A New Type of Jovian Hectometric Radiation Powered by Monoenergetic Electron Beams

In this study, we statistically analyze the Jovian auroral radio sources detected in situ by Juno/Waves at frequencies f below the electron cyclotron frequency f_ce. We first conduct a survey of Juno/Waves data over 1-40 MHz from 2016 to 2022. The 15 detected HectOMetric (HOM) sources all lie within 1-5 MHz and are both less frequent than the radio sources commonly observed slightly above f_ce and clustered in the southern hemisphere, within ∼90-270° longitudes. We analyze these emission regions with a growth rate analysis in the framework of the Cyclotron Maser Instability (CMI), which we apply to JADE-E high cadence electron measurements. We show that the f < f_ce emissions correspond to crossed radio sources, ∼300 km wide. They are located in a hot and highly depleted auroral plasma environment, along flux tubes colocated with upward field-aligned current and at the equatorward edge of the main auroral oval. The wave amplification is consistent with the CMI and its free energy source consists of a shell-type electron distribution function (EDF) with characteristic energies of 0.2-5keV. More energetic, 5-50 keV, shell-type EDFs were systematically observed at higher latitudes but without any radio counterpart. Various parameters for the f < f_ce HOM sources, reminiscent of the ones at Earth/Saturn, are compared. Other CMI-unstable EDFs, primarily loss cone ones, are systematically observed during the same intervals, giving rise to emission observed at f_ce < f < f_ce + 0.5%. Our analysis thus reveals that different portions of the same EDF can be CMI-unstable and simultaneously amplify radio waves below and above f_ce.