Abstract. 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 radio arcs. We analyze the frequency-time shape of nine such arcs, corresponding to four observation geometries (A,B,C,D) and detected over their full frequency extent in joint Nanqay and Wind data. We compute the 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 made regarding the radio source beaming, and its location is only constrained 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), shit•ed by an average of 10 ø (in the south) and 25 ø (in the north) with respect to the IFT. This lag must be accumulated before Io's magnetic perturbation reaches the radio emission region, at high latitudes, at altitudes 5 1 R• above the Jovian surface. Radio emission is found to be beamed in a hollow cone of average half-apex angle 70ø-75 ø and thickness --1 ø. Arc shapes are fully determined by the geometry of observation. Radio fringes preceding the main Io B arcs are well explained by multiple reflections of the magnetic perturbation between Jupiter's ionosphere and Io's torus. The weak trailing part of Io-B arcs may be accounted for through double beaming of the radio emission or through a frequency-dependent lag of the corresponding radio source. The latter explanation suggests an emission scenario in which electron acceleration "leaks" from the magnetic perturbation on its way to Jupiter. Jovian magnetic field models are compared and evaluated in the analysis.

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