Lead angles and emitting electron energies of Io-controlled decameter radio arcs

Abstract

The Io-controlled radio arcs are emissions in the decametric radio range which appear arc shaped in the time–frequency plane. Their occurrence is controlled by Io's position, so it has been for long inferred that they are powered by the Io–Jupiter electrodynamic interaction. Their frequency ranges correspond to the electron cyclotron frequencies along the Io Flux tube, so they are expected to be generated by cyclotron maser instability (CMI). The arc shape was proposed to be a consequence of the strong anisotropy of the decametric radio emissions beaming, combined with the topology of the magnetic field in the source and the observation geometry. Recent papers succeeded at reproducing the morphologies of a few typical radio arcs by modeling in three dimensions the observation geometry, using the best available magnetic field model and a beaming angle variation consistent with a loss-cone driven CMI. In the continuation of these studies, we present here the systematic modeling of a larger number of observations of the radio arcs emitted in Jupiter's southern hemisphere (including multiple arcs or arcs exhibiting abrupt changes of shape), which permits to obtain a statistical determination of the emitting field line localization (lead angle) relative to the instantaneous Io field line, and of the emitting particle velocities or energies. Variations of these parameters relative to Io's longitude are also measured and compared to the location of the UV footprints of the Io–Jupiter interaction. It is shown that the data are better organized in a reference frame attached to the UV spot resulting from the main Alfvén wing resulting from the Io–Jupiter interaction. It is proposed that the radio arcs are related to the first reflected Alfvén wing rather than to the main one.