Modelling the Io‐related DAM emission by modifying the beaming angle

Abstract

The Io–Jupiter interaction generates strong decametric radio emissions (DAM), which appear as arcs in the time–frequency plane. These emissions are beamed at an angle from the magnetic field lines, which may vary with frequency and longitude amongst other properties. Empirical models of this beaming angle describe the shape of the DAM arcs and offer insight into the emission mechanism for DAM. Several studies have investigated the variation in the emission beaming angle. The studies span a range of frequencies which depend on the observational means (spacecraft, ground‐based radio telescopes) used to obtain data. Subsequently, because of the varying assumptions made (e.g. relativistic vs. non‐relativistic electrons for the wave polarization), methods used (e.g. prescribing a beaming angle function vs. determining a beaming angle function from observational geometry) and frequency ranges observed, different results have been found in each study. In the present paper, we model the shape of the emission with an empirical beaming angle function and adjust the parameters to best fit the emission arcs. However, our model builds on previous models by taking into account the location of Io in the Jovian magnetic field. We also look at a broader frequency range than many of the intermediate studies. We find that a simple empirical beaming angle function describes the shape of the A, B, and D arcs and that the beaming angle function must decrease at high and low frequencies. We then propose a simple explanation for the beaming angle profile, deduced from cyclotron maser theory.