Occurrence diagram of Jovian decameter radiation in elliptic coordinates
Résumé
Four zones of enhanced probability are revealed in the CML-Io phase diagram, where the occurrence of the Jovian decameter radiation is plotted versus the central meridian longitude (CML) and the phase of the satellite Io. These zones are usually named Io-controlled sources Io-A, Io-B (emitted from the northern hemisphere), and Io-C, Io-D (emitted from the south). Supposing that the radio emission is generated near the local gyro-frequency (by the cyclotron maser instability) along a magnetic field line carried away by Io during its revolution around Jupiter (with a lead angle δ), we examine the occurrence probability of the radiation in a polar diagram defined by the local magnetic field. The location of the four Io-controlled sources (Io-A, Io-B, Io-C and Io-D) are plotted as a function of the colatitude angle θ relative to the gradient of the magnetic field (playing the role of an optical axis for the wave propagation) and an azimuth angle ψ linked to the direction of magnetic field vector. The angular distribution of the sources clearly shows that the angle θ is not constant, so that the Jovian decameter emission controlled by Io is radiated in a hollow cone which is not axi-symmetrical around the magnetic field gradient but flattened in the direction of the magnetic field vector. A precise oblate beaming cone has been computed for each Jovian hemisphere using elliptic coordinates. Our result is compatible with the existence of a sector of active longitude favouring the radiation in regions of the CML-Io phase diagram that are in good agreement with the observations.