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Banded chorus—A new type of VLF radiation observed in the magnetosphere by OGO 1 and OGO 3


  • W. J. Burtis,

  • R. A. Helliwell


Satellites OGO 1 and OGO 3 observe VLF discrete emissions in the magnetosphere primarily in a single, variable frequency band. The frequency ƒ of this ‘banded chorus’ depends on the equatorial electron gyrofrequency ƒHO for the field line passing through the satellite, typical ratios of ƒ/ƒHO being 0.2–0.5. Evidently the emissions are produced near the equator at a fraction of the electron gyrofrequency, as predicted by electron cyclotron resonance generation mechanisms. A secondary dependence of the banded chorus frequency on dipole latitude, such that the lower ratios of ƒ/ƒHO are found at higher latitudes, is interpreted to mean that the emissions are generated at about half the electron gyrofrequency, but deviate inward from the field line to lower L values as they propagate earthward. Theoretical support is given by ray tracings showing the inward deviation of nonducted whistler-mode radiation due to the curvature of the magnetic field. Banded chorus has been observed at all local times, but is most common in the morning magnetosphere, outside the plasmapause.

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