Discrete VLF emissions (7–9 kHz) displaying unusual banded and periodic structure

Abstract

Rising frequency VLF emissions having unusually high frequency and exhibiting banded structure were recorded between 14.55 and 15.30 U.T. on 28 June 1972 by the VLF goniometer receiver at Halley, Antarctica. The risers were split into two frequency groups, one with frequencies in the range 6.0–7.7 kHz and the other with frequencies between 7.8 and 9.4 kHz, the former being more numerous. The gap between the lower and upper frequency risers is superficially similar to, though at a higher frequency than, the missing bands in emissions observed by satellites. However, it is found to be unlikely that the risers received at Halley can be explained by any of the mechanisms advanced to explain the banded satellite-observed emissions. Several other explanations are considered and it is shown that the most likely is partial suppression by magnetospheric line radiation propagating in the same duct. The risers are interpreted as being generated via cyclotron resonance with counterstreaming electrons. A computer program based on Helliwell's (1967) phenomenological theory is used to determine the generation region and electron energies involved. Both frequency groups of risers display a 4 min periodicity in occurrence. It is shown that this time period is consistent with that required to replenish the flux of resonant electrons, by eastwards drift into the duct, after the emissions have been quenched due to the reduction of the flux by pitch-angle diffusion into the loss cone.