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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]

The present OSS (Outer Solar System) mission continues a long and bright tradition by associating the communities of fundamental physics and planetary sciences in a single mission with ambitious goals in both domains. OSS is an M-class mission to explore the Neptune system almost half a century after the flyby of the Voyager 2 spacecraft. Several discoveries were made by Voyager 2, including the Great Dark Spot (which has now disappeared) and Triton's geysers. Voyager 2 revealed the dynamics of Neptune's atmosphere and found four rings and evidence of ring arcs above Neptune. Benefiting from a greatly improved instrumentation, a mission as OSS would result in a striking advance in the study of the farthest planet of the solar system. Furthermore, OSS would provide a unique opportunity to visit a selected Kuiper Belt object subsequent to the passage of the Neptunian system. OSS would help consolidate the hypothesis of the origin of Triton as a Kuiper Belt object captured by Neptune, and to improve our knowledge on the formation of the solar system. The OSS probe would carry instruments allowing precise tracking of the spacecraft during the cruise. It would facilitate the best possible tests of the laws of gravity in deep space. These objectives are important for fundamental physics, as they test General Relativity, our current theoretical description of gravitation, but also for cosmology, astrophysics and planetary science, as General Relativity is used as a tool in all these domains. In particular, the models of solar system formation uses General Relativity to describe the crucial role of gravity. OSS is proposed as an international cooperation between ESA and NASA, giving the capability for ESA to launch an M-class mission towards the farthest planet of the solar system, and to a Kuiper Belt object. The proposed mission profile would allow to deliver a 500 kg class spacecraft. The design of the probe is mainly constrained by the deep space gravity test in order to minimize the perturbation of the accelerometer measurement. © 2012 Springer Science+Business Media B.V.

At Jupiter, part of the auroral radio emissions are induced by the Galilean moons Io, Europa and Ganymede. Until now, except for Ganymede, they have been only remotely detected, using ground–based radio–telescopes or electric antennas aboard spacecraft. The polar trajectory of the Juno orbiter allows the spacecraft to cross the range of magnetic flux tubes which sustain the various Jupiter–satellite interactions, and in turn to sample in situ the associated radio emission regions. In this study, we focus on the detection and the characterization of radio sources associated with Io, Europa and Ganymede. Using electric wave measurements or radio observations (Juno/Waves), in situ electron measurements (Juno/JADE–E), and magnetic field measurements (Juno/MAG) we demonstrate that the Cyclotron Maser Instability (CMI) driven by a loss–cone electron distribution function is responsible for the encountered radio sources. We confirmed that radio emissions are associated with Main (MAW) or Reflected Alfvén Wing (RAW), but also show that for Europa and Ganymede, induced radio emissions are associated with Transhemispheric Electron Beam (TEB). For each traversed radio source, we determine the latitudinal extension, the CMI–resonant electron energy, and the bandwidth of the emission. We show that the presence of Alfvén perturbations and downward field–aligned currents are necessary for the radio emissions to be amplified. Plain Language Summary: At Jupiter, the auroras are much more intense and long‐lasting than on Earth, and some are influenced by Jupiter's three largest moons: Io, Europa, and Ganymede. We're particularly interested in the radio signals from these auroras. Until recently, these signals were mainly studied from a distance, using Earth‐based telescopes or spacecraft passing by Jupiter. However, since 2016, the Juno spacecraft has been orbiting Jupiter, flying through the auroral zone. Our study investigates the creation of these radio auroras using Juno's instruments to measure radio waves, particles, and magnetic fields. Our research strongly suggests that a phenomenon called the Cyclotron Maser Instability is the cause of these radio signals. This instability happens because some electrons are not coming back from Jupiter after causing Ultraviolet aurora on top of Jupiter's atmosphere. These radio signals are connected to the moons' ultraviolet auroras. Additionally, our research highlights the importance of specific perturbations in Jupiter's magnetic field, known as Alfvén perturbations, and currents that link Jupiter to these moons. This study deepens our understanding of Jupiter‐moon interactions and sheds light on Jupiter's fascinating auroras. Key Points: All Jupiter‐moon radio emissions are shown to be similarly triggered by the CMIThe crossed radio sources are colocated with either MAW, RAW or TEB footprintsThe crossed radio sources coincide with downward field‐aligned currents and Alfvén perturbations [ABSTRACT FROM AUTHOR]

Saturn Kilometric Radiation (SKR) is a non‐thermal auroral emission with peak emission occurring at 100–400 kHz. Its properties have been extensively studied since Cassini's arrival at Saturn until mission end with its Radio and Plasma Wave Science (RPWS) experiment. Low Frequency Extensions (LFEs) of SKR which consist of global intensifications of SKR accompanied by extensions of the main SKR band down to lower frequencies have been studied in particular. Low Frequency Extensions result from internally driven tail reconnection and from solar wind compressions of the magnetosphere, which also trigger tail reconnection. They have been cataloged through visual inspection with two approaches, using an intensity threshold for LFEs in 2006 (Reed et al., 2018, https://doi.org/10.1002/2017ja024499) and more recently O'Dwyer et al. (2023a, https://doi.org/10.25546/103103) produced a sample of LFEs detected by Cassini/RPWS by fitting their exact frequency‐time coordinates with polygons. In this study we use the latter catalog of LFEs as a training set for an image based machine learning algorithm to classify all LFEs detected by Cassini/RPWS. The inputs to the model are multi‐channel images consisting of spectrogram images in flux density and degree of circular polarization. The outputs of the model are binary masks showing the exact location of the LFE in frequency‐time space. The median Intersection Over Union across the testing and training set were calculated to be 0.97 and 0.98, respectively. The output of this study is a list of all 4,874 LFEs detected using this method. The list of LFE frequency‐time coordinates is available for use amongst the scientific community. Plain Language Summary: We are using radio observations from the Cassini spacecraft that was in orbit around the planet Saturn for 13 years. We want to search for characteristic features of Saturn's auroral radio emissions (called Saturn Kilometric Radiation or SKR) in the data stream from the radio instrument—specifically events called Low Frequency Extensions (LFEs). The edges of these events can be tracked in time‐frequency spectrograms of Cassini radio observations. We find several hundred examples of the LFEs that we're looking for, and feed these into a computer algorithm which learns what they look like. The algorithm can then be applied to new/unseen data and we allow it to search for similar events. The end result is an extensive catalog of all the LFEs observed throughout the 13‐year near‐Saturn mission by the radio instrument of Cassini. This catalog can be used by the scientific community as a basis for statistical studies of Saturn's radio emissions. The machine learning aspect of this work can be adapted through something known as transfer learning to other planets where we look for similar features in data. Key Points: Supervised learning applied to database of labeled polygons marked on radio spectrogramsFocus on Low Frequency Extensions of Saturn Kilometric Radiation to return a full catalog from the Cassini missionA modified U‐Net architecture achieved median Intersection over Union values of 0.98 and 0.97 across the training and testing set [ABSTRACT FROM AUTHOR]

The dynamics of the Jovian magnetosphere is controlled by the interplay of the planet's fast rotation, its solar-wind interaction and its main plasma source at the Io torus, mediated by coupling processes involving its magnetosphere, ionosphere, and thermosphere. At the ionospheric level, these processes can be characterized by a set of parameters including conductances, field-aligned currents, horizontal currents, electric fields, transport of charged particles along field lines including the fluxes of electrons precipitating into the upper atmosphere which trigger auroral emissions, and the particle and Joule heating power dissipation rates into the upper atmosphere. Determination of these key parameters makes it possible to estimate the net transfer of momentum and energy between Jovian upper atmosphere and equatorial magnetosphere. A method based on a combined use of Juno multi-instrument data and three modeling tools was developed by Wang et al. (2021, https://doi.org/10.1029/2021ja029469) and applied to an analysis of the first nine orbits to retrieve these parameters along Juno's magnetic footprint. We extend this method to the first 30 Juno science orbits and to both hemispheres. Our results reveal a large variability of these parameters from orbit to orbit and between the two hemispheres. They also show dominant trends. Southern current systems are consistent with the generation of a region of sub-corotating ionospheric plasma flows, while both super-corotating and sub-corotating plasma flows are found in the north. These results are discussed in light of the previous space and ground-based observations and currently available models of plasma convection and current systems, and their implications are assessed. [ABSTRACT FROM AUTHOR]

We study 10 years (1995–2004 inclusive) of auroral kilometric radiation (AKR) radio emission data from the Wind spacecraft to examine the link between AKR and terrestrial substorms. We use substorm lists based on parameters including ground magnetometer signatures and geosynchronous particle injections as a basis for superposed epoch analyses of the AKR data. The results for each list show a similar, clear response of the AKR power around substorm onset. For nearly all event lists, the average response shows that the AKR power begins to increase around 20 min prior to expansion phase onset, as defined by the respective lists. The analysis of the spectral parameters of AKR bursts show that this increase in power is due to an extension of the source region to higher altitudes, which also precedes expansion phase onset by 20 min. Our observations show that the minimum frequency channel that observes AKR at this time, on average, is 60 kHz. AKR visibility is highly sensitive to observing spacecraft location, and the biggest radio response to substorm onset is seen in the 21:00–03:00 hr local time sector. Plain Language Summary: Substorms are an energetic disturbance to the magnetic environment of the Earth. They represent the driving of the terrestrial magnetosphere by particles from the Sun and the subsequent response in various parts of this environment, in both its inner and outer boundaries. These effects are mostly constrained to the nightside of Earth, and can be observed by both ground‐based and remote sensing instruments. In this work, we select auroral kilometric radiation (AKR) observations from 10 years (from 1995 to 2004 inclusive) of radio data from the Wind/WAVES instrument, and compare this with lists of substorm expansion phase onsets that are derived from various observational signatures. After accounting for visibility of the radio sources, we show that the AKR response correlates with the size/strength of the substorm, based on the sensitivity of the list. Our results show that the AKR source region tends to increase in size along magnetic field lines while the emission intensifies, using a longer data set to corroborate previous results. Key Points: Auroral kilometric radiation (AKR) observations made over 10 years are compared with four event lists of substorm onsets using superposed epoch analysesOn average, AKR power increases and source region extends to higher altitudes in the 20 min prior to onsetThe occurrence of AKR power at higher altitudes is sensitive to the substorm size [ABSTRACT FROM AUTHOR]

Auroral Kilometric Radiation (AKR) is the strongest terrestrial radio emission, and emanates from the same electron acceleration regions from which particles precipitate into the ionosphere, exciting the aurorae and other phenomena. As such, AKR is a barometer for the state of solar wind ‐ magnetosphere ‐ ionosphere coupling. AKR is anisotropically beamed in a hollow cone from a source region generally found at nightside local times, meaning that a single source region cannot be viewed from all local times in the magnetosphere. In radio data such as dynamic spectra, AKR is frequently observed simultaneously to other radio emissions which can have a similar intensity and frequency range, making it difficult to automatically detect. Building on a previously published pipeline to extract AKR emissions from Wind/WAVES data, in this paper a novel automated AKR burst detection technique is presented and applied to Wind/WAVES data. Over a five year interval, about 5000 AKR bursts are detected with median burst length ranging from about 30 to 60 min. During detected burst windows, higher solar wind velocity is observed, and the interplanetary magnetic field clock angle is observed to tend toward BZ < 0, BY < 0, when compared with the entire statistical interval. Additionally, higher geomagnetic activity is observed during burst windows at polar, high and equatorial latitudes. Plain Language Summary: Auroral Kilometric Radiation (AKR) is a terrestrial radio emission which is excited by the same electrons which enhance the aurorae. Due to a combination of complex beaming, and the statistical position of the source region, an AKR event cannot be observed at all positions in the Earth's magnetosphere. A combination of different radio emissions are simultaneously observed in the radio data, including both AKR and non‐AKR sources. Building on previous work, in this paper individual AKR burst events are automatically detected from Wind/WAVES data over a five year interval. About 5000 events are detected over the interval, during which the observed geomagnetic activity was higher. Higher solar wind velocity and differences in the morphology of the interplanetary magnetic field are also observed during burst windows, both of which are known to excite magnetospheric dynamics. Key Points: A novel technique has been developed to detect individual Auroral Kilometric Radiation bursts in Wind/WAVES dataWhen the technique is applied to 2000–2004 data, about 5000 bursts are detected with median duration 30–60 minDuring burst windows, higher solar wind velocity, more negative IMF BZ and greater geomagnetic activity is observed [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]

Auroral Kilometric Radiation (AKR) is terrestrial radio emission that originates in particle acceleration regions along magnetic field lines, coinciding with discrete auroral arcs. AKR viewing geometry is complex due to the confinement of the source regions to nightside local times (LTs) and the anisotropy of the beaming pattern, so observations are highly dependent on spacecraft viewing position. We present a novel, empirical technique that selects AKR emission from observations made with the spin‐axis aligned antenna of the Wind/WAVES instrument, based on the rapidly varying amplitude of AKR across spacecraft spin timescales. We apply the technique to Wind/WAVES data during 1999 day of year 227–257, when the Cassini spacecraft flew past Earth and provided an opportunity to observe AKR from two remote locations. We examine the AKR flux and power, with observations made from LTs of 1700–0300 hr having an average power up to 104 Wsr‐1 larger than those on the dayside and an increasing AKR power observed at higher magnetic latitudes. We perform a linear cross‐correlation between the Wind AKR power and the spacecraft magnetic latitude, showing positive then negative correlation as Wind travels from the Northern to Southern magnetic hemisphere. Statistically significant diurnal modulations are found in the whole 30‐day period and in subsets of the data covering different local time sectors, indicative of a predominantly geometrical effect for remote AKR viewing. The reproduction of well‐known features of the AKR verifies the empirical selection and shows the promise of its application to Wind/WAVES observations. Plain Language Summary: Auroral Kilometric Radiation (AKR) is naturally occurring radio emission from the Earth's Northern and Southern polar regions, which becomes more intense as the aurora brightens. In this work, we examine data from the Wind spacecraft WAVES instrument from a 30‐ day interval in 1999 when a second spacecraft, Cassini, was also flying near Earth and measuring the AKR from a different viewpoint. In this work, we select the AKR using an empirical measure of the variability observed by the WAVES instrument, and compare the distribution and time profile of AKR intensity. Comparing measurements of this radio emission from different spacecraft positions help us to understand how the AKR is best viewed and illustrate the constrained beaming of the emission. This information is important for anyone wanting to attempt to interpret measurements of the AKR. Key Points: Novel, empirically based method to extract Auroral Kilometric Radiation (AKR) from Wind/WAVES is presented and applied to observations made during the Cassini flybySelected data show a distribution of AKR power with expected longitudinal and latitudinal visibility constraintsDiurnal temporal modulation observed, suggesting the dominance of a geometric viewing effect and agreeing with previous AKR observations [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]

Within the solar system, X-ray emissions have been detected from every planet except the Ice Giants: Uranus and Neptune. We analyze the three archival Chandra X-ray observations of Uranus (each 24-30 ks duration) to date: a stand-alone Advanced CCD Imaging Spectrometer (ACIS) observation on August 7, 2002 and two High Resolution Camera (HRC) observations on November 11 and 12, 2017 coordinated with optical observations. For the earlier ACIS observation, the Uranus-coincident photons were clustered in the 0.6-1.1 keV spectral range, consistent with emission from Jupiter and Saturn. To test the significance of the detected signal, we distributed a grid of ??10,000 Uranus-sized regions across the field of view (FoV). The number of Uranus-coincident X-ray photons in the 0.5-1.2 keV range exceeded 99.9% of Uranus-sized regions across the FoV (10.2 standard deviations > FoV mean; probability of chance occurrence ~10-6-10-7). However, the planetary signal was low with only 5 ± 2.2 X-ray photons against a FoV mean background of 0.16 photons. Without the possibility of energy filtering, the recent HRC observations had a much brighter background (FoV mean ~10 photons). Consequently, neither of the new observations provided a second unambiguous Uranus detection, although a 40-min interval of brightening on November 12, 2017 did produce a signal above 99.9% of the FoV. The observed Uranus X-ray fluxes of 10-15-10-16 erg/cm²/s are consistent with previous observational limits and modeling predictions. These fluxes exceed expectations from scattered solar emission alone, suggesting either a larger X-ray albedo than Jupiter/Saturn or the possibility of additional X-ray production processes at Uranus. Further observations are needed to test this. [ABSTRACT FROM AUTHOR]

We examine the final 44 orbits of the Cassini spacecraft traversing the midnight sector of Saturn's magnetosphere to distances of ~21 Saturn radii, to investigate responses to heliospheric conditions inferred from model solar wind and Cassini galactic cosmic ray flux data. Clear storm responses to anticipated magnetospheric compressions are observed in magnetic field and energetic particle data, together with Saturn kilometric radiation (SKR), auroral hiss, and ultraviolet auroral emissions. Most compression events are associated with corotating interaction regions, producing ~2-3.5 day intervals of magnetospheric activity that are recurrent with the ~26 day solar rotation period (one or two such events per rotation), though one on the final pass is related to a nonrecurrent interplanetary shock possibly associated with an earlier X-class solar flare. The response to compressions is modulated by the concurrent relative phasing of the northern and southern planetary period oscillation (PPO) systems, with long (>1 planetary rotation) SKR low-frequency extension (LFE) intervals associated with strong field-aligned coupling currents being favored when the two PPO systems act together to thin and thicken the tail plasma sheet during each PPO cycle. LFE onsets/intensifications are then favored at thin plasma sheet phases most unstable to reconnection, producing energetic nightside particle injections and poleward contractions of dawn-brightened auroras. Correspondingly, solar rotation recurrent intervals of magnetospheric quiet conditions also occur with weak energetic particle fluxes and auroral emissions, associated with extended solar wind rarefactions. Overall, the results emphasize how strongly activity in Saturn's magnetosphere is modulated by concurrent heliospheric conditions. [ABSTRACT FROM AUTHOR]

We compare Chandra and XMM‐Newton X‐ray observations of Jupiter during 2007 with a rich multi‐instrument data set including upstream in situ solar wind measurements from the New Horizons spacecraft, radio emissions from the Nançay Decametric Array and Wind/Waves, and ultraviolet (UV) observations from the Hubble Space Telescope. New Horizons data revealed two corotating interaction regions (CIRs) impacted Jupiter during these observations. Non‐Io decametric bursts and UV emissions brightened together and varied in phase with the CIRs. We characterize three types of X‐ray aurorae: hard X‐ray bremsstrahlung main emission, pulsed/flared soft X‐ray emissions, and a newly identified dim flickering (varying on short time scales, but quasi‐continuously present) aurora. For most observations, the X‐ray aurorae were dominated by pulsed/flaring emissions, with ion spectral lines that were best fit by iogenic plasma. However, the brightest X‐ray aurora was coincident with a magnetosphere expansion. For this observation, the aurorae were produced by both flickering emission and erratic pulses/flares. Auroral spectral models for this observation required the addition of solar wind ions to attain good fits, suggesting solar wind entry into the outer magnetosphere or directly into the pole for this particularly bright observation. X‐ray bremsstrahlung from high energy electrons was only bright for one observation, which was during a forward shock. This bremsstrahlung was spatially coincident with bright UV main emission (power > 1 TW) and X‐ray ion spectral line dusk emission, suggesting closening of upward and downward current systems during the shock. Otherwise, the bremsstrahlung was dim, and UV main emission power was also lower (<700 GW), suggesting their power scaled together. Key Points: We characterize three types of X‐ray aurorae (main oval, ir/regular pulses, and flickering aurorae) and compare with radio, UV, and solar wind dataNon‐Io decametric bursts occurred with UV auroral brightening, and UV and hard X‐ray main auroral emission also brightened contemporaneouslySoft X‐ray aurora was best fit by iogenic (S, O) spectral lines except during magnetospheric expansion when solar wind ion lines were needed [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]

An analytical method is developed by which measurements made by the Cassini spacecraft in Saturn's magnetosheath can be used to infer the upstream solar wind parameters, specifically the solar wind speed (Vsw) and the dynamic pressure (Pd). The method is validated by comparing the results with other estimates of these parameters, including the mSWiM MHD model and magnetopause and bow shock models applied to observed boundary crossings. The comparisons suggest that the new inferred Vsw are on average ~40 km/s lower than the mSWiM values, and the dynamic pressure values are slightly lower as well. We find few of the lower Pd values predicted by mSWiM, probably because Cassini would have been inside the expanded magnetosphere under such conditions. Systematic temporal variations such as interplanetary shocks do seem to be captured well, with arrival times within several days of the MHD prediction. Compared to dynamic pressures estimated from boundary crossings with well‐known magnetopause and bow shock models, the magnetosheath‐inferred dynamic pressure tends to be somewhat lower, but within the uncertainties of the analytical derivation. Comparison of the inferred dynamic pressure with observed Saturn's kilometric radiation (SKR) activity reveals several episodes of very good temporal tracking between dynamic pressure and SKR intensity, with relatively short time delays (4–5 hr), suggesting rather direct driving. Such good tracking intervals occur almost exclusively on the dawnside of the magnetosphere, where the dominant SKR source is visible. When the tracking is good, the SKR fluxes vary roughly as the square of the dynamic pressure. Plain Language Summary: The extent to which Saturn's magnetosphere may be driven by variations in the upstream solar wind is not well understood, in part because single‐spacecraft missions provide no monitor of the upstream plasma to accompany the in situ magnetospheric measurements. One global measure of magnetospheric activity is Saturn's kilometric radiation (SKR). Remote observations of SKR can be combined with solar wind measurements to explore the solar wind's influence on the magnetosphere. We propose a method for greatly increasing the amount of time for which the upstream conditions can be known by using the large database of measurements made by Cassini when it was in Saturn's magnetosheath, the shocked solar wind that coats the sunward side of the magnetosphere. The resulting parameters are validated by comparison with other estimates. Then comparison with Cassini SKR observations reveals that at some times the SKR fluxes clearly track the variations in the solar wind dynamic pressure, suggesting a rather direct solar wind driving of at least some of the magnetospheric processes that produce SKR. Key Points: Analytical development of method to estimate upstream solar wind parameters from Cassini measurements in Saturn's magnetosheathResults in generally good agreement with solar wind speed and dynamic pressure estimated by other meansSome several‐day intervals are found where there is good tracking between the inferred dynamic pressure and SKR intensities [ABSTRACT FROM AUTHOR]

The upper atmosphere of Uranus has been observed to be slowly cooling between 1993 and 2011. New analysis of near-infrared observations of emission from H3+ obtained between 2012 and 2018 reveals that this cooling trend has continued, showing that the upper atmosphere has cooled for 27 years, longer than the length of a nominal season of 21 years. The new observations have offered greater spatial resolution and higher sensitivity than previous ones, enabling the characterization of the H3+ intensity as a function of local time. These profiles peak between 13 and 15 h local time, later than models suggest. The NASA Infrared Telescope Facility iSHELL instrument also provides the detection of a bright H3+ signal on 16 October 2016, rotating into view from the dawn sector. This feature is consistent with an auroral signal, but is the only of its kind present in this comprehensive dataset. This article is part of a discussion meeting issue 'Advances in hydrogen molecular ions: H3+, H5+ and beyond'. [ABSTRACT FROM AUTHOR]

Cassini's final orbits in 2016 and 2017 provided unprecedented spatial resolution of Saturn's polar regions from near‐polar spacecraft viewing geometries. Long‐wavelength channels of Cassini's Ultraviolet Imaging Spectrograph instrument detected Saturn's UV‐dark north polar hexagon near 180 nm at planetocentric latitudes near 75°N. The dark polar hexagon is surrounded by a larger, less UV‐dark collar poleward of planetocentric latitude 65°N associated with the dark north polar region seen in ground‐based images. The hexagon is closely surrounded by the main arc of Saturn's UV aurora. The UV‐dark material was locally darkest on one occasion (23 January 2017) at the boundary of the hexagon; in most Ultraviolet Imaging Spectrograph images the dark material more uniformly fills the hexagon. The observed UV‐dark stratospheric material may be a hydrocarbon haze produced by auroral ion‐neutral chemistry at submicrobar pressure levels. Ultraviolet Imaging Spectrograph polar observations are sensitive to UV‐absorbing haze particles at pressures lower than about 10–20 mbar. Key Points: Saturn's polar hexagon is visible in Cassini UVIS dataSaturn's polar hexagon is adjacent to Saturn's auroral emissionsIt is likely that auroral ion chemistry provides a source of dark material for Saturn's polar hexagon [ABSTRACT FROM AUTHOR]

Context. Earth and outer planets are known to produce intense non-thermal radio emissions through a mechanism known as cyclotron maser instability (CMI), requiring the presence of accelerated electrons generally arising from magnetospheric current systems. In return, radio emissions are a good probe of these current systems and acceleration processes. The CMI generates highly anisotropic emissions and leads to important visibility effects, which have to be taken into account when interpreting the data. Several studies have shown that modelling the radio source anisotropic beaming pattern can reveal a wealth of physical information about the planetary or exoplanetary magnetospheres that produce these emissions. Aims. We present a numerical tool, called ExPRES (Exoplanetary and Planetary Radio Emission Simulator), which is able to reproduce the occurrence in a time-frequency plane of R−X CMI-generated radio emissions from planetary magnetospheres, exoplanets, or star–planet interacting systems. Special attention is given to the computation of the radio emission beaming at and near its source. Methods. We explain what physical information about the system can be drawn from such radio observations, and how it is obtained. This information may include the location and dynamics of the radio sources, the type of current system leading to electron acceleration and their energy, and, for exoplanetary systems, the orbital period of the emitting body and the strength, rotation period, tilt, and the offset of the planetary magnetic field. Most of these parameters can only be remotely measured via radio observations. Results. The ExPRES code provides the proper framework of analysis and interpretation for past, current, and future observations of planetary radio emissions, as well as for future detection of radio emissions from exoplanetary systems (or magnetic, white dwarf–planet or white dwarf–brown dwarf systems). Our methodology can be easily adapted to simulate specific observations once effective detection is achieved. [ABSTRACT FROM AUTHOR]

We compare periods and phases of Saturn planetary period oscillations determined from Cassini magnetic field and Saturn kilometric radiation (SKR) data from the beginning of 2016 to the end of mission in mid‐September 2017, encompassing northern summer solstice in May 2017. Both data sets show that the periods are almost unchanging, varying by only ~ ±0.01 hr about 10.79 hr for the northern system and 10.68 hr for the southern system, close to values attained by mid‐2015 after period coalescence between mid‐2013 and mid‐2014. The mean absolute differences between the magnetic and SKR periods are ~0.0036 hr (~13 s), consistent with estimated magnetic measurement uncertainties, while the overall mean difference is less than 0.001 hr (~2–3 s), at the limit of resolution. The relative phasing between magnetic and SKR modulations is correspondingly near constant and such that the equatorial planetary period oscillation fields of the northern/southern systems point radially outward near‐oppositely at ~14.3/2.5 hr local time at corresponding SKR maxima, with upward planetary period oscillation currents located ~2 hr postdawn for both systems, consistent with previous intervals having dawnside spacecraft apoapsides. Southern SKR emissions are found to be significantly dual modulated at both southern and northern periods in data limited to lie well within the southern shadow zone of the northern sources. These northern period modulations are shown to be approximately in phase with those in the northern emissions, consistent with a recent suggestion that bidirectional auroral electron acceleration may generate in phase SKR emissions in both hemispheres. Key Points: Magnetic and SKR periods are near constant at 10.79/10.68 hr (±0.01 hr) for northern/southern PPOs during 2016–2017, agreeing to within ~ ±15 sNorthern/southern upward PPO currents are centered postdawn at related SKR maxima as for other mission intervals with dawnside apoapsidesSouthern hemisphere SKR emissions are dual modulated at the northern period approximately in phase with the northern emissions [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]

The Hubble Space Telescope imaged Saturn's northern ultraviolet auroras during February–June 2014, when Saturn's northern and southern magnetic perturbation fields were locked in antiphase and matched in rotation period (~10.69 hr). During this coalescence period, we test for evidence of rotational modulation of the auroras using the latest rotating current system model and kilometric radio phases derived from Cassini measurements. While we see modulation of auroral intensity in the rotating frames of the planetary period current systems, the pattern is opposite to that expected and is dominated by an asymmetric local time profile that peaks at dawn. Enhancement of the north emission by rotating upward field aligned currents (FACs) is expected to peak at magnetic longitudes of ~90°, whereas here the intensity increased at ~270°. This unexpected finding is attributed to the presence of nonplanetary period oscillation dynamics having affected the auroral morphology, together with insufficient sampling of the rotational system orientations provided during such Hubble Space Telescope campaigns. Rotational modulation is clearest at dawn regardless of the pattern's orientation, suggesting that the physical relationship between rotating FACs and auroral intensity is not direct, having a local time dependence that is not generally observed in the rotating FAC magnitudes. We also find no statistically significant planetary period oscillation of the auroral circle position, but the mean center was offset from the spin pole by ~3° latitude toward early morning local times. Mean auroral boundaries were located at equatorward and poleward colatitudes of 15.0 ± 2.8° and 12.4 ± 3.0°, respectively. Key Points: The 2014 Hubble Space Telescope imagery of Saturn's northern FUV auroras provides updated picture of statistical boundary locationsModulation of auroral intensity by rotating current systems was clearest at dawn local times during the north‐south system coalescenceThis questions the idea that auroral and SKR emission enhancements simply track the rotating current systems [ABSTRACT FROM AUTHOR]

Abstract: On 15 September 2017 the Cassini spacecraft plunged into Saturn's atmosphere after 13 years of successful exploration of the Saturnian system. The day before, the Ultraviolet Imaging Spectrograph (UVIS) on board Cassini observed Saturn's northern aurora for about 14 hr. During these observations, several auroral structures appeared, providing clues about processes simultaneously occurring in Saturn's magnetosphere. The observed dawn auroral enhancement together with the magnetic field and plasma wave data suggest that an intense flux closure process took place in the magnetotail. This enhanced magnetotail reconnection is likely caused by a magnetospheric compression induced by an interplanetary shock. Additionally, a polar arc is observed on the duskside, tracked for the first time from its growth until its quasi‐disappearance and used as an indicator of reconnection location on the dayside magnetopause. Observation of an atypical auroral arc at very high latitudes supports the interplanetary shock scenario. [ABSTRACT FROM AUTHOR]

Abstract: Reconnection is a fundamentally important process in planetary magnetospheres, with both local and global effects. At Saturn, observations of the magnetotail reconnection site (or x‐line) are rare, with only one in situ encounter reported to date. In this work, an extensive database of plasmoids and dipolarizations (Smith et al., 2016, https://doi.org/10.1002/2015JA022005) was investigated from a multi‐instrument perspective in order to probe the location and variability of the magnetotail x‐line. Several clear intervals were identified in which the x‐line location could be indirectly inferred to move on relatively short timescales. Two case studies are presented, the first of which concerns short‐lived flows, suggesting the reconnection sites can be either short‐lived (∼10 minutes) or extremely azimuthally limited (∼3RS/0.4 hr of local time). The second interval concerns the tailward motion of the reconnection site (or sites), inferred from the increasing electron temperature (and diminishing electron density) associated with the flows. This tailward motion occurs over ∼2.5 hr (approximately a quarter of a planetary rotation). The composition of the suprathermal plasma suggests that this could be an example of the gradual depletion of mass‐loaded flux tubes (that must occur prior to lobe reconnection). These case studies are consistent with previous statistical work that suggested that the site of reconnection in the Kronian magnetotail can be highly dynamic. [ABSTRACT FROM AUTHOR]

Abstract: Saturn Kilometric Radiation (SKR) is an auroral radio emission which can be detected quasi‐continuously by the Cassini spacecraft. It has been shown to respond to magnetotail reconnection and to changes in solar wind conditions and thus offers the potential to be used as a remote proxy for magnetospheric dynamics. This work has developed criteria for the selection of low‐frequency extensions (LFEs), powerful intensifications of the main SKR emission, accompanied by an expansion of the SKR to lower frequencies. Upon examination of data from the Cassini Radio and Plasma Wave Science instrument, we detect 282 LFE events which are further grouped into two categories. Shorter events (<20 h) associated with tail reconnection have a median waiting time of ∼10 h, a median duration of 3.1 h and a strong correlation with the northern and southern SKR phase systems. The 60% of the short LFEs have a reconnection event within the preceding 6 h. Longer events (>20 h), associated with increases in solar wind dynamic pressure, can last multiple planetary rotations, have a median waiting time of ∼20 days, and show no relationship with SKR phase. An analysis of the power emitted during LFEs suggests that tail reconnection is not always observed or detected in situ which may partially explain the low correlation between LFEs and tail reconnection. We conclude that short LFEs are a good proxy for reconnection in the tail. [ABSTRACT FROM AUTHOR]

We compare observations from the Juno/Waves radio experiment with simulations of radio «arcs» in the time-frequency plane resulting from the Io-Jupiter interaction, performed with the ExPRES code. We identify the hemisphere of origin of the observed arcs directly from simulations and confirm this identification through comparison with Juno, Nançay, and Wind observations. The occurrence and shape of observed arcs are well modeled, at low latitudes with their usual shapes as seen from Earth, as well as at high latitudes with longer, bowl-shaped, arcs observed for the first time. Predicted emission is actually observed only when the radio beaming angle θ = ( k, B) ≥ 70° ± 5°, providing new constraints on the generation of the decameter emission by the Cyclotron Maser Instability. Further improvements of ExPRES are outlined, which will then be applied to Juno and Earth-based observations of radio emissions induced by other Galilean satellites or associated to the main auroral oval. [ABSTRACT FROM AUTHOR]

The Jovian high-latitude radio emissions produced by Jupiter's magnetosphere extend from a few kilohertz to 40 MHz. Part of the decametric (DAM) emissions are driven by the Galilean moon Io (Io-DAM). As UV aurorae have been detected at the footprint of Europa and Ganymede, we expect that these moons drive Jovian radio emissions as well. To check this assumption, we used the ExPRES simulation code (Exoplanetary and Planetary Radio Emissions Simulator) to predict dynamic spectrum (time-frequency spectograms) of the radio emissions controlled by the four Galilean moons. Then we compared the simulations to the Voyager/PRA and Cassini/RPWS radio observations of Jupiter (1979, and between 2000 and 2003, respectively). We present the first clear evidence for the existence of decametric emissions controlled by Europa and Ganymede. Their statistical analysis allows us to describe the average properties of the Europa-DAM and Ganymede-DAM emissions such as their spectrum, temporal variability, and occurrence as a function of moon phase and subobserver's longitude. [ABSTRACT FROM AUTHOR]