Whistler Mode and Theory of VLF Emission

Abstract

The object of the present lectures is to give an account of the theory of the natural VLF emissions. One of the main characteristics of the VLF emissions is that they are narrow band. The frequency at a given instant is generally very well defined, therefore, requiring a very specific mechanism for their emissions. The emissions lie principally in the range from 2 to 10 Kc/sec. This is remarkably low, corresponding to a ratio f/f_H = 2.7×10⁻³ near the earth’s surface (typical values: f = 4 Kc/sec, H = 0.4 to 0.5 gauss, f_H ≃ 1.5 Mc/sec). Since the two natural frequencies for a plasma are the plasma frequency f_p, which in the Earth’s exosphere and ionosphere is everywhere larger than 100 Kc/sec, and the electron gyrofrequency f_H, both considerably larger than the observed frequencies almost everywhere in the magnetosphere, the VLF emissions are certainly not due to resonance phenomena by local excitation of these frequencies in the ambient plasma surrounding the earth. Far away from the Earth the resonance at the gyrofrequency could give right values for the frequencies of VLF emissions, but the absorption for electromagnetic waves propagating at the gyrofrequency in a plasma is so large that there is no propagation.