Power threshold for growth of coherent VLF signals in the magnetosphere

Abstract

It is well known that coherent VLF signals injected into the magnetosphere from Siple Station, Antarctica, often show temporal growth of 20-30 dB and emission triggering, as observed at the conjugate point, near Roberval, Quebec.In a new kind of experiment it has been found that when the input power to the transmitting antenna is reduced below a 'threshold' value Pt, growth and triggering cease. Below Pt the output is proportional to the input. In one typical experiment the peak received power increased by 24 dB when the input power to the antenna crossed the threshold value of 1000 W (estimated radiated power of 10 W). The value of Pt varied widely depending on the duct involved and on the time, changing as much as 10 dB in less than 1 hour. As the power was lowered during multipath propagation, the last duct to be cut off was found to terminate nearly overhead at Siple, as was expected, assuming all ducts to be equally active. Minimum radiated power for growth and triggering was 1 W. One possible explanation for the threshold effect is background noise (e.g., plasmaspheric hiss) that prevents the instability from getting started.Another is a drop in temporal growth rate to below zero at low signal level.Measurement of Pt might possibly serve as a groundbased diagnostic for magnetospheric flux levels, assuming calibration by satellite particle detectors.

Mitigation of energetic electrons in the magnetosphere by amplified whistler wave under double cyclotron resonances

“…1 is rather low (to be consistent with that of previous experiments (Helliwell et al, 1980)). In practical application for achieving significant electron precipitation, the incident wave field has to increase considerably.…”

“…1 is for a direct comparison with the early Siple experimental result (Helliwell et al, 1980;Helliwell and Inan, 1982;Helliwell, 1983). In Siple experiments (Helliwell et al, 1980), injected Siple signals of 3 kHz, propagating along the L ∼ =4 shell, were often amplified by 10 to 30 dB and oscillated in time. The numerical result presented in Fig.…”

“…Indeed, it has been observed (Helliwell et al, 1980;Helliwell and Inan, Published by Copernicus Publications on behalf of the European Geosciences Union and the American Geophysical Union. 774 S. P. Kuo: Whistler wave-electron interaction in the magnetosphere 1982; Helliwell, 1983) that trapped energetic (keV) electrons in the magnetosphere can significantly amplify whistler waves.…”

Identifying power line harmonic radiation from an electrical network

“…We calculate the total radiated power from the Dunedin 1050 Hz ripple control signal to be about 4 µW. Clearly, this radiated power is much lower than the 1 W triggering threshold found in active experiments (Helliwell et al, 1980), and thus the Dunedin ripple control signal is not a significant "pollutant" of geospace.…”

“…These experiments indicated a radiated power threshold of ∼1 W (Helliwell et al, 1980), where higher powers lead to the observation of nonlinear triggered emissions (Park and Chang, 1978) indicating strong interactions between waves and trapped particles. While it seems likely that PLHR is not currently significant to geospace in general, we expect electrical power consumption to grow world-wide and the harmonic content in transmission lines to grow faster still, hence increasing PLHR levels.…”