Predictions for Uranus-moons radio emissions and comparison with Voyager 2/PRA observations

Abstract

Jupiter's and Saturn's moons are known to induce auroral emission in the Ultraviolet wavelengths. At Jupiter, the moons Io, Europa and Ganymede are also responsible for powerful decametric radio emissions, driven by the cyclotron maser instability, sustained by weakly relativistic electrons energized through an Alfvénic acceleration process. At Uranus, periodic hectometric radio emissions, observed by the radio instrument Planetary Radio Astronomy experiment of Voyager 2 just after the closest approach of Uranus on 24th Jan. 1986, were tentatively attributed by Kistler (1988) to a similar Alfvénic interaction between the moon Ariel and the planet. Taking advantage of recent prediction and modeling efforts devoted to understand the Jupiter-satellite decametric emissions, and in the context of preparing a future Uranus Orbiter, we calculate an estimate of the theoretical power generated as Alfvén waves by the electrodynamic interaction between Uranus and its three closest large satellites Miranda, Ariel and Umbriel of 10^5 W. We then present simulations of the expected radio emission induced by these moons, using the Exoplanetary and Planetary Radio Emission Simulator (ExPRES). The simulations are compared to the high resolution measurements of the radio instrument onboard Voyager 2 recently refurbished in a digital format. Our conclusion conrms that the arc-shaped intermittent structure identied by Kistler (1988) is consistent with emissions induced by the Ariel-Uranus interaction, produced through the cyclotron maser instability driven by a loss cone-type electron distribution function and a resonant electron population of 20 keV.