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The occurrence probability of the Jovian decameter radio emissions depends on two essential parameters: the central meridian longitude (CML) and the orbital phase of the satellite Io. Four main zones of enhanced occurrence probability emerge from the CML-Io phase diagram: the so-called Io-controlled sources Io-A, Io-B, Io-C and Io-D. We study the compatibility of the location of these sources with the existence of a specific active longitude range, anchored in Jupiter’s magnetic field, and favoring the radio emissions. A theoretical model, based on the cyclotron maser instability (CMI), was proposed a few years ago in order to explain the existence of such active longitudes, assuming that the radiation was emitted at the local gyrofrequency in a hollow cone of constant angle, along a magnetic field line carried away by Io through its revolution around Jupiter. Unfortunately this model was not able to justify the dimension in longitude of all the Io-controlled sources, in particular those located in the Jovian southern hemisphere (Io-C and Io-D). We show that the azimuthal distribution of the four occurrence regions (Io-A, Io-B, Io-C and Io-D) around the gradient of the local magnetic field is not constant so that the emission cone (in each Jovian hemisphere) presents a significant flattening in the direction of the magnetic field vector. Introducing a beaming cone with an elliptical section makes the location and extension in longitude of the sources (in the CML-Io phase diagram) compatible with the existence of an active longitude. A theory of the CMI, acting in an inhomogeneous medium in which the magnetic field vector and the gradient of its modulus are not aligned, shall be required in order to justify the flattening of the emission cone.

Intense saturnian radio emission has been observed since June 2004, and until today, by the Radio and Plasma Waves Experiment (RPWS) on board Cassini spacecraft. During this long period of about six years the spacecraft was orbiting principally in the planetary equatorial plane. However in 2007 and 2008 orbits reach latitudes higher than 50? which lead us to investigate sub-auroral saturnian radio emissions. In this study we examine the spectral distinctions between the saturnian radio emissions observed in the equatorial plane and those in latitudes close to the southern and northern auroral regions. We consider the three components reported by Galopeau et al. [2007, J. Geophys. Res., 112, A11213] principally dominated by the so-called saturnian kilometric radiation (80 kHz - 900 kHz). We analyze the occurrence probability of these components taking into consideration different geometric configurations between the spacecraft, the planet and the Sun. We discuss the spectral alteration and change, from the equatorial plane to the high latitudes, of Saturnian radio emissions. Also we examine the spectral distinction between the radio emissions emitted from the northern and southern hemispheres, in particular in the case of the saturnian kilometric radiation (SKR). Our results are discussed and compared with those already reported in the literature dealing with the Voyager and the Cassini missions.

Recent observations performed by the radio and plasma wave science (RPWS) experiment on board the Cassini spacecraft have revealed the presence of two distinct and variable spin modulation periods (10.6 hours and 10.8 hours) in Saturn’s radio emissions emanating from the northern and southern hemispheres respectively. The main time modulation of planetary radio emissions has always been attributed to the effect on the inner magnetosphere of the internal magnetic field which rigidly rotates with the deep interior of the planet. The magnetospheric plasma is supposed to be frozen in this magnetic field so that a north/south asymmetry in the radio modulation period should never be observed. However Saturn’s magnetic field is very particular since its dipolar moment is nearly aligned with the rotation axis of the planet. Such an alignment could bring out some phenomena in the internal structure which are masked in the case of other magnetized planets the magnetic dipole of which is significantly tilted. The existence of two separated and slowly varying periods in the saturnian magnetic field could be the signature of a dynamo the dynamics of which is governed by a Rikitake system.