Direction finding of half‐gyrofrequency VLF emissions in the off‐equatorial region of the magnetosphere and their generation and propagation

Abstract

The spectra of VLF/ELF emissions in the off‐equatorial region in the outer magnetosphere are, on occasion, found to consist of upper and lower bands with a frequency gap between them. The wave normal directions of the upper band VLF emissions have been determined by means of wave distribution function analyses based first on the measurement of three magnetic field components only, and then with the additional use of an electric field component. It is found that the wave distribution functions are composed of two peaks whose central wave normal angles θ with respect to the magnetic field at a geomagnetic latitude of ∼17° are close to the local oblique resonance cone θres but whose values of θ are always about 15°–20° less than θres at a higher latitude of ∼26°. This observed wave normal behavior is compared with the theoretical prediction from direct ray tracing studies using the hypothesis deduced from a previous investigation that the waves are generated at the equator with wave normals close to θres. A combination of this comparison and inverse ray tracing in which the initial wave normal directions are those determined from the wave distribution analysis has yielded the result that the θ values of the upper band VLF emissions are very close to θres at the equator and that the normalized frequency there, Λeq, is above 0.5. Hence the upper band VLF emissions are identified as being “half‐gyrofrequency” VLF emissions generated in the vicinity of the magnetic equator, with their wave normals close to θres. The characteristics of propagation between the equatorial source region and the observing positions have been investigated by direct ray tracing. The generation mechanism is also discussed.