Brieuc Collet

Saturn’s Kilometric Radiation (SKR) is a powerful non-thermal radio auroral emission produced by the Cyclotron Maser Instability (CMI), which generates waves close to the local cyclotron frequency in a highly magnetized plasma from mildly relativistic electrons. The source of free energy for SKR was identified by Cassini to be the monoenergetic shell (horseshoe) distributions of 6-12 keV. Since then, Juno identified loss cone and shell distributions as the main and secondary CMI sources of free energy at Jupiter. We reanalyze Cassini measurements during SKR source crossings. We identify similar empty loss cones between 1.6 and 8.9 keV on 6 occasions alongside shell distributions. We show that loss cone distributions are unstable by computing the CMI growth rate. The amplified waves are predicted to have frequencies between 1.003 fce and 1.017 fce with a beaming angle close to 80°. Finally, we compare our findings with those obtained at Jupiter.

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.

Among the zoo of Jovian auroral radio emissions, the broadband kilometric (bKOM) component has been the least studied. Taking advantage of Juno in situ measurements within the auroral regions, we surveyed the Juno/Waves radio observations over the 60 first orbits to identify 7 bKOM source candidates. These were mostly detected during dawn storm auroral episodes (4/7) and three were found to be colocated with auroral cavities. By applying a growth rate analysis based on JADE-E electron measurements, we show that the observed waves are driven by the Cyclotron Maser Instability from two free energy sources. The main emission, produced slightly above the electron gyrofrequency fce, is amplified by conics-type electron distribution functions (EDF) with 2─30 keV electron characteristic energies. Sporadic bursts, produced slightly below fce, are driven by shell-type EDF of 0.1─10 keV. These results are analyzed comparatively to those obtained previously for Jovian hectometric and decametric emissions

Auroral radio emissions from Earth, Saturn, and Jupiter are now known to all be generated by the cyclotron maser instability near the electron cyclotron fundamental frequency f_ce. This common generation mechanism results in similar wave properties in terms of beaming and polarization. However, while harmonics at 2 f_ce and 3 f_ce have been identified in the terrestrial and kronian cases, none of the components of Jupiter’s auroral radio spectrum had been associated with harmonics. In this study, we confirm the existence of first harmonic emissions for the Jovian broadband-kilometric (bKOM) and hectometric (HOM) components (200-700 kHz) in 6 cases using in situ data from Juno/Waves observations close to the source. Among these cases, 2 second harmonics were also identified. These harmonics are three orders of magnitude weaker than the associated fundamental and were identified in regions where the f_pe/f_ce(f_pe the electron plasma frequency) ratio is of the order of 10^-2. This discovery confirms the universality of the CMI and suggests that harmonics in the decametric range could exist.