Role of Magnetic Field Gradient on the Beaming Cone Flattening of Io-Controlled Jovian Decameter Radiation

Abstract:

A recent study of the angular distribution of the Jovian decameter radiation occurrence probability, relatively to the local magnetic field B and its gradient Grad B in the source region, revealed that the radio emission is beamed in a hollow cone presenting a flattening in a certain direction. In the present work, we investigate some physical reasons for the existence of such a flattening in the beam geometry. 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 electron population of the plasma and the electromagnetic waves with right circular polarization of the X mode. In a medium with axial symmetry, i.e., where B and Grad B are parallel, the maximum amplification is obtained 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. The angle corresponding to the maximum amplification is not constant anymore when B and Grad B are not parallel, so that the emission cone does not have any axial symmetry and then presents a flattening in a privileged direction. The topology of the magnetic field models (O6, VIP4, VIT4, VIPAL) developed for describing the Jovian magnetospheric environment seems to indicate that such a geometry exists in the case of the Io-controlled Jovian decameter radiation.