Jupiter’s magnetic field and the generation and control of decameter radiation observed by the Juno spacecraft during the prime mission

Decametric radio emissions (DAM) originating in Jupiter’s polar magnetosphere ought to originate on magnetic field lines at the local electron gyrofrequency. The Io-related DAM have received the most attention since the 1980’s. The maximum frequency of these emissions ought to be bounded by the maximum magnetic field strength above the footprint of the instantaneous Io Flux Tube (IFT). However, there remains a lack of agreement between the frequency extent of Io-related decameter radiation and the frequency extent predicted by Jovian magnetic field models. Here, we analyze peak frequencies and source locations of Io and non-Io-related DAM observed by Juno during the prime mission (~10,600 events) and show how the latest magnetic field model can accommodate and control Io-DAM. We note that the observed peak frequencies appear to be truncated at 37 MHz although the magnetic field in the northern hemisphere would allow events to 55 MHz at some longitudes. Lower frequencies than the ones allowed by the magnetic field are consistently observed for most of the Io’s longitude. To reconcile this discrepancy, we analyze the upper electron density limit distribution along the magnetic field lines, the possible existence of plasma cavities and the locations in the magnetosphere where the extraordinary mode is no longer achieved. For this, we make use of beaming angles of Io-DAM and the geometry of the Jovian magnetic field.