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International audience; A recent study of the Io-controlled Jovian decameter radiation revealed that the radio emission is beamed in a hollow cone which presents a flattening in a spe- cific direction linked to the local magnetic field in the source. We investigate some reasons for the existence of such a flattening. The Jovian decameter radiation, like the other auroral radio emissions emanating from the magnetized planets in the solar system, is known to be produced by the cyclotron maser instability (CMI). This mechanism allows the direct amplification of the waves through a resonant coupling between the elec- tron population of the plasma and the electromagneticwaves with right circular polarization of the X mode. In a medium with axial symmetry, i.e., where B and ∇B are parallel, this amplification is maximum for a particular value of the emergence angle relatively to the local magnetic field B. We suppose that the plasma is constituted of a cold component which supports the wave propagation and an energetic component which takes part in the growth of the waves by supplying the CMI with free energy. When B and ∇B are not parallel, the angle corresponding to the maximum am- plification is not constant anymore, so that the emis- sion cone does not have any axial symmetry and then presents a flattening; it is the case of the Io-controlled Jovian decameter radiation.

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.

Long-term observations of Jupiter's decametric (DAM) radiation have shown that a large part of emission is modulated by two dominant factors: the planetary rotation and the orbital phase of Io. The first one indicates that the occurrence probability of the radiation depends on the observer's longitude, while the second factor points to a control of part of the radio emission by Io. Spectropolarimeter measurements showed that the radiation emitted from the northern and southern hemispheres are right-hand and left-hand circular polarized, respectively. Here we compare the Jovian decametric polarization observations to those derived from the active longitude model proposed by Galopeau et al. (JGR, 109, 2004). This model gives evidence of the presence of a longitude range in the northern and southern hemispheres favoring the radio decametric emission and leading to a higher occurrence probability. We discuss the common polarization features between the model and the observations, and we show that the behaviors of the southern and northern sources are not alike despite a same generation mechanism.

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, reveals four zones of enhanced probability: the so-called 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 gyrofrequency (by the cyclotron maser instability) along a magnetic field line carried away by Io during its revolution around Jupiter (with a lead angle ), we study the occurence probability in a polar diagram defined by the local magnetic field. More precisely we examine the location of the four Io-controlled sources (Io-A, Io-B, Io-C and Io- D) as a function of the colatitude angle relative to the gradient of the magnetic field (playing the role of an optical axis for the wave propagation) and an azimuth angle linked to the direction of magnetic field vector. The angular distribution of the sources clearly shows that the angle is not constant and 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. The use of elliptic coordinates allows us to compute with precision an oblate beaming cone for each Jovian hemisphere. This new result is compatible with the existence of a sector of active longitude favouring the radiation in regions of the CML-Io phase diagram in good agreement with the observations.

International audience; We study the frequency and time variations of Jovian hectometric emissions (HOM) recorded by the Radio and Plasma Wave Science (RPWS) experiment onboard the Cassini spacecraft during its Jupiter flyby. The capabilities of the RPWS experiment enable us to analyze the intensity extinction of HOM radiation, the so-called attenuation band. Using about 7 weeks of RPWS data obtained around the closest approach, a statistical analysis investigates the spectral variations of this phenomenon with respect to the spacecraft magnetic latitude and central meridian longitude. We show that the 'trace' of the attenuation band is usually not a full sinusoid as reported in previous studies and only parts of the curve are observed. Also, the intensity extinction can occur when the spacecraft is not in the planetary magnetic equator plane. This specific feature appears or vanishes when the observer (e.g., Cassini or Galileo spacecraft) is far from or close to the planet, respectively. The regular and systematic observations of the attenuation band suggest that the plasma medium at the origin of these features is steady and stable. The Io torus may be considered to be the most probable plasma medium where HOM emission is refracted through its raypath propagation. Furthermore, intensity extinction at frequencies higher than 3 MHz and up to 5 MHz implies the presence of particular electronic density irregularities in the Io torus. Volcanic activity, particularly in the northern hemisphere of the Io satellite, may be the source of such Io torus plasma irregularities.

International audience; The RPWS experiment onboard Cassini spacecraft provides, since mid 2004, regular and continuous observations of Saturnian radio emissions in the frequency range from few Hz to 1.3 MHz. A first attempt to classify and to recognize the different components have been reported by Galopeau et al. (JGR, 2007). It have been shown that three main components are observed principally dominated by the so-called Saturnian Kilometric Radiation (80 kHz - 900 kHz), and two others at low (5 kHz - 80 kHz) and high (900 kHz - 1.2 MHz) frequencies. These components are observed when the spacecraft orbits were mostly in the equatorial plane of the planet. In this contribution we re-analyse these components by considering another time interval in which the Cassini orbits are above the equatorial plane, in particular at latitudes close to the Southern and Northern auroral regions. We study the probability of occurrence of these components taking into consideration the distance to Saturn, the latitude, the Saturnian Longitude System (SLS), and the geometric configuration between the spacecraft, the planet and the Sun. We show a spectral alteration of Saturnian radio emissions when the Cassini positions (i.e. from the equatorial plane to the high latitudes) is close to the auroral regions. 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 by the Voyager and Cassini missions.

International audience; The occurrence probability of the Io-controlled Jovian decameter radio emissions depends on the central meridian longitude and the orbital phase of the satellite Io. Investigations by Galopeau et al. (2004, 2007) have shown that some specific Jovian “active” longitudes favor the radiation. The authors proposed a model which involves the cyclotron maser instability (CMI) as the mechanism at the origin of the Jovian radio emissions produced near the local gyrofrequency, along an active magnetic field line carried away by Io through its revolution around Jupiter. Those studies brought out the existence of an active longitude anchored in Jupiter's magnetic field and directly related to the efficiency of the CMI. In the present analysis, we model the four occurrence regions associated to Io-controlled sources. This approach is in a reverse way of the parametric method developed by Galopeau et al. (2007). The properties of the propagation and polarization of the radio wave, derived from the CMI, are the key ingredients for the study of the beaming cone. It is shown that the lead angle of the active magnetic field line relative to Io has a significant effect on both the selection of the propagation conditions and the limit between the right- and left-hand polarization states. The modeled and observed occurrence regions are found to be similar for a lead angle of about 20°. However, it seems that the behaviors of the southern and northern sources are not alike despite a common generation mechanism.

International audience; Recent investigations by Galopeau et al. (J. Geophys. Res. 112, 2007) have shown that some specificJovian ‘active’ longitudes favour the Io-controlled Jovian decameter radiation by considering that theemissions are produced near the local gyrofrequency by the cyclotron maser instability (CMI), along anactive magnetic field line carried away by Io along its revolution around Jupiter, within a hollow cone ofconstant angle θ. The active field line is supposed to present a lead angle δ relatively to Io’s position. Thismodel can explain why the occurrence probability of the radio emission is larger in some specific regionsof the central meridian longitude (CML)-Io phase diagram: the so-called Io-A, Io-B, Io-C and Io-D‘sources’. Long term observations allow us to define four boxes representing the typical Io-controlledsource regions. The precise location of these four boxes strongly constrains the range of possible activelongitude for both northern and southern radio emissions. Our model also allows us to derive the half-angle θ of the hollow cone describing the emission beaming. We analyse the effect of the emissionfrequency and the value of the lead angle δ on the active longitude range and emission angle θ. Our studyshows that a common active longitude can only explain part of the emissions coming from restrictedzones of the CML-Io phase diagram. It is particularly clear for the Io-D source

International audience; We report on the Jovian radio wave propagationthrough the Io torus plasma. We select cases ofJovian hectometric (HOM) emission which aresubject to attenuation effect. The morphology ofsuch phenomena is characterized by the use ofRPWS experiment observations recorded duringthe Cassini Jupiter flyby. This attenuation isshown to depend on the Jovian magnetic fieldwhere clear modulations appear at two centralmeridian longitudes around ~180° and ~50°,corresponding respectively to the position of themagnetic field axis in the Northern and Southernhemispheres. A detailed analysis of the dynamicspectra leads us to confirm (previous work byLecacheux, JGR, 85, 1980) the presence ofrefraction effect caused by the Io plasma torus.The Jovian radio wave presents different spectralfeatures when it propagates through the Io plasmatorus. We note in particular three main cases: atotal emission extinction, a partial decrease of theradiation and an enhancement of the intensitybefore and after the attenuation of HOM emission.The origins of such spectral effects depend on: thesource location (in the Northern or Southernhemisphere), the observer position (e.g. Cassinispacecraft) with regard to the magnetic equator ofthe planet, and the electronic density in the Iotorus. We discuss our results and compare them toprevious studies based on data recorded byVoyager, Galileo and Cassini spacecraft

International audience; The occurrence probability of the Io-controlled Jovian decameter radio emissions depends on both the central meridian longitude (CML) and the phase of the satellite Io. Recent investigations by Galopeau et al. (J. Geophys. Res. 112, 2007) have shown that some specific Jovian 'active' longitudes favour the radiation by considering that the emissions are produced near the local gyrofrequency, along an active magnetic field line carried away by Io along its revolution around Jupiter, within a hollow cone of constant angle. This model involves the cyclotron maser instability as the mechanism at the origin of the radio emission. The observed occurrence probability happens to be larger in some specific regions of the CML-Io phase diagram, the so-called Io-controlled emissions and well-known as Io-A, Io-B, Io-C and Io-D 'sources'. In the present investigation, we model four boxes representing the typical Io-controlled source regions in this diagram. We discuss the consequences and the constraints on the location of the Jovian active longitude at the origin of the Io-controlled radiation simultaneously for both hemispheres. The first results show that the active longitude model can only explain part of the emissions coming from restricted zones of the CML-Io phase diagram. This is particular the case for the emissions of the Io-D source.