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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]

We investigate the beaming of 11 Io‐Jupiter decametric (Io‐DAM) emissions observed by Juno/Waves, the Nançay Decameter Array, and NenuFAR. Using an up‐to‐date magnetic field model and three methods to position the active Io Flux Tube (IFT), we accurately locate the radiosources and determine their emission angle θ from the local magnetic field vector. These methods use (a) updated models of the IFT equatorial lead angle, (b) ultraviolet (UV) images of Jupiter's aurorae, and (c) multi‐point radio measurements. The kinetic energy Ee− of source electrons is then inferred from θ in the framework of the Cyclotron Maser Instability. The precise position of the active IFT achieved from methods (b and c) can be used to test the effective plasma density of the Io torus. Simultaneous radio/UV observations reveal that multiple Io‐DAM arcs are associated with multiple UV spots and provide the first direct evidence of an Io‐DAM arc associated with a trans‐hemispheric beam UV spot. Multi‐point radio observations probe the Io‐DAM sources at various altitudes, times and hemispheres. Overall, θ varies a function of frequency (altitude), by decreasing from 75°−80° to 70°−75° over 10−40 MHz with slightly larger values in the northern hemisphere, and independently varies as a function of time (or longitude of Io). Its uncertainty of a few degrees is dominated by the error on the longitude of the active IFT. The inferred values of Ee− also vary as a function of altitude and time. For the 11 investigated cases, they range from 3 to 16 keV, with a 6.6 ± 2.7 keV average. Plain Language Summary: The auroral decametric emissions of Jupiter induced by Io (Io‐DAM) are radiated along high latitude magnetic field lines at large aperture angles from the local magnetic field vector, forming a thin hollow cone. In this study, we determine the emission angle θ of 11 cases of Io‐DAM emissions observed by Juno/Waves, the Nançay Decameter Array and the NenuFAR radiotelescope with an up‐to‐date magnetic field model and three different methods aimed at minimizing the uncertainty on θ. These methods accurately position the active Io magnetic Flux Tube (IFT) which hosts the decametric radiosources by using (a) models of the active IFT, (b) ultraviolet images of Jupiter's aurorae, and (c) multi‐point radio measurements. most notably, we found that θ varies within 70°–80° as a function of the source altitude along the field line and independently as a function of time. Assuming that the Io‐DAM emissions are driven by the Cyclotron Maser Instability from energetic electrons, we infer from the measured θ the kinetic energy Ee− of the source electrons accelerated by the Io‐Jupiter interaction. The obtained values of Ee− also depend on altitude and time and vary between 3 and 16 keV, with a ∼6.5 keV average, in agreement with Juno in situ measurements. Key Points: We derive the Io‐decametric emission angle θ from Juno, Nançay Decameter Array, and NenuFAR data using 3 methods to locate the radio sourcesθ(f) decreases from 75°−80° to 70°−75° over 10–40 MHz and varies both as a function of frequency (or altitude) and time (or longitude of Io)The inferred electron energies amplifying Io‐decametric waves range from 3 to 16 keV also vary as a function of altitude and time [ABSTRACT FROM AUTHOR]

Reanalyzing Cassini radio observations performed during Jupiter's flyby of 2000–2001, we study the internal (rotational) versus external (solar wind) control of Jupiter's radio emissions, from kilometer to decameter wavelengths, and the relations between the different auroral radio components. For that purpose, we build a database of the occurrence of Jovian auroral radio components bKOM, HOM, and DAM observed by Cassini, and then frequency‐longitude stacked plots of the polarized intensity of these radio components. Comparing the results obtained inbound and outbound, as a function of the Observer's or Sun's longitude, we find that HOM & DAM are dominantly rotation‐modulated (i.e., emitted from searchlight‐like sources fixed in Jovian longitude), whereas bKOM is modulated more strongly by the solar wind than by the rotation (i.e., emitted from sources more active within a given Local Time sector). We propose a simple analytical description of these internal and external modulations and evaluate its main parameters (the amplitude of each control) for HOM + DAM and bKOM. Comparing Cassini and Nançay Decameter Array data, we find that HOM is primarily connected to the decameter emissions originating from the dusk sector of the Jovian magnetosphere. HOM and DAM components form a complex but stable pattern in the frequency‐longitude plane. HOM also seems to be related to the "lesser arcs" identified by Voyager. bKOM consists of a main part above ∼40 kHz in antiphase with HOM occurrence, and detached patches below ∼80 kHz in phase with HOM. The frequency‐longitude patterns formed by DAM, HOM and bKOM remain to be modeled. Key Points: We build synthetic frequency‐longitude maps of polarized Jovian auroral radio emissions, DAM, HOM, and bKOMJovian HOM and DAM are dominantly rotation‐modulated, bKOM dominantly solar wind‐modulatedHOM appears primarily connected to decameter sources from the dusk side of the Jovian magnetosphere [ABSTRACT FROM AUTHOR]

Since the discovery of Jovian auroral radio emissions, the question arises of the source positions of the different components (broadband kilometric, hectometric, and decametric) and their association with far ultraviolet (FUV) auroral emissions. We surveyed Juno's first 15 perijoves to track local radio sources from in situ Juno/Waves measurements (50 Hz to 40 MHz). This allowed us to study the 3‐D spatial distribution of the broadband kilometric, hectometric, and decametric radio sources. These sources are carried by the same magnetic field lines, with the bulk of them at apex M ranging from 15 to 60 (distance measured in RJ at the magnetic equator). Finally, comparisons with images of the Jovian FUV aurorae simultaneously acquired by the Hubble Space Telescope (HST) reveal a partial spatial colocation of the FUV main oval emission with the identified local radio sources. Plain Language Summary: Jupiter produces auroral emission at radio and far ultraviolet (FUV) wavelengths. These emissions have been studied for more than half a century, from an equatorial point of view. The Juno spacecraft, in orbit around Jupiter since July 2016, passes above the northern and southern poles once per orbit, and thus inside the regions where the auroral radio emission occurs. Using the first 15 Juno's orbits, we found that the sources, from the kilometer to the decameter wavelengths, are all colocated. The comparison with simultaneous images on the FUV wavelengths, acquired by the Hubble Space Telescope, reveals that the source of Jovian auroral radiation at the kilometer and decameter wavelengths are magnetically connected to region of FUV emissions. Key Points: The position of the sources of the broadband Kilometric, Hectometric and Decametric auroral radio emissions is statistically determined from in situ source crossings by Juno/WavesAll radio sources are found to be located on the same set of magnetic field lines, with M ranging from 10 to 60, and are connected to the main UV ovalCorrespondence between auroral radio and ultraviolet emissions validated by simultaneous auroral observations by Juno/Waves (radio) and HST/STIS (FarUltraviolet) [ABSTRACT FROM AUTHOR]

Throughout 2017, the Hubble Space Telescope (HST) observed the northern far‐ultraviolet aurorae of Saturn at northern solstice, during the Cassini Grand Finale. These conditions provided a complete viewing of the northern auroral region from Earth and a maximal solar illumination, expected to maximize the ionosphere‐magnetosphere coupling. We analyze 24 HST images concurrently with Cassini measurements of Saturn's kilometric radiation and solar wind parameters predicted by two magnetohydrodynamic models. The aurorae reveal highly variable components, down to timescales of minutes, radiating 7 to 124 GW. They include a nightside‐shifted main oval, unexpectedly frequent and bright cusp emissions, and a dayside low‐latitude component. On average, these emissions display a strong local time dependence with two maxima at dawn and premidnight, the latter being newly observed and attributed to nightside injections possibly associated with solstice conditions. These results provide a reference frame to analyze Cassini in situ measurements, whether simultaneous or not. Plain Language Summary: In 2017, the Hubble Space Telescope regularly observed the northern ultraviolet aurorae of Saturn in coordination with Cassini in situ measurements obtained during the Grand Finale, when the spacecraft flew across magnetic field lines connected to the aurorae. Hubble imaged Saturn's aurorae at 24 occasions spread over 7 months during northern solstice, when the northern auroral region was both fully visible from Earth and permanently illuminated by the Sun. The observed aurorae display a variety of components observed poleward of 68° latitude with different properties, some of which were unreported before. These emissions strongly vary with time, down to a few minutes, and radiate from 7 to 124 GW. On average, the auroral intensity also strongly varies with local time (a Sun‐referenced frame) and peaks at dawn, as previously observed, and also premidnight, pointing to a recurrent nightside activity of the magnetosphere. These results provide a reference basis to analyze Cassini in situ measurements. Key Points: Saturn's northern UV aurorae at solstice were sampled from HST observations coordinated with Cassini's Grand FinaleThe observed aurorae are highly variable with powerful events, radiating up to 124 GW, controlled by solar wind and planetary rotationThe average auroral brightness strongly varies with LT with two maxima at dawn (previously known) and premidnight (newly identified) [ABSTRACT FROM AUTHOR]