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 central meridian longitude (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.

The low-frequency source of Saturn’s kilometric radiation

“…Figure S4E shows an alternative representation of q( f) in the time-frequency plane. This systematic change from perpendicular to oblique emission with increasing frequency is most likely due to refraction of R-X mode waves on the nearby iso-f X surface, where f X~fce is the R-X mode cutoff frequency (8), as proposed for auroral radio emissions of Earth and Jupiter (34,35). In this case, the apparent R-X mode beaming angle should vary as a function of the k-vector direction projected in a plane perpendicular to B, with the strongest refraction expected for wave propagation toward the equator.…”

Detection of Jupiter decametric emissions controlled by Europa and Ganymede with Voyager/PRA and Cassini/RPWS

“…These differences slightly change the maximum frequency of the emission from a few tenths to a few MHz, depending on the longitudes of the sources. As in Hess et al [], and unlike in Ray and Hess [], we did not simulate any refraction effect, suspected to play a nonnegligible role, especially close to the source [ Galopeau and Boudjada , ]. The refraction effects need to be taken into account for a better spectral simulation.…”

“…This continuity at high frequencies is not observed in practice for near‐equatorial observations, with a range of Io phases where no Io‐DAM emission is visible [see Marques et al , ]. A possibility is that the emission cone is likely oblate, with a reduced value of θ toward the magnetic equator, making it invisible for near‐equatorial observers, or observers in the hemisphere opposed to the source [ Galopeau and Boudjada , ; Louis et al , ].…”

Statistical study of latitudinal beaming of Jupiter's decametric radio emissions using Juno

“…A difference between our fixed thin hollow cone and their oblique hollow cone on Jovian northern radio sources is that the z axis of our model is parallel to B , not −∇ B . Using Galopeau and Boudjada []'s model, we tested whether the J‐shaped non‐Io‐B arcs at 19.5 MHz and 16.5 MHz can be accounted for. Insofar as we assume the beaming angles are acute, there were no solutions to match this J‐shaped pattern.…”