ8 and this result is discussed in terms of changes in wave-particle interactions and duct size. 1 Introduction Clilverd et al. (1996) reported the ®rst observed occurrence of simultaneous common-duct whistlermode VLF signals at Dunedin, New Zealand …46 S; 171 E† and Faraday, Antarctica …65 S; 64 W†, some 7Mm apart. The signals which came from VLF transmitters situated in North America and propagated along ®eld lines near L ˆ 2:2 (Clilverd et al., 1992a) were observed between 1992± 1994. The locations of the duct exit regions were determined and a subionospheric propagation model was used to estimate signal input and output powers. Two apparent groups of events were reported. One group was described as the `Atlantic' group and was characterised by ducts with entry longitudes near Faraday conjugate …65 ÿ85 W† with whistler-mode signals originating from NSS. The other group was described as the `Paci®c' group and was characterised by ducts with entry longitudes near NLK …120 ÿ140 W† and whistler-mode signals originating from NLK. The extremes in wave energy transmission eciency coincided with extremes in magnetic activity, i.e. the lowest eciency duct occurred after the lowest magnetic activity. Both of the extreme values came from the Paci®c data and a logarithmic relationship of increasing eciency with increasing magnetic disturbance was apparent. This is consistent with the energetic plasma ̄ux and anisotropy increasing with magnetic disturbance (Tanaka et al., 1989). No explanation was proposed for the reason why a similar relationship could not be observed in the events that occurred at Atlantic longitudes. This paper reports a study of whistler-mode signals, with L-values between 1.8 and 2.6, which have propagated in the same longitude ranges as the previous work, but originating from the opposite transmitters (i.e NLK at Atlantic longitudes and NSS at Paci®c longitudes). Hence this study allows the elimination of both a longitude and a transmitter e€ect in the relationship Correspondence to: N. R. Thomson This article was processed using Springer-Verlag TEX ann geo-macro package 1.0 and the AMS fonts, developed by the American Mathematical Society. Ann. Geophysicae 15, 999±1004 (1997) Ó EGS ± Springer-Verlag 1997