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In the Jovian magnetosphere, quasi‐periodic phenomena, with quasi‐periods on the order of 10–60 min, are frequently identified using different data sets. These pulsations are a branch of ultra‐low frequency (ULF) waves, which are believed to play a crucial role in driving the energy circulation within Jupiter's magnetosphere. In this study, we utilize magnetic field data collected by Juno between 2016 and 2022 to perform a comprehensive global statistical analysis of the spatial distribution and periodic characteristics of ULF waves in the Jovian magnetosphere. Our findings reveal distinct periodic features observed at different latitudes and distances, providing valuable insights into the generation mechanisms of ULF waves. Furthermore, we establish a close relationship between the presence of these ULF wave fluctuations and the magnetospheric state, such as under conditions of solar wind compression. By combining contemporaneous ultraviolet aurora observations from the Hubble Space Telescope (HST) and magnetic field data obtained by Juno, we have discovered that the compressed magnetospheres exhibit more pronounced ULF waves and enhanced auroral activity. These results provide a global picture of the distribution, implying potential generation of ULF waves in the Jovian magnetosphere, and shedding light on the processes behind the 10–60‐min energy releases. In the Jovian system, there are patterns of magnetic waves with periods of 10–60 min. These waves are part of a larger category called ultra‐low‐frequency waves and play an important role in how energy moves around Jupiter. Here, we study these waves using data collected by the Juno spacecraft from 2016 to 2022, investigating where these waves occur and how often they occur. The waves are different depending on the location and distance from Jupiter, which gives us clues about how they are created. These waves also appear to be more common when Jupiter's magnetic field is compressed by the solar wind. We have also combined data from the Hubble Space Telescope and Juno, and found that the compressed magnetic field leads to more of these waves and stronger auroras on Jupiter. This research helps us to understand how these 10–60‐min bursts of energy work in Jupiter's magnetosphere. We investigate the spatial distribution and periodic characteristics of ultra‐low frequency (ULF) waves in the Jovian magnetosphereWe examine and compare the occurrence rate of ULF waves under different solar wind compression statesThe connection between ULF waves and auroral morphology provides key constraints on wave generation mechanisms We investigate the spatial distribution and periodic characteristics of ultra‐low frequency (ULF) waves in the Jovian magnetosphere We examine and compare the occurrence rate of ULF waves under different solar wind compression states The connection between ULF waves and auroral morphology provides key constraints on wave generation mechanisms

We present simultaneous observations of aurorae at Jupiter from the Hubble Space Telescope and Hisaki, in combination with the in situ measurements of magnetic field, particles, and radio waves from the Juno Spacecraft in the outer magnetosphere, from ~ 80RJto 60RJduring 17 to 22 March 2017. Two cycles of accumulation and release of magnetic flux, named magnetic loading/unloading, were identified during this period, which correlate well with electron energization and auroral intensifications. Magnetic reconnection events are identified during both the loading and unloading periods, indicating that reconnection and unloading are independent processes. These results show that the dynamics in the middle magnetosphere are coupled with auroral variability. Accumulation and release of magnetic flux in the middle Jovian magnetosphere modulate auroral intensificationsMagnetic reconnection process occurs independently of Jupiter's global loading and unloading of magnetic fluxWe provide direct evidence that unloading of magnetic flux causes enhancements of auroral kilometric emissions

We present comparisons of precipitating electron flux and auroral brightness measurements made during several Juno transits over Jupiter's auroral regions in both hemispheres. We extract from the ultraviolet spectrograph (UVS) spectral imager H2emission intensities at locations magnetically conjugate to the spacecraft using the JRM09 model. We use UVS images as close in time as possible to the electron measurements by the Jupiter Energetic Particle Detector Instrument (JEDI) instrument. The upward electron flux generally exceeds the downward component and shows a broadband energy distribution. Auroral intensity is related to total precipitated electron flux and compared with the energy‐integrated JEDI flux inside the loss cone. The far ultraviolet color ratio along the spacecraft footprint maps variations of the mean energy of the auroral electron precipitation. A wide diversity of situations has been observed. The intensity of the diffuse emission equatorward of the main oval is generally in fair agreement with the JEDI downward energy flux. The intensity of the ME matches exceeds or remains below the value expected from the JEDI electron energy flux. The polar emission may be more than an order of magnitude brighter than associated with the JEDI electron flux in association with high values of the color ratio. We tentatively explain these observations by the location of the electron energization region relative to Juno's orbit as it transits the auroral region. Current models predict that the extent and the altitude of electron acceleration along the magnetic field lines are consistent with this assumption. We compare precipitated electron flux measured with JEDI to the auroral intensity and FUV color ratio observed with UVS at Juno's footprintThe different UV auroral features generally map well with the corresponding structures measured in the precipitated electron energy fluxComparison of energy flux at the two levels reveal a diversity of situations probably related to the location of the acceleration region

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. 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

The two main ultraviolet‐signatures resulting from the Io‐magnetosphere interaction are the local auroras on Io's atmosphere, and the Io footprints on Jupiter. We study here how Io's daily eclipses affect the footprint. Previous observations showed that its atmosphere collapses in eclipse. While remote observers can observe Io's local auroras briefly when Io disappears behind Jupiter, Juno is able to follow the Io footprint in the unlit hemisphere. Theoretical models of the variability of the energy flux fed into the Alfvén wings, ultimately powering the footprints, are not sufficiently constrained by observations. For the first time, we use observations of Io's footprint from the Ultraviolet Spectrograph (UVS) on Juno recorded as Io went into eclipse. We benchmark the trend of the footprint brightness using observations by UVS taken over Io's complete orbit and find that the footprint emitted power variation with Jupiter's rotation shows fairly consistent trends with previous observations. Two exploitable data sets provided measurements when Io was simultaneously in eclipse. No statistically significant changes were recorded as Io left and moved into eclipse, respectively, suggesting either that (i) Io's atmospheric densities within and outside eclipse are large enough to produce a saturated plasma interaction, that is, in the saturated state, changes in Io's atmospheric properties to first order do not control the total Alfvénic energy flux, (ii) the atmospheric collapse during the Juno observations was less than previously observed, or (iii) additional processes of the Alfvén wings in addition to the Poynting flux generated at Io control the footprint luminosity. Juno‐UVS observed for the first time the Io footprints while Io went into eclipseThe effect of Io's atmospheric collapse on the footprint emitted power is investigatedNo significant variation of the footprint emitted power is observed during eclipse

One of the most intriguing discoveries of Juno is the quasi‐systematic detection of upgoing electrons above the auroral regions. Here we discuss a by‐product of the most energetic component of this population: a contamination resembling bar codes in the Juno‐UVS images. This pattern is likely caused by bursts of ∼10 MeV electrons penetrating the instrument. These events are mostly detected when Juno's magnetic footprint is located poleward of the main emission relative to the magnetic pole. The signal is not periodic, but the bursts are typically  0.1–1 s apart. They are essentially detected when Juno‐UVS is oriented toward Jupiter, indicating that the signal is due to upgoing electrons. The event detections occur between 1 and 7 Jovian radii above the 1‐bar level, suggesting that the electron acceleration takes place close to Jupiter and is thus both strong and brief. Someone's noise is always someone else's signal. Here we use the Juno‐UVS ultraviolet spectrograph on board the Juno spacecraft as a particle detector rather than a photon collector. We analyze artifacts looking like a bar code in ultraviolet images of Jupiter's aurora to investigate a very specific kind of radiation above the poles. These bar code events are caused by very sporadic bursts of relativistic electrons arising from the aurora itself, rather than the radiation belts. This indicates the existence of a very efficient mechanism accelerating electrons upward right above Jupiter's atmosphere. Rapidly varying noise caused by ∼10 MeV penetrating electrons creates bar codepatterns in Juno‐UVS dataThe typical timescale is ∼0.1‐1 sThe magnetic footprints of these bar code events are mostly located poleward of the main auroral emission relative to the magnetic pole

The Juno spacecraft crossed flux tubes connected to the Io footprint tail at low Jovian altitudes on multiple occasions. The transits covered longitudinal separations of approximately 10° to 120° along the footprint tail. Juno's suite of magnetospheric instruments acquired detailed measurements of the Io footprint tail. Juno observed planetward electron energy fluxes of ~70 mW/m2near the Io footprint and ~10 mW/m2farther down the tail, along with correlated, intense electric and magnetic wave signatures, which also decreased down the tail. All observed electron distributions were broad in energy, suggesting a dominantly broadband acceleration process, and did not show any broad inverted‐Vstructure that would be indicative of acceleration by a quasi‐static, discrete, parallel potential. Observed waves were primarily below the proton cyclotron frequency, yet identification of a definitive wave mode is elusive. Beyond 40° down the footprint tail, Juno observed depleted upward loss cones, suggesting that the broadband acceleration occurred at distances beyond Juno's transit distance of 1.3 to 1.7 RJ. For all transits, Juno observed fine structure on scales of approximately tens of kilometers and confirmed independently with electron and wave measurements that a bifurcated tail can intermittently exist. The Juno spacecraft crossed regions magnetically connected to auroral structures associated with Jupiter's moon Io on multiple occasions. The transits covered longitudinal separations of approximately 10° to 120° along Io's auroral tail. Juno's suite of instruments acquired detailed measurements of these auroral structures. Juno directly observed the electrons that sustain these auroral features before they crash into the atmosphere and generate the brilliant aurora. The flux of these electrons decreased as Juno transited the tail farther from Io's longitude. While there are two main explanations for Io's auroral signatures, the nature of the observed electrons in this work favors one mechanism over the other. When Juno was far from Io's longitude, the observations suggest that the spacecraft was below the point at which the electrons are accelerated into the atmosphere. For all transits, Juno observed fine structure on scales of approximately tens of kilometers and confirmed that a bifurcated tail can intermittently exist. Juno crossed flux tubes connected to Io's footprint tail aurora 10° to 120° downstream of the main Alfven wing (MAW) spotBroad power law‐like electron energy distributions suggest a primarily broadband acceleration mechanismJuno observed structure with ~20‐km resolution, bifurcated tails, and broadening with longitudinal separation from the MAW spot

Auroral emissions equatorward of the main emission at Jupiter are suggested to reflect the dynamics of the plasma in the middle magnetosphere. Here we examine the motion of the auroral signatures of magnetospheric injections appearing in Hubble Space Telescope (HST) images. Our results suggest that the injected plasma moves planetward and lags behind corotation. We then compare the characteristics of the observed signatures with simulations of the auroral precipitation related to injections due to pitch angle scattering. These results indicate that the lifetime of the auroral structures lies between a half and a full rotation of Jupiter. Ultraviolet spectrally resolved images acquired with HST are then used to highlight the energy‐dependent drift of the electrons in auroral injection signatures. Comparison of these observations with simulations of the energy‐dependent drift of injected particles suggests that these structures are ~3‐hr old. Finally, we extend our investigations toward larger and less structured outer emissions possibly associated with younger plasma injections. The motion and evolution of these features are similar to those of the small and compact ones considered in the first part of the study. Pitch angle scattering is the most probable auroral mechanism generating auroral signatures of plasma injections at JupiterThe auroral signatures of plasma injection have a lifetime between one half and a full Jovian rotationThere is an offset between the maxima in the brightness and color ratio profiles across auroral signature of plasma injections

Planetary magnetospheres receive plasma and energy from the Sun or moons of planets and consequently stretch magnetic field lines. The process may last for varied timescales at different planets. From time to time, energy is rapidly released in the magnetosphere and subsequently precipitated into the ionosphere and upper atmosphere. Usually, this energy dissipation is associated with magnetic dipolarization in the magnetosphere.This process is accompanied by plasma acceleration and field‐aligned current formation, and subsequently auroral emissions are often significantly enhanced. Using measurements from multiple instruments on board the Cassini spacecraft, we reveal that magnetic dipolarization events at Saturn could reoccur after one planetary rotation and name them as recurrent dipolarizations. Three events are presented, including one from the dayside magnetosphere, which has no known precedent with terrestrial magnetospheric observations. During these events, recurrent energizations of plasma (electrons or ions) were also detected, which clearly demonstrate that these processes shall not be simply attributed to modulation of planetary periodic oscillation, although we do not exclude the possibility that the planetary periodic oscillation may modulate other processes (e.g., magnetic reconnection) which energizes particles. We discuss the potential physical mechanisms for generating the recurrent dipolarization process in a comprehensive view, including aurora and energetic neutral atom emissions. Using measurements from the Cassini spacecraft, we reveal a new feature of magnetic dipolarization at Saturn, that is, the magnetic signature repeat after one planetary rotation, which is named recurrent dipolarization. Up to hundreds of kiloelectron volt electrons and ions are identified for the recurrent dipolarization events, suggesting that these particles have experienced efficient acceleration and cannot be purely due to planetary modulation. It remains a mystery why the magnetic dipolarization process associated with energetic ions and electrons could reoccur after one planetary rotation. Moreover, dipolarization process in Saturn's dayside magnetosphere is reported for the first time at Saturn, which has no known precedent with terrestrial or other planetary magnetospheric observations. The results demonstrate that magnetosphere‐ionosphere coupling dynamics at Saturn and Earth have fundamental similarities and differences. We report a recurrent type of magnetic dipolarization at SaturnThe associated processes could efficiently accelerate electrons and ions to tens to hundreds of kiloelectron voltsCorotation of magnetosphere and planetary periodic oscillation are suggested as possible mechanisms

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. The plasma dynamics within Saturn's magnetosphere and the interactions of the plasma with the planetary magnetic field and the atmosphere are very complex. The magnetospheric dynamics can be investigated using the observations of the auroral emissions in the upper atmosphere of the polar regions of Saturn, since these polar lights are generated by precipitation into the atmosphere of electrons guided by the magnetic field lines. These electrons originating from various regions of the magnetosphere, the auroral emissions provide a global picture of the magnetospheric processes all around the planet. Here we use the final observations of Saturn's aurora by the Cassini spacecraft to understand the processes which occur in Saturn's magnetosphere on the day before the end of the Cassini mission in September 2017. In particular, a brightening and a broadening of one part of the aurora might be the response to a compression of the magnetosphere induced by an interplanetary shock in the solar wind. Additionally, an arc‐like auroral structure was tracked for the first time from its growth until its disappearance, revealing the mechanism producing it. Finally, the aurora exhibits another arc almost at the pole which had never been observed before. An intense auroral storm indicates the occurrence of magnetotail reconnection induced by an ICMEFirst observational evidence of the formation of a nightside polar arcMost polar auroral arc ever observed at Saturn

We present high‐sensitivity Hubble Space Telescope (HST) Cosmic Origins Spectrograph and HST Space Telescope Imaging Spectrograph measurements of atmospheric OI 130.4‐nm and OI] 135.6‐nm emissions at Ganymede, which exhibit significant spatial and temporal variability. These observations represent the first observations of Ganymede using HST Cosmic Origins Spectrograph and of both the leading and trailing hemispheres within a single HST campaign, minimizing the potential influence of long‐term changes in the Jovian plasma sheet or in Ganymede's atmosphere on the comparison of the two hemispheres. The mean disk‐averaged OI] 135.6‐nm/OI 130.4‐nm observed intensity ratio was 2.72 ± 0.57 on the leading hemisphere and 1.42 ± 0.16 on the trailing hemisphere. The observed leading hemisphere ratios are consistent with an O2atmosphere, but we show that an atomic oxygen component of ~10% is required to produce the observed trailing hemisphere ratios. The excess 130.4‐nm emission on the trailing hemisphere relative to that expected for an O2atmosphere was ~11 R. The O column density required to produce this excess is determined based on previous estimates of the electron density and temperature at Ganymede and exceeds the limit for an optically thin atmosphere. The implication that the O atmosphere is optically thick may be investigated in future by observing Ganymede as it moves into eclipse or by determining the ratio of the individual components within the 130.4‐nm triplet. Ganymede's FUV oxygen emissions were observed on both orbital leading and trailing hemispheres in the same HST campaign for the first timeThe observed hemispheric difference in the brightness of both emissions and in their ratio therefore cannot be caused by long‐term changesThe OI] 135.6‐nm/OI 130.4‐nm ratio on the trailing hemisphere suggests that an optically thick O atmosphere is present there with O/O2~ 10%

We define the spatial clustering of X‐rays within Jupiter's northern auroral regions by classifying their distributions into “X‐ray auroral structures.” Using data from Chandra during Juno's main mission observations (24 May 2016 to 8 September 2019), we define five X‐ray structures based on their ionospheric location and calculate the distribution of auroral photons. The morphology and ionospheric location of these structures allow us to explore the possibility of numerous X‐ray auroral magnetospheric drivers. We compare these distributions to Hubble Space Telescope (HST) and Juno (Waves and MAG) data, and a 1D solar wind propagation model to infer the state of Jupiter's magnetosphere. Our results suggest that the five sub‐classes of “X‐ray structures” fall under two broad morphologies: fully polar and low latitude emissions. Visibility modeling of each structure suggests the non‐uniformity of the photon distributions across the Chandra intervals are likely associated with the switching on/off of magnetospheric drivers as opposed to geometrical effects. The combination of ultraviolet (UV) and X‐ray morphological structures is a powerful tool to elucidate the behavior of both electrons and ions and their link to solar wind/magnetospheric conditions in the absence of an upstream solar monitor. Although much work is still needed to progress the use of X‐ray morphology as a diagnostic tool, we set the foundations for future studies to continue this vital research. The mechanism that allows precipitation of ions into Jupiter's atmosphere and generate pulsed X‐ray auroral emissions is still under debate today. Previous studies have linked this driver to possible activity in Jupiter's outer magnetosphere (the interface between the solar wind and Jupiter) and have observed the emissions to exhibit variable behavior. More recent studies have suggested a wide range of physical phenomena causing these emissions. Here we explore this idea in more detail by introducing five “X‐ray auroral structures” that map to different regions in the Jovian system. Using data from the Chandra X‐ray Observatory during Juno's main mission allows us to calculate the distribution of X‐rays from Jupiter's northern auroral region. We compare our X‐ray results with the ultraviolet emissions (“UV auroral families”) observed from simultaneous Hubble Space Telescope (HST) data and infer the conditions at Jupiter using models and Juno observations. These “X‐ray structures” provide us with many ways to observe variable behavior and provide a possible tool to monitor the solar wind conditions, when used in tandem with the HST “UV auroral families.” We present the morphology of new “X‐ray auroral structures,” observed on Jupiter via Chandra's high spatial resolution cameraWe show that non‐uniform auroral photon distributions are likely from “switching on/off” of auroral driver(s) opposed to planetary tiltWe show that combination of X‐ray and ultraviolet (UV) “auroral families” may be a useful proxy to determine the magnetospheric conditions at Jupiter We present the morphology of new “X‐ray auroral structures,” observed on Jupiter via Chandra's high spatial resolution camera We show that non‐uniform auroral photon distributions are likely from “switching on/off” of auroral driver(s) opposed to planetary tilt We show that combination of X‐ray and ultraviolet (UV) “auroral families” may be a useful proxy to determine the magnetospheric conditions at Jupiter

We compare Hubble Space Telescope observations of Jupiter's FUV auroras with contemporaneous conjugate Juno in situ observations in the equatorial middle magnetosphere of Jupiter. We show that bright patches on and equatorward of the main emission are associated with hot plasma injections driven by ongoing active magnetospheric convection. During the interval that Juno crossed the magnetic field lines threading the complex of auroral patches, a series of energetic particle injection signatures were observed, and immediately prior, the plasma data exhibited flux tube interchange events indicating ongoing convection. This presents the first direct evidence that auroral morphology previously termed “strong injections” is indeed a manifestation of magnetospheric injections, and that this morphology indicates that Jupiter's magnetosphere is undergoing an interval of active iogenic plasma outflow. Auroras, known as the “Northern (or Southern) Lights” on Earth, are spectacular manifestations of energetic processes occurring in the space environment of a planet. The behavior of Jupiter's magnetosphere is dominated by the planet's rapid rotation, along with the centrifugally‐driven outflow of plasma (ionized gas) originating from active volcanoes on the moon Io. A prominent auroral feature on Jupiter has for many years been interpreted as a sign that Jupiter's magnetosphere is undergoing active convection, in which plasma from Io “falls” away from the planet, to be replaced by hot, relatively empty “bubbles” known as injections, moving inward. This feature comprises prominent patches of bright emission that are often observed in Jupiter's auroras, though the evidence associating them with injections has been largely circumstantial. Here we show that the NASA Juno spacecraft flew through such injections in the equatorial magnetosphere on magnetic field lines mapping to a cluster of auroral patches as observed by HST. The Juno data also indicated the interval was characterized by signatures of convection and outflow of plasma originating from Io. This demonstrates that auroral patches are signatures of injections, and that auroral emissions are an important tool for diagnosing the behavior of planetary magnetospheres. Bright FUV auroral patches on Jupiter are associated with magnetospheric injections and magnetospheric convectionHubble Space Telescope and Juno equatorial data show a cluster of patches is magnetically conjugate with energetic particle injectionsThe interval also exhibits flux tube interchange and lagging magnetic field associated with plasma mass outflow Bright FUV auroral patches on Jupiter are associated with magnetospheric injections and magnetospheric convection Hubble Space Telescope and Juno equatorial data show a cluster of patches is magnetically conjugate with energetic particle injections The interval also exhibits flux tube interchange and lagging magnetic field associated with plasma mass outflow

We present Cassini auroral observations obtained on 11 November 2016 with the Ultraviolet Imaging Spectrograph at the beginning of the F‐ring orbits and the Grand Finale phase of the mission. The spacecraft made a close approach to Saturn's southern pole and offered a remarkable view of the dayside and nightside aurora. With this sequence we identify, for the first time, the presence of dusk/midnight arcs, which are azimuthally spread from high to low latitudes, suggesting that their source region extends from the outer to middle/inner magnetosphere. The observed arcs could be auroral manifestations of plasma flows propagating toward the planet from the magnetotail, similar to terrestrial “auroral streamers.” During the sequence the dawn auroral region brightens and expands poleward. We suggest that the dawn auroral breakup results from a combination of plasma instability and global‐scale magnetic field reconfiguration, which is initiated by plasma flows propagating toward the planet. Alternatively, the dawn auroral enhancement could be triggered by tail magnetic reconnection. We identify a novel type of auroral arc at Saturn: “azimuthally extended polar to equator arc”The arc could be ionospheric signature of moving plasma flow released from tail reconnection, similar to the terrestrial auroral streamerDawn auroral enhancements and poleward expansion could be initiated by the auroral arc

We present the first comparison of Jupiter's auroral morphology with an extended, continuous, and complete set of near‐Jupiter interplanetary data, revealing the response of Jupiter's auroras to the interplanetary conditions. We show that for ∼1–3 days following compression region onset, the planet's main emission brightened. A duskside poleward region also brightened during compressions, as well as during shallow rarefaction conditions at the start of the program. The power emitted from the noon active region did not exhibit dependence on any interplanetary parameter, though the morphology typically differed between rarefactions and compressions. The auroras equatorward of the main emission brightened over ∼10 days following an interval of increased volcanic activity on Io. These results show that the dependence of Jupiter's magnetosphere and auroras on the interplanetary conditions are more diverse than previously thought. Jupiter's auroras (northern lights) are the brightest in the solar system, over a hundred times brighter than the Earth's. Auroras on Earth are driven by the solar wind, a million mile‐per‐hour stream of charged particles flowing away from the Sun, hitting the Earth's magnetic field, and stirring it around, but it is not known whether the solar wind causes any significant auroras on Jupiter. The main reason for this uncertainty is a lack of observations of the planet's auroras obtained while spacecraft have been near Jupiter and able to supply a full and continuous set of measurements of the solar wind and its accompanying magnetic field. In early mid‐2016 Juno approached Jupiter, providing such an interplanetary data set, and we obtained over a month's worth of observations of Jupiter's auroras using the Hubble Space Telescope. We saw several solar wind storms, each causing auroral fireworks on Jupiter. We captured the most powerful auroras observed by Hubble to date, brightened main oval emissions, and flashing high‐latitude patches of auroras during the solar wind storms. These results indicate that Jupiter's auroral response to the solar wind is more diverse than we previously have thought. We present the first comparison of Jupiter's auroras with an extended and complete set of near‐Jupiter interplanetary dataDuring compressions, the well‐defined sector of Jupiter's emission and the dusk poleward region brightened, the latter pulsatingThe power emitted from the noon active region did not exhibit dependence on any interplanetary parameter, though the morphology changed

Juno ultraviolet spectrograph (UVS) observations of Jupiter's aurora obtained during approach are presented. Prior to the bow shock crossing on 24 June 2016, the Juno approach provided a rare opportunity to correlate local solar wind conditions with Jovian auroral emissions. Some of Jupiter's auroral emissions are expected to be controlled or modified by local solar wind conditions. Here we compare synoptic Juno‐UVS observations of Jupiter's auroral emissions, acquired during 3–29 June 2016, with in situ solar wind observations, and related Jupiter observations from Earth. Four large auroral brightening events are evident in the synoptic data, in which the total emitted auroral power increases by a factor of 3–4 for a few hours. Only one of these brightening events correlates well with large transient increases in solar wind ram pressure. The brightening events which are not associated with the solar wind generally have a risetime of ~2 h and a decay time of ~5 h. Juno ultraviolet spectrograph (UVS) observations of Jupiter's aurora obtained during approach are presented. Jupiter's auroras are thought to be controlled in part by the solar wind, a stream of charged particles flowing outward from the Sun. The Juno approach was a rare opportunity to compare solar wind conditions near Jupiter with the Jovian ultraviolet aurora, using observations made during June 2016. Although Jupiter's aurora is always present, four brightening events were seen in the data, in which the total auroral power increased to several times the typical level for a few hours. Only one of these brightening events appears well connected with solar wind activity. The brightening events which are not associated with the solar wind all increase in brightness in about 2 h and then dim back down again in about another 5 h. Jupiter's aurora and the solar wind have a complex relationshipThe single solar wind structure that correlated with an auroral brightening event was a CIRAuroral brightening events which are not related to solar wind conditions have a similar time evolution

The electromagnetic interaction between Io, Europa, and Ganymede and the rotating plasma that surrounds Jupiter has a signature in the aurora of the planet. This signature, called the satellite footprint, takes the form of a series of spots located slightly downstream of the feet of the field lines passing through the moon under consideration. In the case of Io, these spots are also followed by an extended tail in the downstream direction relative to the plasma flow encountering the moon. A few examples of a tail for the Europa footprint have also been reported in the northern hemisphere. Here we present a simplified Alfvénic model for footprint tails and simulations of vertical brightness profiles for various electron distributions, which favor such a model over quasi‐static models. We also report here additional cases of Europa footprint tails, in both hemispheres, even though such detections are rare and difficult. Furthermore, we show that the Ganymede footprint can also be followed by a similar tail. Finally, we present a case of a 320° long Io footprint tail, while other cases in similar configurations do not display such a length. The length of the footprint tail is not a reliable parameter to differentiate quasi‐static and Afvénic tail generation modelsMonte Carlo simulations favor the Alfvénic electron acceleration scenario over the quasi‐static electric field scenario for the tailThe Europa and Ganymede footprints also have a tail, in both hemispheres

A multiple auroral onset substorm on 28 March 2010 provides an opportunity to understand the physical mechanism in generating auroral intensifications during a substorm expansion phase. Conjugate observations of magnetic fields and plasma from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft, of field‐aligned currents (FACs) from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) satellites, and from ground‐based magnetometers and aurora are all available. The comprehensive measurements allow us to further our understanding of the complicated causalities among dipolarization, FAC generation, particle acceleration, and auroral intensification. During the substorm expansion phase, the plasma sheet expanded and was perturbed leading to the generation of a slow mode wave, which modulated electron flux in the outer plasma sheet. During this current sheet expansion, field‐aligned currents formed, and geomagnetic perturbations were simultaneously detected by ground‐based instruments. However, a magnetic dipolarization did not occur until about 3 min later in the outer plasma sheet observed by THEMIS‐A spacecraft (THA). We believe that this dipolarization led to an efficient Fermi acceleration to electrons and consequently the cause of a significant auroral intensification during the expansion phase as observed by the All‐Sky Imagers (ASIs). This Fermi acceleration mechanism operating efficiently in the outer plasma sheet during the expansion phase could be a common explanation of the poleward auroral development after substorm onset. These results also show a good agreement between the upward FAC derived from AMPERE measurements and the auroral brightening observed by the ASIs. We present simultaneous measurements of field‐aligned currents from magnetosphere, ionosphere, and ground station for a substorm eventFermi acceleration related to the shrinkage of magnetic field in outer plasma sheet could lead to poleward auroral intensificationSlow mode wave is generated during near‐Earth current sheet expansion

Saturn's dayside aurora displays a number of morphological features poleward of the main emission region. We present an unusual morphology captured by the Hubble Space Telescope on 14 June 2014 (day 165), where for 2 h, Saturn's FUV aurora faded almost entirely, with the exception of a distinct emission spot at high latitude. The spot remained fixed in local time between 10 and 15 LT and moved poleward to a minimum colatitude of ~4°. It was bright and persistent, displaying intensities of up to 49 kR over a lifetime of 2 h. Interestingly, the spot constituted the entirety of the northern auroral emission, with no emissions present at any other local time—including Saturn's characteristic dawn arc, the complete absence of which is rarely observed. Solar wind parameters from propagation models, together with a Cassini magnetopause crossing and solar wind encounter, indicate that Saturn's magnetosphere was likely to have been embedded in a rarefaction region, resulting in an expanded magnetosphere configuration during the interval. We infer that the spot was sustained by reconnection either poleward of the cusp or at low latitudes under a strong component of interplanetary magnetic field transverse to the solar wind flow. The subsequent poleward motion could then arise from either reconfiguration of successive open field lines across the polar cap or convection of newly opened field lines. We also consider the possible modulation of the feature by planetary period rotating current systems. Saturn's dawn arc auroral emission was observed to fade strongly for several hoursThis may have been attributable to reduced plasma flow shear during a prolonged solar wind rarefactionAn isolated auroral spot emission is evidence of dayside reconnection at an expanded magnetosphere

Auroral emissions serve as a powerful tool to investigate the magnetospheric processes at Saturn. Solar wind and internally driven processes largely control Saturn's auroral morphology. The main auroral emission at Saturn is suggested to be connected with the magnetosphere‐solar wind interaction, through the flow shear related to rotational dynamics. Dawn auroral enhancements are associated with intense field‐aligned currents generated by hot tenuous plasma carried toward the planet in fast moving flux tubes as they return from tail reconnection site to the dayside. In this work we demonstrate, based on Cassini auroral observations, that the main auroral emission at Saturn, as it rotates from midnight to dusk via noon, occasionally stagnates near noon over a couple of hours. In half of the sequences examined, the auroral emission is blocked close to noon, while in three out of four cases, the blockage of the auroral emission is accompanied with signatures of dayside reconnection. We discuss some possible interpretations of the auroral “blockage” near noon. According to the first one, it could be related to local time variations of the flow shear close to noon. Auroral local time variations are also suggested to be initiated by radial transport process. Alternatively, the auroral blockage at noon could be associated with a plasma circulation theory, according to which tenuously populated closed flux tubes as they return from the nightside to the morning sector experience a blockage in the equatorial plane and they cannot rotate beyond noon. The auroral emission at Saturn as it rotates from midnight to dusk via noon occasionally is blocked near noonThe blockage of the emission near noon is often accompanied by dayside magnetopause reconnectionThe auroral blockage could be related to flow shear variations or/and to fast rotating flux tubes which experience a blockage at noon

On 27 August 2016, the NASA Juno spacecraft performed its first close‐up observations of Jupiter during its perijove. Here we present the UV images and color ratio maps from the Juno‐UVS UV imaging spectrograph acquired at that time. Data were acquired during four sequences (three in the north, one in the south) from 5:00 UT to 13:00 UT. From these observations, we produced complete maps of the Jovian aurorae, including the nightside. The sequence shows the development of intense outer emission outside the main oval, first in a localized region (255°–295° System III longitude) and then all around the pole, followed by a large nightside protrusion of auroral emissions from the main emission into the polar region. Some localized features show signs of differential drift with energy, typical of plasma injections in the middle magnetosphere. Finally, the color‐ratio map in the north shows a well‐defined area in the polar region possibly linked to the polar cap. During Perijove 1, Juno‐UVS acquired the first spatially and spectrally resolved Far‐UV observations of Jupiter's nightside auroraeDuring the 10 h sequences, large and intense outer emissions progressively developed and a protrusion formed with the southern main ovalA well‐defined area identified in the northern polar region resembles the shape, location, and area of the predicted open field line region

In early 2014, continuous monitoring with the Hisaki satellite discovered transient auroral emission at Jupiter during a period when the solar wind was relatively quiet for a few days. Simultaneous imaging made by the Hubble Space Telescope (HST) suggested that the transient aurora is associated with a global magnetospheric disturbance that spans from the inner to outer magnetosphere. However, the temporal and spatial evolutions of the magnetospheric disturbance were not resolved because of the lack of continuous monitoring of the transient aurora simultaneously with the imaging. Here we report the coordinated observation of the aurora and plasma torus made by Hisaki and HST during the approach phase of the Juno spacecraft in mid‐2016. On day 142, Hisaki detected a transient aurora with a maximum total H2emission power of ~8.5 TW. The simultaneous HST imaging was indicative of a large “dawn storm,” which is associated with tail reconnection, at the onset of the transient aurora. The outer emission, which is associated with hot plasma injection in the inner magnetosphere, followed the dawn storm within less than two Jupiter rotations. The monitoring of the torus with Hisaki indicated that the hot plasma population increased in the torus during the transient aurora. These results imply that the magnetospheric disturbance is initiated via the tail reconnection and rapidly expands toward the inner magnetosphere, followed by the hot plasma injection reaching the plasma torus. This corresponds to the radially inward transport of the plasma and/or energy from the outer to the inner magnetosphere. By monitoring of Jupiter with Hisaki and HST we discovered that dawn storm is followed by outer emission during transient auroraThe monitoring with Hisaki indicated hot electron injection in the plasma torus during the declining phase of the transient auroraEnergy for these disturbances is likely released via tail reconnection and transported to the plasma torus within a few Jupiter rotations

Solar wind energy is transferred to planetary magnetospheres via magnetopause reconnection, driving magnetospheric dynamics. At giant planets like Saturn, rapid rotation and internal plasma sources from geologically active moons also drive magnetospheric dynamics. In both cases, magnetic energy is regularly released via magnetospheric current redistributions that usually result in a change of the global magnetic field topology (named substorm dipolarization at Earth). Besides this substorm dipolarization, the front boundary of the reconnection outflow can also lead to a strong but localized magnetic dipolarization, named a reconnection front. The enhancement of the north‐south magnetic component is usually adopted as the indicator of magnetic dipolarization. However, this field increase alone cannot distinguish between the two fundamentally different mechanisms. Using measurements from Cassini, we present multiple cases whereby we identify the two distinct types of dipolarization at Saturn. A comparison between Earth and Saturn provides new insight to revealing the energy dissipation in planetary magnetospheres. Two fundamentally different types of magnetic dipolarization are identified in SaturnThe two types of dipolarization at Saturn share many similarities with Earth's dipolarization processesInternally driven process may produce both types of the magnetic dipolarization

One of the auroral features of Jupiter is the emission associated with the orbital motion of its moon Io. The relative velocity between Io and the surrounding plasma trigger perturbations that travels as Alfvén waves along the magnetic field lines toward the Jovian ionosphere. These waves can accelerate electrons into the atmosphere and ultimately produce an auroral emission, called the Io footprint. The speed of the Alfvén waves—and hence the position of the footprint—depends on the magnetic field and on the plasma distribution along the field line passing through Io, whose SO2‐rich atmosphere is the source of a dense plasma torus around Jupiter. Since 2016, the Jovian InfraRed Auroral Mapper (JIRAM) onboard Junohas been observing the Io footprint with a spatial resolution of ∼few tens of km/pixel. JIRAM detected evidences of variability in the Io footprint position that are not dependent on the System III longitude of Io. The position of the Io footprint in the JIRAM images is compared with the position predicted by a model of the Io Plasma Torus and of the magnetic field. This is the first attempt to retrieve quantitative information on the variability of the torus by looking at the Io footprint. The results are consistent with previous observations of the density and temperature of the Io Plasma Torus. However, we found that the plasma density and temperature exhibit considerable non‐System III variability that can be due either to local time asymmetry of the torus or to its temporal variability. Juno‐JIRAM detected evidence of variability in the Io footprint that are not related to the System III longitude of IoQuantitative information on the state of the Io Plasma Torus and its variability are inferred from the Io footprint positionThe Io Plasma Torus electron density varies between <2,000 and ∼2,750 cm−3, while the thermal ion temperature varies between 40–100 eV Juno‐JIRAM detected evidence of variability in the Io footprint that are not related to the System III longitude of Io Quantitative information on the state of the Io Plasma Torus and its variability are inferred from the Io footprint position The Io Plasma Torus electron density varies between <2,000 and ∼2,750 cm−3, while the thermal ion temperature varies between 40–100 eV

Juno's arrival at Jupiter in 2016 revealed unprecedented details about Jupiter's ultraviolet aurorae thanks to its unique suite of remote sensing and in situ instruments. Here we present results from in situ observations during Juno flybys above specific bright auroral spots in Jupiter's polar aurora. We compare data observed by Juno‐UVS, JEDI, JADE, Waves, and MAG instruments when Juno was magnetically connected to bright polar auroral spots (or their immediate vicinity) during perijove 3 (PJ3), PJ15, and PJ33. The highly energetic particles observed by JEDI show enhancements dominated by upward electrons, which suggests that the particle acceleration region takes place below the spacecraft. Moreover, brightness and upward particle flux were higher for the northern bright spot in PJ3 than the southern spots found in PJ15 and PJ33. In addition, we notice the intensification of whistler‐mode waves at the time of the particle enhancements, suggesting that wave‐particle interactions contribute to the acceleration of particles that cause the UV aurorae. The MAG data reveal magnetic perturbations during the PJ3 spot detection by Juno, which suggests the presence of significant field‐aligned electric currents. While the stable positions of the bright spots in System III suggest that the phenomenon is fixed to the planet's rotation, the presence of field‐aligned currents leaves the possibility of an origin rooted much farther in the magnetosphere. Jupiter's auroral bright spot emissions observed by Juno‐Ultraviolet Spectrograph were simultaneously measured with the JADE, JEDI, Waves, and MAG instrumentsFor each event, we observe characteristic changes in particle distributions, wave emissions, and magnetic field disturbancesWhistler waves and electric currents appear to play a role in generating bright auroral polar spots Jupiter's auroral bright spot emissions observed by Juno‐Ultraviolet Spectrograph were simultaneously measured with the JADE, JEDI, Waves, and MAG instruments For each event, we observe characteristic changes in particle distributions, wave emissions, and magnetic field disturbances Whistler waves and electric currents appear to play a role in generating bright auroral polar spots

Jupiter's auroral emission is a spectacular phenomenon that provides insight into energy release processes related to the coupling of its magnetosphere and ionosphere. This energy release is influenced by solar wind conditions. Using joint observations from Juno and the Hubble Space Telescope (HST), we statistically investigate the relationship between auroral power and current sheet variations under different solar wind conditions. In this study, we reveal that during global main auroral brightening events that are closely connected to solar wind compressions, the dawn side current sheet is substantially thinner than during times when a quiet auroral morphology is present. Furthermore, the total current intensity in the current sheet is found to increase under solar wind compression conditions compared to the quiet period. These findings provide important observational evidence for how magnetospheric dynamics driven by solar wind behavior affect auroral activity, deepening our understanding of the coupling between Jupiter's magnetosphere and ionosphere. Jupiter, the largest planet in our solar system, has a fascinating and powerful auroral emission that can help us understand the interactions between its magnetic field and the charged particles in its atmosphere. These auroral emissions are influenced by solar wind conditions, which are streams of charged particles coming from the Sun. In this study, we used observations from the Juno spacecraft and the Hubble Space Telescope to investigate the relationship between Jupiter's auroral emissions and changes in the planet's magnetic field. We found that during periods of increased solar wind pressure, the magnetic field layer, known as the current sheet, becomes thinner compared to times when the aurora is quiet. These findings offer valuable evidence of how Jupiter's magnetic field is affected by solar wind behavior and improve our understanding of the relationship between the planet's magnetic field and its aurora. This research helps us better comprehend the complex processes occurring in Jupiter's magnetosphere and can potentially enhance our knowledge of similar phenomena occurring on other planets. The features of the current sheet in Jupiter's dawnside magnetosphere are highly relevant to auroral contextsDuring a solar wind compression event, the dawnside current sheet becomes substantially thinner than during quiet timesThe total current intensity of the current sheet is much higher during solar wind compression events than the quiet times The features of the current sheet in Jupiter's dawnside magnetosphere are highly relevant to auroral contexts During a solar wind compression event, the dawnside current sheet becomes substantially thinner than during quiet times The total current intensity of the current sheet is much higher during solar wind compression events than the quiet times

Jupiter's satellite auroral footprints are a consequence of the interaction between the Jovian magnetic field with co‐rotating iogenic plasma and the Galilean moons. The disturbances created near the moons propagate as Alfvén waves along the magnetic field lines. The position of the moons is therefore “Alfvénically” connected to their respective auroral footprint. The angular separation from the instantaneous magnetic footprint can be estimated by the so‐called lead angle. That lead angle varies periodically as a function of orbital longitude, since the time for the Alfvén waves to reach the Jovian ionosphere varies accordingly. Using spectral images of the Main Alfvén Wing auroral spots collected by Juno‐UVS during the first 43 orbits, this work provides the first empirical model of the Io, Europa, and Ganymede equatorial lead angles for the northern and southern hemispheres. Alfvén travel times between the three innermost Galilean moons to Jupiter's northern and southern hemispheres are estimated from the lead angle measurements. We also demonstrate the accuracy of the mapping from the Juno magnetic field reference model (JRM33) at the completion of the prime mission for M‐shells extending to at least 15 RJ. Finally, we shows how the added knowledge of the lead angle can improve the interpretation of the moon‐induced decametric emissions. The interaction between the Jovian magnetospheric plasma and the Galilean moons gives rise to a complex set of phenomena, including the generation of auroral spots magnetically related to the moons and the generation of radio emissions. The magnetic perturbations local to the moons propagate at a finite speed along the magnetic field lines, and reach the northern and southern Jovian hemispheres where they produce the auroral spots. Studying the position of these auroral spots and how they vary over a complete Jovian rotation provides information about the magnetic mapping, as they map directly to the actual physical positions of the moons. The magnetic field model derived from Juno's prime mission is in good agreement with the observation of the satellite footprints. This paper provides information about how the electromagnetic perturbation resulting from the interaction propagates at a finite speed to create auroral spots, leading to an angular shift between the instantaneously magnetically‐mapped position of the moon and the auroral footprint, a quantity also known as the ”equatorial lead angle”. The present work provides an empirical fit of the equatorial lead angle for Io, Europa, and Ganymede derived from Juno data. Over 1,600 ultraviolet spectral images of the Io, Europa, and Ganymede footprints from Juno are analyzedEmpirical formulae for the Io, Europa, and Ganymede equatorial lead angles derived from Juno data are providedAlfvén travel time estimates are derived, constraining the Alfvénic interaction at the three innermost Galilean moons Over 1,600 ultraviolet spectral images of the Io, Europa, and Ganymede footprints from Juno are analyzed Empirical formulae for the Io, Europa, and Ganymede equatorial lead angles derived from Juno data are provided Alfvén travel time estimates are derived, constraining the Alfvénic interaction at the three innermost Galilean moons

During the Juno orbit 34 Ganymede encounter, the ultraviolet spectrograph mapped UV sunlight reflected by Ganymede from a closest approach altitude of 1,044 km, allowing us to study spatial variations in Ganymede's far ultraviolet reflectance at higher resolution than has previously been possible. We find that a characteristic signature of water ice seen around 165 nm in laboratory spectra is absent over much of the observed area, but is detectable in the north high latitude region. We suggest that the spectral difference between the high latitudes and other icy regions, such as Tros crater, may be explained by the presence of additional UV‐absorbing contaminants such as NH3at lower latitudes. We also note a decrease in the relative reflectance of the high latitude regions at wavelengths >190 nm, which may be the start of a previously observed ozone absorption feature. The ultraviolet spectrograph (UVS) instrument on the Juno spacecraft observed ultraviolet sunlight reflected by the surface of Jupiter's largest moon, Ganymede. Although we know from previous observations that Ganymede's surface contains a large amount of water ice, we found that Ganymede does not reflect ultraviolet light in the way we expected: while water ice is very reflective at ultraviolet wavelengths longer than 165 nm, Ganymede's surface does not reflect well at these wavelengths. The northern high latitudes were found to be more reflective than the low latitudes, suggesting that the polar regions are more water‐ice rich than parts of the surface closer to the equator. The bright Tros crater, which is known to contain a large amount of water ice, had a low UV reflectance, which we suggest may be due to UV‐absorbing material trapped within the ice there. These results will be useful for planning future investigations of Ganymede's surface composition by the UVS instrument on the upcoming JUICE mission, which will tour all three of Jupiter's icy moons before entering orbit around Ganymede. Juno ultraviolet spectrograph mapped UV sunlight reflected by Ganymede's surface during the Juno orbit 34 Ganymede flybyThe UV signature of pure water ice was detected in the north polar regions but not within other ice‐rich regions including Tros craterAdditional absorption features observed may be related to ammonia and ozone Juno ultraviolet spectrograph mapped UV sunlight reflected by Ganymede's surface during the Juno orbit 34 Ganymede flyby The UV signature of pure water ice was detected in the north polar regions but not within other ice‐rich regions including Tros crater Additional absorption features observed may be related to ammonia and ozone

On 7 June 2021, Juno‐UVS mapped Ganymede's auroral emissions near a closest approach altitude of 1,046 km. The high spatial resolution map exhibits bright, 200–1,000 R, oxygen emissions organized into northern and southern auroral ovals. Though the map has incomplete global coverage, UVS observed longitudinal structure similar to that described by McGrath et al. (2013), https://doi.org/10.1002/jgra.50122and latitudinal and vertical structure never before resolved. The mapped auroral emissions (a) display an intense narrow auroral curtain with a sharp poleward boundary, (b) have a more slowly decreasing equatorial edge on the leading hemisphere, (c) appear to originate near the surface with a vertical extent of 25–50 km, and (d) are slightly brighter in the north than the south. Additionally, we present UVS observations from the more distant Juno Ganymede flyby on 20 July 2021. We describe the observations, compare them to previous Hubble Space Telescope observations and current model predictions of the open‐closed‐field line‐boundary. Observations of Ganymede by Juno‐UVS during a close flyby (during Juno's 34th Jupiter orbit) captured unique high‐spatial‐resolution measurements of Ganymede's auroral emissions. Organized into two polar ovals, the positions and intensities of the auroral emissions are consistent with previous Hubble Space Telescope observations. The observed morphology of the auroras on the leading hemisphere of Ganymede exhibits latitudinal structure never before resolved. Previous studies suggest the poleward edge of the emissions trace the poleward most position of the magnetic field lines that have both ends rooted to Ganymede. We show that a magnetic field model of Ganymede within Jupiter's larger magnetosphere predicts last‐closed field‐lines very close to the observed auroral emissions, as expected. More distant observations taken during Juno's following orbit (orbit 35) capture auroral emissions at slightly different longitudes. They too show similar agreement with previous observations and current magnetic field models. The high spatial resolution observations revealed auroral emissions of over 1000 Rayleighs, brighter than previously observedThe leading hemisphere aurora exhibits an intense auroral curtain with a sharp poleward boundary and more slowly tapering equatorial edgeJuno‐UVS observed Ganymede's auroral emissions extending up to a maximum of 50 km altitude The leading hemisphere aurora exhibits an intense auroral curtain with a sharp poleward boundary and more slowly tapering equatorial edge

Jupiter displays the most powerful auroral emissions in our solar system, which result from strong energy dissipation in Jupiter's surrounding space environment. Although mass and energy in Jupiter's magnetosphere mostly come from the innermost Galilean moon Io's volcanic activity and Jupiter's rotation, solar wind perturbations can play crucial roles in releasing magnetospheric energy. The systematic response of the aurora to a solar wind compression remains poorly understood because of timing uncertainties. Here we report the analysis of a set of auroral images from the Hubble Space Telescope with contemporaneous in situ magnetopause detections from Juno, allowing for a more direct comparison. By analyzing the dawn side main auroral emission, we distinguish two non‐mutually exclusive types of auroral enhancements: auroral dawn storm (ADS) featured with latitudinal extension in limited longitudes, and a long‐lasting main auroral brightening with limited extension in latitudes while extending over a large longitude range. Only the latter systematically appears under a compressed magnetopause, while the dawn storms could occur whatever the state of the magnetopause. The results could provide important constraints to improve theoretical models and numerical simulations. During expanded magnetopause conditions, Jupiter's aurora displayed either quiet or dawn storm morphology. The result is consistent with recent discovery of the initiation of ADSs in midnight and post‐midnight, possibly driven by magnetic reconnection plasma instabilities in night magnetotail. Our results show that some typical auroral morphologies could be used as a diagnostic of solar wind conditions at Jupiter. Planetary aurorae are an image of the perturbations of energetic particles in the planetary magnetosphere. Jupiter has the largest magnetosphere in our solar system, and it produces the most powerful auroral emission. Unlike the terrestrial magnetosphere that is mainly driven by solar wind activities, the major plasma source in Jupiter's magnetosphere comes from the innermost Galilean moon Io's volcanic activity. The respective impact of the solar wind and internal plasma sources on Jupiter's magnetosphere and aurorae has been under debate for decades, mostly due to the lack of direct connection between solar wind conditions and auroral morphologies. Using contemporaneous measurements from the Hubble Space Telescope and Juno spacecraft, we can systematically determine the relation between auroral morphologies and magnetopause compression at Jupiter. The results could crucially constrain the physical interpretation of Jupiter's main aurora. We quantify two types of auroral intensifications at JupiterDuring solar wind compression, Jupiter's main aurora was systematically enhanced globallyDuring Quiet solar wind conditions, aurora could have either quiet or dawn storm morphologies We quantify two types of auroral intensifications at Jupiter During solar wind compression, Jupiter's main aurora was systematically enhanced globally During Quiet solar wind conditions, aurora could have either quiet or dawn storm morphologies

The main auroral emission at Jupiter results from the ionosphere‐magnetosphere coupling current system associated with the corotation breakdown of iogenic plasma in the current sheet. The morphology and brightness of the main auroral emission are generally suggested to be stable during time intervals of the order of an hour. Here we reveal a transient small‐scale structure in the main emission that is characterized by a localized brightness enhancement close to noon local time. The evolution of this small‐scale structure is investigated in both hemispheres on the basis of far UV images obtained with the Hubble Space Telescope between 1997 and 2007. Our observations indicate that the transient feature vary within a few tens of minutes. As one plausible explanation based on Galileo observations, we suggest that the localized enhancement of the field‐aligned currents associated with the transient structure is due to the shear induced by intermittent inward plasma flow near noon in the equatorial plane. Report on a transient small‐scale auroral structure at JupiterIts magnetospheric source is fixed in local time (noon)Plausible explanation: inward plasma flow in the equatorial plane

Nightside polar arcs are some of the most puzzling auroral emissions at Earth. They are features which extend from the nightside auroral oval into the open magnetic field line region (polar cap), and they represent optical signatures of magnetotail dynamics. Here we report the first observation of an arc at Saturn, which is attached at the nightside main oval and extends into the polar cap region, resembling a terrestrial transpolar arc. We show that Earth‐like polar arcs can exceptionally occur in a fast rotational and internally influenced magnetosphere such as Saturn's. Finally, we discuss the possibility that the polar arc at Saturn is related to tail reconnection and we address the role of solar wind in the magnetotail dynamics at Saturn. Discovery of a nightside polar arc at SaturnEarth‐like transpolar arc can occur in internally influenced magnetosphereSaturn's polar arc might be related to solar wind driven tail reconnection

We report on the first analysis of magnetospheric cusp observations at Saturn by multiple in situ instruments onboard the Cassini spacecraft. Using this we infer the process of reconnection was occurring at Saturn's magnetopause. This agrees with remote observations that showed the associated auroral signatures of reconnection. Cassini crossed the northern cusp around noon local time along a poleward trajectory. The spacecraft observed ion energy‐latitude dispersions—a characteristic signature of the terrestrial cusp. This ion dispersion is “stepped,” which shows that the reconnection is pulsed. The ion energy‐pitch angle dispersions suggest that the field‐aligned distance from the cusp to the reconnection site varies between ∼27 and 51 RS. An intensification of lower frequencies of the Saturn kilometric radiation emissions suggests the prior arrival of a solar wind shock front, compressing the magnetosphere and providing more favorable conditions for magnetopause reconnection. We observe evidence for reconnection in the cusp plasma at SaturnWe present evidence that the reconnection process can be pulsed at SaturnSaturn's cusp shows similar characteristics to the terrestrial cusp

We present 14 simultaneous Chandra X‐ray Observatory (CXO)‐Hubble Space Telescope (HST) observations of Jupiter's Northern X‐ray and ultraviolet (UV) aurorae from 2016 to 2019. Despite the variety of dynamic UV and X‐ray auroral structures, one region is conspicuous by its persistent absence of emission: the dark polar region (DPR). Previous HST observations have shown that very little UV emission is produced by the DPR. We find that the DPR also produces very few X‐ray photons. For all 14 observations, the low level of X‐ray emission from the DPR is consistent (within 2‐standard deviations) with scattered solar emission and/or photons spread by Chandra's Point Spread Function from known X‐ray‐bright regions. We therefore conclude that for these 14 observations the DPR produced no statistically significant detectable X‐ray signature. Jupiter produces the most powerful ultraviolet (UV) and X‐ray aurorae in the solar system. While the UV and X‐ray aurora of the planet have each been explored independently, previous work exploring the spatial connection between Jupiter's X‐ray and UV aurorae was limited to a single simultaneous Chandra X‐ray Observatory (CXO) and Hubble Space Telescope (HST) observation from 2003. Since 2016, the arrival of the Juno spacecraft at Jupiter has led to extensive observing campaigns with CXO and HST, resulting in many new simultaneous X‐ray‐UV observations. Here, we present the analysis of 14 simultaneous CXO and HST observations of Jupiter's Northern aurorae from 2016 to 2019. While much of the emission from these bright aurorae is dynamic, there is one region that is dim for all 14 observations: Jupiter's dark polar region. Previous observations of this region have shown that it produces very little UV light. However, the extent to which X‐ray light is produced by the region remained unexplored and could provide the key to identifying why the region is so dark and what this tells us about the Jovian system. Across these 14 observations, we find that the region produces no significant X‐ray emission. We compare 14 simultaneous Chandra X‐ray Observatory and Hubble Space Telescope UV observations of Jupiter's Northern AuroraeWe detect no statistically significant X‐ray emission from the UV dark polar region (DPR)The DPR expands and the X‐ray and UV emission on the swirl region boundary shifts when the aurora show solar wind compression morphology We compare 14 simultaneous Chandra X‐ray Observatory and Hubble Space Telescope UV observations of Jupiter's Northern Aurorae We detect no statistically significant X‐ray emission from the UV dark polar region (DPR) The DPR expands and the X‐ray and UV emission on the swirl region boundary shifts when the aurora show solar wind compression morphology

We analyze Hubble Space Telescope observations of Ganymede made with the Space Telescope Imaging Spectrograph between 1998 and 2017 to generate a brightness map of Ganymede's oxygen emission at 1,356 Å. Our Mercator projected map demonstrates that the brightness along Ganymede's northern and southern auroral ovals strongly varies with longitude. To quantify this variation around Ganymede, we investigate the brightness averaged over 36°‐wide longitude corridors centered around the sub‐Jovian (0° W), leading (90° W), anti‐Jovian (180° W), and trailing (270° W) central longitudes. In the northern hemisphere, the brightness of the auroral oval is 3.7 ± 0.4 times lower in the sub‐Jovian and anti‐Jovian corridors compared to the trailing and leading corridors. The southern oval is overall brighter than the northern oval, and only 2.5 ± 0.2 times fainter on the sub‐ and anti‐Jovian corridors compared to the trailing and leading corridors. This demonstrates that Ganymede's auroral ovals are strongly structured in auroral crescents on the leading side (plasma downstream side) and on the trailing side (plasma upstream side). We also find that the brightness is not symmetric with respect to the 270° meridian, but shifted by ∼20° towards the Jovian‐facing hemisphere. Our map will be useful for subsequent studies to understand the processes that generate the aurora in Ganymede's non‐rotationally driven, sub‐Alfvénic magnetosphere. Northern lights often illuminate the night sky in a shimmering green or red tone at high geographic latitudes. This emission, scientifically referred to as aurora, is a result of electrically charged particles that move along Earth's magnetic field lines and interact with its atmosphere to produce auroral emission. Apart from the Earth, multiple other planets in our solar system also exhibit auroral emission. By characterizing the brightness and structure of these lights, we are therefore able to deduce insights about a planet's atmosphere, magnetic field and the physical processes occurring along the field lines from afar. In this work, we used observations from the Hubble Space Telescope to analyze the auroral emission of Jupiter's largest moon Ganymede. We combined multiple images of Ganymede to create the first complete map that displays the auroral brightness. Our map revealed that the emission on Ganymede's auroral ovals varies strongly in brightness with divisions into two distinct bright and faint regions. They resemble two auroral crescents in the north and south respectively, and demonstrate the uniqueness of Ganymede's aurora in comparison with the auroral ovals of other planets in the solar system. Brightness map of Ganymede's ultraviolet auroral emission has been constructed based on Hubble Space Telescope observations from 1998 to 2017Auroral ovals are structured in upstream and downstream “crescents”Brightness on sub‐Jovian and anti‐Jovian side is strongly reduced by a factor of 3–4 compared to upstream and downstream side Brightness map of Ganymede's ultraviolet auroral emission has been constructed based on Hubble Space Telescope observations from 1998 to 2017 Auroral ovals are structured in upstream and downstream “crescents” Brightness on sub‐Jovian and anti‐Jovian side is strongly reduced by a factor of 3–4 compared to upstream and downstream side

Jupiter's satellite auroral footprints are a manifestation of the satellite‐magnetosphere interaction of the Galilean moons. Juno's polar elliptical orbit enables crossing the magnetic flux tubes connecting each Galilean moon with their associated auroral emission. Its payload allows measuring the fields and particle population in the flux tubes while remotely sensing their associated auroral emissions. During its thirtieth perijove, Juno crossed the flux tube directly connected to Ganymede's leading footprint spot, a unique event in the entire Juno prime mission. Juno revealed a highly‐structured precipitating electron flux, up to 316 mW/m2, while measuring both a small perturbation in the magnetic field azimuthal component and small Poynting flux with an estimated total downward current of 4.2 ± 1.2 kA. Based on the evolution of the footprint morphology and the field and particle measurements, Juno transited for the first time through a region connected to the transhemispheric electron beam of the Ganymede footprint. The interaction between Jupiter's corotating plasma torus and the Galilean satellites generates a set of complex magnetospheric processes. One such interaction produces permanent auroral spots around Jupiter's northern and southern poles, known as footprints. For each close flyby, Juno's in situ instruments can measure such interaction. During its thirtieth perijove, Juno crossed the magnetic field lines connecting the interaction region of Ganymede with one of its auroral spots on Jupiter. This study describes and analyzes the set of measurements associated with that unique event. Juno crossed the magnetic flux tube connected to Ganymede auroral footprint and recorded a multi‐instrument set of measurementsJuno measured ∼316 mW/m2of precipitating electrons while magnetically tied to Ganymede's leading auroral footprint spotThe associated Juno measurements suggest that it transited through a region linked to the transhemispheric electron beam Juno crossed the magnetic flux tube connected to Ganymede auroral footprint and recorded a multi‐instrument set of measurements Juno measured ∼316 mW/m2of precipitating electrons while magnetically tied to Ganymede's leading auroral footprint spot The associated Juno measurements suggest that it transited through a region linked to the transhemispheric electron beam

A model coupling an electron energy degradation code with a detailed synthetic spectrum of the H2 Lyman and Werner band system is used to calculate the emerging auroral ultraviolet spectra from Jupiter's atmosphere excited by electrons with different initial energy distributions. The atmospheric model is adapted from the vertical P-T profile measured by the Galileo probe and mid-latitude model hydrocarbon photochemistry. Each altitude layer, with its own gas temperature, contributes to the emergent ultraviolet spectrum and the absorbers are vertically distributed within the source region of the auroral emissions. Examples of the calculated spectra are shown to validate the synthetic spectrum and to illustrate the importance of the electron energy distribution and the vertical structure. The model is then applied to the analysis of seven HST/GHRS spectra of the 1200–1700 A˚ region obtained with 5-A˚ resolution at various locations in the north and south Jovian aurora. These spectra have different color ratios which characterize the energy of the precipitated electrons, although they do not have a high enough spectral resolution to permit a determination of the H2 temperature. We find that the characteristic energy of the assumed initial Maxwellian distribution ranges between 17 and 40 keV. A clear signature of acetylene absorption is observed near 1520, 1480, and 1440 A˚ where the C2H2 cross section shows strong absorption peaks. The acetylene column abundance overlying the emission peak varies from 0.02 to 0.2 of the methane column. A better fit is obtained for some spectra when ethane absorption is added. The C2H6 column abundance varies from 0 to 0.5 of the methane column. These changes relative to methane are presumably the result of perturbations by heat released by the fast electron thermalization and/or perturbations to the hydrocarbon chemistry resulting from the production of H atoms by the aurora. A spectrum of the Io flux tube footprint and its trailing tail shows an ultraviolet color and hydrocarbon absorption quite similar to some of the main oval spectra. This observation suggests that the electrons of the Io flux tube are energized to a few tens of keV, similar to the electron precipitated in the main ovals and polar caps. Echelle spectra between 1216 and 1220 A˚ at 0.07 A˚ resolution are also compared with the model fitting best the mid-resolution spectra. It is found that the effective H2 rovibrational temperature associated with the echelle spectra are significantly higher than predicted by the mid-latitude model. A large vertical temperature gradient just above the methane homopause due to large heating by auroral precipitation is a plausible explanation for this difference.

A model simulating Earth views of UV auroral arcs and diffuse emissions in the Jovian north polar region is described. It assumes a Chapman profile of vertical brightness distribution and various horizontal structures. Simple geometric cases are described to illustrate the dependence on the altitude, atmospheric scale height, and central meridian planetary longitude (CML) of an idealized auroral morphology seen from Earth orbit. The numerical simulation makes it possible to assess the importance of limb brightening and the contribution from high altitude auroral emission located behind the planetary limb. As an application of the simulation model, four images obtained with the Wide Field and Planetary Camera 2 (WFPC2) on board the Hubble Space Telescope are used to determine the characteristics of their auroral (discrete and diffuse) structures. The apparent brightness distribution along the arcs can only be reproduced if intrinsic longitudinal (or local time) variations are introduced, in addition to the path length effects of the viewing geometry. A composite average auroral distribution is built by mapping 10 WFPC2 images from the same dataset. It illustrates the dichotomy frequently observed between a narrow single structure arc at System III longitudes larger than 180° (or morning sector) and the multiple arc and broad diffuse emission at longitudes less than 180° (or afternoon sector). It is shown that the equatorial auroral emission boundary is located between the 6RJand the 30RJmagnetic field line footprints of the Goddard Space Flight Center (GSFC) O6model.

Jupiter's powerful auroral emission is usually divided into the polar, main, and equatorward components. The driver of Jupiter's main aurora is a central question for the community. Previous investigations reveal many distinct substructures on the main auroral oval, which are indicators of fundamentally different magnetospheric processes. Understanding these substructures could provide key constraints for uncovering the driver of Jupiter's main aurora emission. In this study, we show the evolution of a double‐auroral arc on the dawnside from observations by the Hubble Space Telescope (HST). Simultaneous in situ observations from the Juno spacecraft provide direct evidence of magnetic reconnection and magnetic dipolarization. By analyzing the datasets from Juno and HST, we suggest that the evolution of the double‐arc structure is likely a consequence of the non‐steady progress of magnetic reconnection. Jupiter's most powerful aurora reflects the strong perturbations of energetic particles in the space environment. Besides the relatively steady main aurora, there are rapidly evolving structures that imply the transient plasma processes, for example, magnetic reconnection, wave‐particle interaction, and so on. The relation between transient processes and auroral structures is poorly understood due to limited observations. Using the simultaneous measures from Juno and Hubble Space Telescope, we reveal the magnetospheric driver (i.e., magnetic reconnection) for an evolving double‐arc auroral structure at the dawn sector. This study shows a good example of reconnection signature on the auroral image, which provides key implications to understand planetary space environment from dynamic auroral features. We investigate a double‐arc dawn aurora at Jupiter and the associated reconnection signatures in magnetosphere for the poleward arcPoleward arc evolves rapidly and both arcs move equatorward, suggesting a combination of global reconfiguration and localized reconnectionMultiple reconnection fronts observed by Juno are suggestive evidence of unsteady and/or drizzle‐like reconnection processes We investigate a double‐arc dawn aurora at Jupiter and the associated reconnection signatures in magnetosphere for the poleward arc Poleward arc evolves rapidly and both arcs move equatorward, suggesting a combination of global reconfiguration and localized reconnection Multiple reconnection fronts observed by Juno are suggestive evidence of unsteady and/or drizzle‐like reconnection processes

Ionospheric conductance contributes to regulate the characteristics of the ionospheric current and the closure of the magnetosphere‐ionosphere circuit in the ionosphere. Measurements of Birkeland currents with the Juno magnetometer have indicated that they are statistically larger in the south. It has been suggested that these asymmetries may be the consequence of a higher Pedersen conductance in the southern hemisphere. We have derived the local precipitated electron energy flux and their characteristic energy from 14 multi‐spectral images collected with the ultraviolet spectrograph on board Juno. This information was then used as input to an ionospheric model providing the density of H3+, H+, and hydrocarbon ions to calculate the spatial distribution of the Pedersen conductance for Juno perijoves 1–15. We show that the area‐integrated conductance is closely proportional to the H3+ion content and quasi equal in the north and the south. The mean conductance is 0.47 mho in both hemispheres. However, local variations in the Pedersen conductivity and/or hemispheric differences between the magnetospheric rotation and the rotation velocities of the neutrals can also result in asymmetry of Birkeland currents. The electric conductance of Jupiter’s partly controls the currents that circulate in its ionosphere and the current loops closing in this conducting region. Measurements of the ionospheric current intensity with the magnetometer along Juno’s trajectory have shown that these currents are statistically stronger in the southern than in the northern hemisphere. A possibility has been raised that this asymmetry is related to a higher value of the Pedersen conductivity in the south. To test this hypothesis, we have compared the vertically integrated conductivity in both hemispheres derived from the intensity and spectral composition of images obtained with the Juno Ultraviolet Spectral Imager during its first 15 orbits around Jupiter. Comparison of the pairs of global conductance maps indicates that there is no significant statistical conductance asymmetry as both the area‐integrated and the mean conductivity are quasi equal in the two hemispheres. Possible causes for the observed ionospheric current asymmetries are the local aspect of the magnetometer measurements and/or hemispheric differences between the magnetospheric rotation and the rotation velocities of the neutrals. We have built and compared 28 Pedersen conductance maps based on ultraviolet spectrograph spectral images collected during the 15 first Juno orbitsThe mean conductance is 0.47 mho in both hemispheres, a symmetry in contrast with the asymmetry observed in the ionospheric currentsThe auroral H2 emitted power varies between 1.1 and 3.6 TW but the north/south power ratio is close to unity on the average We have built and compared 28 Pedersen conductance maps based on ultraviolet spectrograph spectral images collected during the 15 first Juno orbits The mean conductance is 0.47 mho in both hemispheres, a symmetry in contrast with the asymmetry observed in the ionospheric currents The auroral H2 emitted power varies between 1.1 and 3.6 TW but the north/south power ratio is close to unity on the average

Jupiter's polar auroral region hosts UV auroral emissions that relate to the magnetospheric dynamics from the outer magnetosphere. Juno‐UVS has discovered intriguing features characterized by expanding emission circles of UV‐brightness <140 kR. These events are located at the border of the previously defined swirl region, nearby the polar dark region. The features expand into a circular shape up to ∼1,000 km in radius, at expansion velocities from 3.3 ± 1.7 up to 7.7 ± 3.5 km/s, as measured over the four best observed cases. Using color ratio measurements as a proxy for the depth of the recorded features, the mean electron energy responsible for these emissions is 80–160 keV. Events occurring in the outer magnetosphere at distances >100 RJare likely causing for these features. Dayside magnetopause reconnection and Kelvin‐Helmholtz instabilities resulting from the shear flows near the magnetopause are expected to generate field‐aligned currents that could potentially be the cause of these features. Juno‐UVS discovered expanding auroral emission circles with typical brightness up to 140 kRThe features are located in the auroral swirl region, magnetically mapping to the outer magnetosphereTheir origin would be consistent with dayside reconnection or signatures of Kelvin‐Helmholtz instabilities Juno‐UVS discovered expanding auroral emission circles with typical brightness up to 140 kR The features are located in the auroral swirl region, magnetically mapping to the outer magnetosphere Their origin would be consistent with dayside reconnection or signatures of Kelvin‐Helmholtz instabilities

Since 2016, the Juno‐UVS (Ultraviolet Spectrograph) instrument has been taking spectral images of Jupiter's auroras in their full extent, including the nightside, which cannot be viewed from Earth. We present a systematic analysis of features in Jupiter's polar auroras called auroral bright spots, which were observed by Juno‐UVS during the first 25 orbits of the spacecraft. An auroral bright spot is an isolated localized and transient brightening in the polar region. Bright spots were identified in 16 perijoves (PJ) out of 24, mostly in either the northern or the southern hemisphere but rarely in both during the same PJ. The emitted power of the bright spots is time variable with peak power ranging from a few tens to a hundred of gigawatts. Moreover, we found that, for some PJs, bright spots exhibit quasiperiodic behavior. The spots, within PJ4 and PJ16, each reappeared within <2,000 km from the previous position in System III with periods of 28 and 22 min, respectively. This period is similar to periods previously identified in X‐rays and various other observations. The bright spot positions are in a specific region in the northern hemisphere in System III, but are scattered around the magnetic pole in the southern hemisphere, near the edge of the swirl region. Furthermore, the bright spots can be seen at any local time, rather than being confined to the noon sector as previously thought from Earth‐based observations. This suggests that the bright spots might not be firmly connected to the noon facing magnetospheric cusp processes. During the first 25 perijoves of the Juno mission, the Ultraviolet Spectrograph instrument observed transient bright spots at the edge of the swirl regionDuring longer continuous observing sequences, the bright spots reoccurred at approximately the same System III location every ∼25 minThe bright spots were not fixed at noon or any other local time, which excludes any explanations involving an Earth‐like noon‐facing cusp During the first 25 perijoves of the Juno mission, the Ultraviolet Spectrograph instrument observed transient bright spots at the edge of the swirl region During longer continuous observing sequences, the bright spots reoccurred at approximately the same System III location every ∼25 min The bright spots were not fixed at noon or any other local time, which excludes any explanations involving an Earth‐like noon‐facing cusp

The main ultraviolet auroral emission at Saturn consists of multiple structures forming a discontinuous ring of emission around the poles, occasionally organized in a global spiral. We present continuous observation of an auroral spiral rotating at ∼85% of rigid corotation during several hours. Simultaneously, energetic neutral atom (ENA) emissions revealed a hot magnetospheric plasma population located in the same local time sector as the ends of the rotating spiral. Following plasma theory, we propose that pressure gradients induced by the energized plasma distorted the magnetospheric current system, resulting in the spiral morphology of the aurora. The rotating hot plasma was several times reenergized in the dusk sector during at least 2 days, generating a long‐lasting auroral spiral. The ultraviolet spiral, the ENA emissions, and the ions revealed by this multi‐instrument data set are three signatures of a magnetosphere‐ionosphere coupling current system and of the associated hot plasma population rotating around Saturn. The main auroral emission forms a spiral observed during two consecutive daysThe spiral morphology is due to the presence of a hot plasma population in the magnetodiscThe auroral spiral winds while the hot plasma bubble expands due to gradient and curvature drifts

Jupiter displays many distinct auroral structures, among which auroral dawn storms and auroral injections are often observed contemporaneously. However, it is unclear if the contemporaneous nature of the observations is a coincidence or part of an underlying physical connection. We show six clear examples from a recent Hubble Space Telescope campaign (GO‐14634) that each display both auroral dawn storms and auroral injection signatures. We found that these conjugate phenomena could exist during intervals of either relatively low or high auroral activity, as evidenced by the varied levels of total auroral power. In situ observations of the magnetosphere by Juno show a strong magnetic reconnection event inside of 45 Jupiter radii (RJ) on the predawn sector, followed by two dipolarization events within the following 2 hr, coincident with the auroral dawn storm and auroral injection event. We therefore suggest that the auroral dawn storm is the manifestation of magnetic reconnection in the dawnside magnetosphere. The dipolarization region is mapped to the auroral injection, strongly suggesting that this was associated with the auroral injection. Since magnetic reconnection and dipolarization are physically connected, we therefore suggest that the often‐conjugate auroral dawn storm and auroral injection events are also physically connected consequences. Jupiter's auroral dawn storm and auroral injection events are conjugate and physically connected phenomenaWe report the recurrent nature of magnetic dipolarization at JupiterThese observations suggest that reconnection manifests auroral dawn storms and subsequent dipolarization produces auroral injection events

Ionospheric conductivity perpendicular to the magnetic field plays a crucial role in the electrical coupling between planetary magnetospheres and ionospheres. At Jupiter, it controls the flow of ionospheric current from above and the closure of the magnetosphere‐ionosphere circuit in the ionosphere. We use multispectral images collected with the Ultraviolet Spectral (UVS) imager on board Juno to estimate the two‐dimensional distribution of the electron energy flux and characteristic energy. These values are fed to an ionospheric model describing the generation and loss of different ion species, to calculate the auroral Pedersen conductivity. The vertical distributions of H3+, hydrocarbon ions, and electrons are calculated at steady state for each UVS pixel to characterize the spatial distribution of electrical conductance in the auroral region. We find that the main contribution to the Pedersen conductance stems from collisions of H3+and heavier ions with H2. However, hydrocarbon ions contribute as much as 50% to Σpwhen the auroral electrons penetrate below the homopause. The largest values are usually associated with the bright main emission, the Io auroral footprint and occasional bright emissions at high latitude. We present examples of maps for both hemispheres based on Juno‐UVS images, with Pedersen conductance ranging from less than 0.1 to a few mhos. One of the quantities characterizing the ability of ionospheres to carry currents perpendicular to the magnetic field is the altitude integrated Pedersen conductivity. On Jupiter, it is an important quantity that partly controls how electric currents can flow between the magnetosphere and the high‐latitude ionosphere. It is therefore a key element in the understanding of how and where the Jovian aurora is formed and an important input to numerical models of auroral precipitation. We use observations from the UltraViolet spectral imager near Juno's closest approach to Jupiter to remotely characterize the flux of energy carried by the auroral electrons and their mean energy. These quantities are evaluated for each instrumental pixel and used as inputs to a model to map the Pedersen integrated conductivity. The main contributions to the conductance are caused by collisions between H3+and hydrocarbon ions such as CH5+and C3Hn+with neutral constituents. We present examples of Pedersen conductance maps for both hemispheres. We find that the conductance is spatially very variable with values ranging from less than 0.1 to several mhos. The largest values are usually associated with the bright main emission, the Io auroral footprint and occasional bright emissions at high latitude. Multispectral auroral observations from UVS‐Juno are used to map the auroral ionospheric Pedersen conductance in both hemispheresH3+and hydrocarbon ions make most of the contribution to auroral ionospheric conductanceThe conductance varies from less than 0.1 up to several mhos in the main aurora, high latitude precipitation, and Io magnetic footprint

We present observations of Jupiter's magnetic field and plasma obtained with the NASA Juno spacecraft during February 2018, along with simultaneous Hubble Space Telescope (HST) observations of the planet's auroras. We show that a few‐day transient enhancement of the azimuthal and radial magnetic fields and plasma temperature was coincident with a significant brightening of Jupiter's dawn‐side main auroral emission. This presents the first evidence of control of Jupiter's main auroral emission intensity by magnetosphere‐ionosphere coupling currents. We support this association by self‐consistent calculation of the magnetosphere‐ionosphere coupling and radial force balance currents using an axisymmetric model, which broadly reproduces the Juno magnetic field and plasma observations and the HST auroral observations. We show that the transient enhancement can be explained by increased hot plasma pressure in the magnetosphere together with increased iogenic plasma mass outflow rate. Overall, this work provides important observational and modeling evidence revealing the behavior of Jupiter's giant magnetosphere. Bright Jovian main auroral emission coincided with enhanced magnetospheric azimuthal and radial magnetic field and increased plasma pressureThis represents the first evidence of control of Jupiter's main auroral emission intensity by magnetosphere‐ionosphere coupling currentsModeling of the magnetosphere‐ionosphere coupling and radial force balance currents yields results consistent with observations

We report the temporal and spectral results of the first XMM‐Newton observation of Jupiter's X‐ray auroras during a clear magnetospheric compression event on June 2017 as confirmed by data from the Jovian Auroral Distributions Experiment (JADE) instrument onboard Juno. The northern and southern auroras were visible twice and thrice respectively as they rotated in and out of view during the ∼23‐hr (almost 2.5 Jupiter rotations) long XMM‐Newton Jovian‐observing campaign. Previous auroral observations by Chandra and XMM‐Newton have shown that the X‐ray auroras sometimes pulse with a regular period. We applied wavelet and fast Fourier transforms (FFTs) on the auroral light curves to show that, following the compression event, the X‐ray auroras exhibited a recurring 23‐ to 27‐min periodicity that lasted over 12.5 hr (longer than a Jupiter rotation). This periodicity was observed from both the northern and southern auroras, suggesting that the emission from both poles was caused by a shared driver. The soft X‐ray component of the auroras is due to charge exchange processes between precipitating ions and neutrals in Jupiter's atmosphere. We utilized the Atomic Charge Exchange (ACX) spectral package to produce solar wind and iogenic plasma models to fit the auroral spectra in order to identify the origins of these ions. For this observation, the iogenic model gave the best fit, which suggests that the precipitating ions are from iogenic plasma in Jupiter's magnetosphere. The solar wind is a continuous stream of charged particles released by the Sun that flows out toward the edge of the Solar System. It meets obstacles along the way, such as the magnetic fields of planets like the Earth to create a magnetic bubble around them called a magnetosphere. The magnetosphere prevents most of these charged particles from reaching the Earth's atmosphere. Those that make their way through interact with the gas molecules in the atmosphere above the polar regions and cause them to glow to produce the auroras or the northern and southern lights. Jupiter's auroras are much more powerful than the Earth's, and they emit different types of radiation, including X‐rays. It is currently unclear as to what causes Jupiter's X‐ray auroras. Its moon, Io, spews volcanic material into the magnetosphere that can be accelerated into the planet's atmosphere. We created models that consisted of the particles found in the solar wind and in the material from Io's volcanoes to see which one was responsible for Jupiter's X‐ray auroras. In this case, it was Io's volcanoes. We also found that the auroras pulsate every ∼23–27 min in the north and ∼23–33 min in the south. XMM‐Newton observed Jupiter on 19 June 2017 during a magnetospheric compression eventFast Fourier transform analysis shows that the X‐ray auroras pulsated over multiple Jupiter rotationsSpectral analysis shows precipitating ions from Io and not the solar wind are responsible for the X‐ray auroras

Although auroral emissions at giant planets have been observed for decades, the physical mechanisms of aurorae at giant planets remain unclear. One key reason is the lack of simultaneous measurements in the magnetosphere while remote sensing of the aurora. We report a dynamic auroral event identified with the Cassini Ultraviolet Imaging Spectrograph (UVIS) at Saturn on 13 July 2008 with coordinated measurements of the magnetic field and plasma in the magnetosphere. The auroral intensification was transient, only lasting for ∼30 min. The magnetic field and plasma are perturbed during the auroral intensification period. We suggest that this intensification was caused by wave mode conversion generated field‐aligned currents, and we propose two potential mechanisms for the generation of this plasma wave and the transient auroral intensification. A survey of the Cassini UVIS database reveals that this type of transient auroral intensification is very common (10/11 time sequences, and ∼10% of the total images). We present simultaneous observations of auroral brightening and plasma perturbations at SaturnWe show evidence of plasma wave conversion in driving a transient auroral brightening at SaturnWe propose two potential mechanisms generating the plasma wave and aurora intensification

The duskside of the polar region of Jupiter's UV aurorae, called the active region, sometimes exhibits quasiperiodic (QP) flares on timescales of 2–3 min. Based on Hubble Space Telescope Far‐UV time‐tag images, we show for the first time that the Northern Hemisphere also displays QP flares. The area covered by these flares can reach up to 2.4 × 108km2(i.e., the whole active region) but often only involves an area an order of magnitude smaller. Using a magnetic field mapping model, we deduced that these areas correspond to the dayside outer magnetosphere. In our data set, quasiperiodic features are only seen on half of the cases, and even on a given observation, a region can be quiet for one half and blinking on the other half. Consecutive observations in the two hemispheres show that the brightening can occur in phase. Combined with the size and location of the flares, this behavior suggests that the QP flares most likely take place on closed magnetic field lines. Two to three minute quasiperiodic (QP) flares are seen in approximately half of the sequencesQP flares can brighten in phase in the two hemispheresThe QP flares occur on closed field lines and map to the outer dayside magnetosphere