Faraday rotation in Jupiter's decametric radio emission used for remote sensing of the terrestrial ionosphere and the emission's source region at Jupiter

Abstract

Faraday rotation in the Jovian decametric radiation (DAM) is a well-known phenomenon when performing observations from ground-based telescopes which are sensitive to linear polarization. It is due to the different phase velocity of the ordinary and extra-ordinary wave modes that results in a rotation of the major axis of the elliptically polarized Jovian decametric emission when propagating through a magneto-active plasma. The amount of rotation is frequency dependent and therefore the Faraday rotation can be well recognized on the dynamic spectra by intensity minima (when the major axis of the polarization ellipse is perpendicular to the antenna elements) and maxima (when the major axis is parallel to the antenna elements) as a function of frequency. The study of Faraday rotation allows remote sensing of the terrestrial ionosphere and/or the source regions of the decametric emission in the Jovian magnetosphere. Investigating Jovian decametric observations containing Faraday rotation made at the Kharkov radio-telescope (Ukraine), it is found that the polarization angle at Jupiter (orientation of the major axis of the polarization ellipse at the radio source) cannot unam-biguously be determined with these data. Results on the polarization angle of Jovian DAM already given in the literature are still contradictory. Also it is not yet clear whether or not the total amount of Faraday rotation observed is due to the terrestrial ionosphere or if there is additionally a considerable contribution from the Jovian magnetosphere. Using modeled Faraday rotation patterns derived by electromagnetic ray-tracing through a realistic terrestrial ionospheric model enable most of the conflicting results to be explained in the literature by systematic errors due to over-simplification made in the respective assumptions. It is concluded that the amount of Faraday rotation observed in Jovian DAM spectra can be explained by the terrestrial ionosphere. At least in the case of low ionospheric plasma densities, the quadratic formula Ω = C 0 + C 1 f 2 + C 2 f 4 is sufficient to describe the number of Faraday fringes Ω as a function of frequency. Also, the polarization angle is confirmed to be parallel to B at the Jovian source position.