IO-CONTROLLED DECAMETER ARCS AND IO-JUPITER INTERACTION

The motion of the satellite Io in Jupiter's magnetic field results in an electrodynamic circuit, approximately fixed in Io's frame, revealed by ultraviolet and infrared spots at the footprints of Io's flux tube (IFT) as well as prominent (so-called ''Io-controlled'') decameter ra dio arcs. We analyze the frequency-time shape of nine such arcs, corre sponding to four observation geometries (A,B,C,D) and detected over th eir full frequency extent in joint NanFay and Wind data. We compute th e radio beaming angle as a function of frequency and lag of the radio- emitting flux tube(s) relative to the IFT. No a priori assumption is m ade regarding the radio source beaming, and its location is only const rained by the emission occurring near the local electron gyrofrequency . We find northern sources for A and B arcs and southern sources for C and D arcs. The shape of all Io arcs is consistent with an origin at a single flux tube (in Io's frame), shifted by an average of 10 degree s (in the south) and 25 degrees (in the north) with respect to the IFT . This lag must be accumulated before Io's magnetic perturbation reach es the radio emission region, at high latitudes, at altitudes less tha n or equal to 1 R-J above the Jovian surface. Radio emission is found to be beamed in a hollow cone of average half-apex angle 70 degrees-75 degrees and thickness approximate to 1 degrees. Are shapes are fully determined by the geometry of observation. Radio fringes preceding the main 10 B arcs are well explained by multiple reflections of the magn etic perturbation between Jupiter's ionosphere and Io's torus. The wea k trailing part of Io-B arcs may be accounted for through double beami ng of the radio emission or through a frequency-dependent lag of the c orresponding radio source. The latter explanation suggests an emission scenario in which electron acceleration ''leaks'' from the magnetic p erturbation on its way to Jupiter. Jovian magnetic field models are co mpared and-evaluated in the analysis.