Effect of limiting polarization in the Jovian inner magnetosphere

Abstract

Jovian decameter emission is known to exhibit almost total polarization. We consider the elliptical polarization to be a consequence of linear-mode coupling in the Jovian magnetosphere outside the source region. We determine conditions of emission propagation along the ray path that are necessary for self-consistent explanatation of the polarization observations and show that the ellipticity (axial ratio of the polarization ellipse) is determined by the magnetospheric plasma density n_e in a small region a distance of about half the Jovian radius from the radiation source. The plasma density in the region is quite low, n_e<0.4 cm⁻³, and the geometrical-optics approximation of emssion propagation in front of the region converts to the vacuum approximation behind it. The latter means that the linear-mode coupling in the Jovian inner magnetosphere is manifested as the effect of limiting polarization. Sources of decameter emission emitting at different frequencies f are located at heights corresponding to gyrofrequency levels f _Be ≅f and at magnetic-force lines that belong to L-shells passing through the satellite Io. The location of the transitional region in the Jovian magnetosphere varies depending on the emission frequency and the time. For each given decameter radio emission storm occupying some region in frequencytime space, we have a number of transitional regions located in a certain region of the Jovian magnetosphere—the interaction region of the magnetosphere (IRM) for the given emission storm. The distribution of magnetospheric plasma in an IRM is found from data of observations of the polarization ellipiicity of the given decameter radio emission storm. By matching the calculated ellipticity of emission with the observed ellipticity at every point of frequency-time space of the emission dynamic spectrum one finds a recurrent relation between the local values of the magnetospheric plasma density N_c and the planetary magnetic field B in the IRM, which allows evaluation of the distribution of plasma density if a definite model of the Jovian magnetic field has been adopted.