Abstract
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 1.8â2.6. 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 >500km 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 Kp=2â8 and this result is discussed in terms of changes in wave-particle interactions and duct size.
Highlights
Clilverd et al (1996) reported the ®rst observed occurrence of simultaneous common-duct whistlermode VLF signals at Dunedin, New Zealand 46SY 171E and Faraday, Antarctica 65SY 64W, some 7 Mm apart
The extremes in wave energy transmission eciency coincided with extremes in magnetic activity, i.e. the lowest eciency duct occurred after the lowest magnetic activity
The entry regions of the ducts are consistent with the existence of preferred paths of propagation from transmitter to nonconjugate receiver, given that NLK's latitude is too high for direct entry
Summary
Clilverd et al (1996) reported the ®rst observed occurrence of simultaneous common-duct whistlermode VLF signals at Dunedin, New Zealand 46SY 171E and Faraday, Antarctica 65SY 64W, some 7 Mm apart. One group was described as theAtlantic' group and was characterised by ducts with entry longitudes near Faraday conjugate 65À85W with whistler-mode signals originating from NSS. The other group was described as thePaci®c' group and was characterised by ducts with entry longitudes near NLK 120À140W and whistler-mode signals originating from NLK. The extremes in wave energy transmission eciency coincided with extremes in magnetic activity, i.e. the lowest eciency duct occurred after the lowest magnetic activity. Both of the extreme values came from the Paci®c data and a logarithmic relationship of increasing eciency with increasing magnetic disturbance was apparent. No explanation was proposed for the reason why a similar relationship could not be observed in the events that occurred at Atlantic longitudes
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