Intensity of Discrete VLF Emissions

Abstract

Very low frequency whistler mode noise from the magnetosphere frequently appears in the form of narrow band tones of variable frequency called discrete VLF emissions. These tones may appear spontaneously or they may be triggered by transmissions from ground based VLF stations (Helliwell, 1965). They can be observed from within the plasmasphere out to the magnetopause and at frequencies from 300 to 30000 Hz. A phenomenological theory of discrete emissions has been advanced, based on cyclotron resonance between energetic electrons and narrow band whistler mode waves traveling along the static magnetic field (Helliwell, 1967). An important feature of this theory is the maintenance of oscillations through feedback between the waves and the electrons over a path that is long compared with the wavelength.