Ground-based evidence of latitude-dependent cyclotron absorption of whistler mode signals originating from VLF transmitters

We report the results of the first multisite campaign using VLF Doppler receivers to detect whistler mode signals in the south Atlantic originating from VLF transmitters in the northern hemisphere. The signals at 24 and 21.4 kHz were recorded by a mobile receiver on board the RRS Bransfield during February-March 1993 and an identical system at Faraday, Antarctica. The data show that the output power of plasmaspheric ducts varies with latitude, decreasing with increasing L shell between 1.6 less than or equal to L less than or equal to 2.7. Using the HOTRAY ray tracing code, we find that the primary cause of the latitudinal variation in duct output power is absorption via cyclotron resonance with energetic electrons in the plasmasphere. The variation is due mainly to the reduction in resonant energy with increasing L shell which brings more particles into resonance with the wave. We demonstrate that ground-based observations of the absorption of whistler mode signals originating from VLF transmitters can be used as a new way of estimating the energetic electron flux between 1 and 40 keV inside the plasmasphere between 1.6 less than or equal to L less than or equal to 2.7. Typically, estimates of electron flux of 1 to 7 times greater than that reported by Schield and Frank [1970] are required to model the experimental data recorded here during nondisturbed geomagnetic conditions.