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 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. Plain Language Summary: Taking advantage of Juno radio, electron and magnetic measurements within the source of Jupiter's auroral radio emissions, we analyze a new type of HectOMectric (HOM, a wavelength of 1 hm matching a frequency of 3 MHz) emissions observed in situ by Juno/Waves at frequencies f below the electron cyclotron frequency fce. We first survey the Juno/Waves radio observations over 1–40 MHz between 2016 and 2022, covering the first 45 orbits. The 15 detected cases of f < fce emissions are much less frequent than the usual HOM emissions observed slightly above fce and their sources are inhomogeneously distributed. We then analyze these events in the framework of the Cyclotron Maser Instability (CMI) by calculating their theoretical growth rate from electron distribution functions simultaneously measured by the Juno/JADE‐E spectrometer. We show that the f < fce HOM sources are definitely consistent with the CMI powered by electron beams of 0.2–5 keV. This new type of Jovian auroral radio emission is reminiscent of the ones prominently observed at Earth and Saturn. These f < fce sources co‐exist with HOM emission at fce < f < fce + 0.5%, which is also driven by the CMI based on different well‐known sources of free energy. Key Points: A survey of Juno/Waves in situ measurement (2016–2022) reveals 15 hectometric sources observed below the local electron cyclotron frequencyWe show with a Cyclotron Maser Instability growth rate analysis using Juno/JADE‐E data that they are generated by 0.2–5 keV shell electronsThis new source of Jovian auroral radio emission is reminiscent of the auroral kilometric radiations of Earth and Saturn [ABSTRACT FROM AUTHOR]