FARADAY-ROTATION OF JUPITER DECAMETRIC RADIATION

Jovian decametric emission (DAM) is known to exhibit, contrary to other low-frequency planetary radio emission, a high degree of linear polarization. As it travels through the Io plasma torus and the terrestrial ionosphere, DAM is subject to Faraday rotation that may exceed approximately 10(2) radian at 30 MHz. We report recent observations of DAM's Faraday rotation obtained with the array of 144 log-periodic antennas at Nancay (France) coupled to a wide band (10-40 MHz) sweep frequency polarimeter. The study of the Faraday effect on Jovian emission shows that the usual simple approximation where the rotation of the polarization plane varies as the inverse of the frequency squared (nu-2) cannot explain the observations. A second order term proportional to (nu-4), resulting from the difference between the X- and O-mode ray paths and the high density gradients of the ionosphere, must be included in order to account for the observations. For the three most accurate measurements, done on three Io-B emissions, the major axis of the polarization ellipse at Jupiter is found, on average, to be at position angle + 15 from the jovian northern pole; unlike, five Io-A events exhibit really a random distribution.