Theoretical model of Saturn's kilometric radiation spectrum

Abstract

A theoretical model is developed here in order to determine an envelope for the average spectrum of the Saturnian kilometric radiation (SKR). The microscopic generation mechanism is supposed to be the so‐called synchrotron (or cyclotron) maser instability. As in recent works on the terrestrial kilometric radiation, the effect of the magnetic field inhomogeneity on the generation process must be taken into account. Then, assuming that the emission is nonlinearly saturated by trapping, our calculation allows us to put an upper limit on the SKR spectral intensity very simply: the maximum level of the wave electric field within the source region is indeed linked to a few macroscopic plasma parameters, which can be derived from the observations (the structure of the magnetospheric magnetic field, the cold plasma density, and the characteristic energy of the hot emitting electrons). We have used a dipolar magnetic field model, while the plasma distribution results from the superposition of two components: an ionospheric population and a plasma disc, whose scale heights have been roughly determined from Voyager measurements. The energetic electrons responsible for the emission are supposed to precipitate along the high‐latitude magnetic field lines where SKR emission is known to take place. The width of the source region can be self‐consistently estimated from the model. A very good agreement is obtained between the theoretical spectrum and the observational radio data. The calculated spectral intensities exceed the most intense observed intensities by up to 1 order of magnitude, suggesting that the SKR emission is only marginally saturated by nonlinear processes.