An oblate beaming cone for Io-controlled Jovian decameter emission

Abstract : We investigate the emission cone of the Io-controlled Jovian decameter radiation from the layout of the sources in the CML-Io phase diagram where the occurrence of the radiation is plotted versus the central meridian longitude (CML) and the phase of the satellite Io. Four zones of enhanced probability are revealed in this diagram and named Io-controlled sources Io-A, Io-B, Io-C and Io-D. We propose an angular representation of the CML-Io phase diagram in a coordinate system linked to the local magnetic field in the radio source and the gradient of the magnetic field. The angular distribution of the sources in such a diagram clearly shows that the radio emission is radiated in a hollow cone which is not axisymmetrical around the magnetic field gradient but flattened in the direction of the magnetic field vector. The use of elliptic coordinates allows us to compute the variable opening angle and the flattening of the cone in both Jovian hemispheres. Finally it comes from our investigation that the flattening of the emission cone allows the theoretical existence of a Jovian active longitude sector much more easily than in the case of an axisymmetrical cone, and the zones of maximum theoretical amplification correspond to the areas of high occurrence probability observed in the CML-Io phase diagram.