Evidence of jovian active longitude: 1. Efficiency of cyclotron maser instability

Long-term observations of Jupiter's decametric radiation have shown that a great 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. Within the framework of the cyclotron maser instability, which is supposed to be the mechanism at the origin of most planetary radio emissions, we estimate the efficiency of this theoretical mechanism at the footprint of the Io flux tube during a complete revolution of the satellite around Jupiter. Our study is based on several simplifying hypotheses: on one hand, we suppose that electrons are accelerated in the neighborhood of Io and follow an adiabatic motion along magnetic field lines carried by the satellite; on the other hand, we assume that a loss cone built up by electrons which have disappeared in Jupiter's ionosphere constitutes the main source of free energy needed by the cyclotron maser instability to produce the radiation. We calculate the maximum growth rate of the waves amplified by the mechanism as a function of the jovicentric longitude of Io. It emerges that some longitudes in the Northern and Southern Hemispheres favor the radio decametric emission and lead to a higher occurrence probability. Our results are compared to the occurrences observed for the sources Io-A, Io-B, Io-C, and Io-D in the usual central meridian longitude-Io phase diagram.