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

Abstract

AbstractIn this study, we statistically analyze the Jovian auroral radio sources detected in situ by Juno/Waves at frequencies f below the electron cyclotron frequency fce. 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 fce 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 < fce 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 < fce 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 fce < f < fce + 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 fce.