A test of incoherent Cerenkov radiation for VLF hiss and other magnetospheric emissions

Abstract

Results of a calculation to ascertain if VLF hiss (several kilohertz to several hundred kilohertz) is explained by incoherent Cerenkov radiation from intense fluxes of electrons (hundreds to thousands of eV) in the magnetosphere are presented. The Cerenkov and cyclotron power emission formulas of Mansfield have been modified to include collisions in order to limit realistically the emitted power and to estimate the spatial attenuation of propagating waves. Ray paths in three dimensions are calculated between the generation region and the observation region where the theoretical power density and fields are evaluated. A comparison is made to a very intense VLF hiss event by using the electron spectrum observed simultaneously with the Injun 5 spacecraft. Although the calculated and observed spectral shapes are in agreement, the power and fields are 2 orders of magnitude too low. It is argued that the source may be a cooperative mechanism, such as partially coherent or amplified Cerenkov emission, which operates in the 3000- to 10,000-km altitude range and which emits waves with initial wave normals near the resonance cone angle (≳80° from the magnetic field direction). Incoherent Cerenkov radiation has also been considered as a possible source for several other emission phenomena. Upper hybrid resonance noise and fast hisslers might be understood in terms of this mechanism, although a more detailed calculation is necessary; V-shaped VLF hiss, saucers, ELF hiss, and Io-related Jovian decametric radiation are not explained. Other suggested mechanisms for these phenomena are discussed. Ray-tracing calculations generally confirm the deduced source regions for these various emission phenomena.