Occurrence properties of ducted whistler‐mode signals from the new VLF transmitter at Siple Station, Antarctica

Abstract

A new VLF transmitter called Jupiter was installed at Siple Station, Antarctica (L ∼4.3) in 1979. A study has been made of the occurrence properties of magnetospheric signals from the transmitter detected at the conjugate station, Roberval, Canada and in the two‐hop mode at Siple in 1980. The diurnal variation of the signals exhibited the previously noted major activity peak near dawn and a secondary peak near dusk. The former is believed to be due to a corresponding peak in resonant particle fluxes within the outer plasmasphere and the latter to destabilization of energetic particles that locally penetrated the evening bulge of the plasmasphere. The propagation paths followed by the better defined 2–4 kHz Siple signals had equatorial radii in the range 4 <L < 4.75. This region maps at 100 km altitude to a north–south range of approximately ±150 km, centered on the latitude of Siple. The east–west range is believed to have been of comparable size. This concentration of signal paths is attributed to the effects of an injected wave power threshold for fast temporal wave growth in the magnetosphere, combined with spatial variation in ionospheric illumination by the transmitter. Also important was a concentration of operator‐selected transmitter frequencies below 4 kHz, which permitted ducted propagation (i.e., f < ƒHeq/2, where ƒHeq is the equatorial gyrofrequency) on paths beginning nearly overhead at ∼ 100 km altitude. The data are consistent with the existence of a relatively narrow north–south separation of ∼ 100 km or less between the ionospheric point of wave injection and the field lines of the duct itself. The Siple transmitter has been able to induce magnetospheric wave growth and triggered emissions over the range 0.2 < ƒ/ƒHeq < 0.5. These results differ from the reported concentration at ƒ/ƒHeq ∼0.5 of discrete emission triggering by whistlers and by certain VLF transmitters. Enhanced Siple signal activity at ground stations has resulted from increases in transmitter power, a notable change from 1978 to 1980 being in the 2–3 kHz range, where Jupiter realized its largest (∼10 dB) power advantage over Zeus.