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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 Juno has 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

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. Key Points: 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 [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

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

Discrete aurora at Mars, characterized by their small spatial scale and tendency to form near strong crustal magnetic fields, are emissions produced by particle precipitation into the Martian upper atmosphere. Since 2014, Mars Atmosphere and Volatile EvolutioN's (MAVEN's) Imaging Ultraviolet Spectrograph (IUVS) has obtained a large collection of UV discrete aurora observations during its routine periapsis nightside limb scans. Initial analysis of these observations has shown that, near the strongest crustal magnetic fields in the southern hemisphere, the IUVS discrete aurora detection frequency is highly sensitive to the interplanetary magnetic field (IMF) clock angle. However, the role of other solar wind properties in controlling the discrete aurora detection frequency has not yet been determined. In this work, we use the IUVS discrete aurora observations, along with MAVEN observations of the upstream solar wind, to determine how the discrete aurora detection frequency varies with solar wind dynamic pressure, IMF strength, and IMF cone angle. We find that, outside of the strong crustal field region (SCFR) in the southern hemisphere, the aurora detection frequency is relatively insensitive to the IMF orientation, but significantly increases with solar wind dynamic pressure, and moderately increases with IMF strength. Interestingly however, although high solar wind dynamic pressures cause more aurora to form, they have little impact on the brightness of the auroral emissions. Alternatively, inside the SCFR, the detection frequency is only moderately dependent on the solar wind dynamic pressure, and is much more sensitive to the IMF clock and cone angles. In the SCFR, aurora are unlikely to occur when the IMF points near the radial or anti‐radial directions when the cone angle (arccos(Bx/|B|)) is less than 30° or between 120° and 150°. Together, these results provide the first comprehensive characterization of how upstream solar wind conditions affect the formation of discrete aurora at Mars. Key Points: Outside the strong crustal field region, high solar wind pressures increase the aurora detection frequency but not the emission brightnessIMF orientation affects the aurora detection frequency more prominently near strong crustal fieldsIn the strong crustal field region, aurora are rare when the IMF points near the radial or anti‐radial directions [ABSTRACT FROM AUTHOR]

The Far Ultra Violet (FUV) ultraviolet imager onboard the NASA‐ICON mission is dedicated to the observation and study of the ionosphere dynamics at mid and low latitudes. We compare O+ density profiles provided by the ICON FUV instrument during nighttime with electron density profiles measured by the COSMIC‐2 constellation (C2) and ground‐based ionosondes. Co‐located simultaneous observations are compared, covering the period from November 2019 to July 2020, which produces several thousands of coincidences. Manual scaling of ionogram sequences ensures the reliability of the ionosonde profiles, while C2 data are carefully selected using an automatic quality control algorithm. Photoelectron contribution coming from the magnetically conjugated hemisphere is clearly visible in FUV data around solstices and has been filtered out from our analysis. We find that the FUV observations are consistent with the C2 and ionosonde measurements, with an average positive bias lower than 1 × 1011e/m3. When restricting the analysis to cases having an NmF2 value larger than 5 × 1011e/m3, FUV provides the peak electron density with a mean difference with C2 of 10%. The peak altitude, also determined from FUV observations, is found to be 15 km above that obtained from C2, and 38 km above the ionosonde value on average. Key Points: We compare ICON‐FUV NmF2 and hmF2 observations with those provided by COSMIC‐2 and ionosondesFar Ultra Violet Imaging Spectrograph (FUV) observations are affected by conjugate photoelectrons mainly around solsticesThe FUV performance during nighttime allows for reliable electron density measurement [ABSTRACT FROM AUTHOR]

Mars Atmosphere and Volatile Evolution (MAVEN)'s Imaging Ultraviolet Spectrograph has identified 278 occurrences of discrete aurora events on Mars, which are patchy, sporadic ultraviolet emissions emanating from the upper atmosphere. We confirm prior results finding that emissions are highly correlated with crustal magnetic fields results, with the brightest and most frequent occurrences located around strong crustal fields in the southern hemisphere. The six‐year data set shows that events can also occur globally, in regions of weak or absent crustal fields. We find that events occur primarily in evening hours, especially during favorable orientations of the interplanetary magnetic field. Under these conditions, auroral events probably occur nightly and last for hours. Optical counterparts to these UV emissions would probably be detectable with present‐day instrumentation, and would be visible to future astronauts. Plain Language Summary: Mars has an unusual magnetic field: it lacks a global field but retains the vestiges of an ancient field locked in regions of the crust. These fields are sufficient to focus charged particle precipitation into the atmosphere and cause aurora. The Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft carries an ultraviolet instrument capable of mapping aurora emissions and correlating them with conditions that favor aurora. We have detected hundreds of auroral events allowing the first statistical study of their behavior. We find that events occur primarily in evening hours, especially during favorable orientations of the interplanetary magnetic field. Under these conditions, auroral events probably occur nightly and last for hours. Astronauts, whenever they reach Mars, would have a good chance to see this natural wonder from above or below. Key Points: Mars Atmosphere and Volatile Evolution (MAVEN)/Imaging UltraViolet Spectrograph (IUVS) has detected hundreds of transient, patchy auroral emission events at Mars, mostly near strong crustal magnetic fieldsEvents are triggered in evening hours during favorable orientations of the interplanetary magnetic field, and may last for hoursVisible counterparts to the ultraviolet emissions could potentially be imaged by spacecraft and eventually seen by astronauts [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 >100RJ are 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 [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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.Key Points: 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 [ABSTRACT FROM AUTHOR]

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 Σp when 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. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

We report results from a study of two consecutive Martian years of imaging observations of nitric oxide ultraviolet nightglow by the Imaging Ultraviolet Spectrograph (IUVS) on the Mars Atmosphere and Volatile Evolution (MAVEN) mission spacecraft. The emission arises from recombination of N and O atoms in Mars' nightside mesosphere. The brightness traces the reaction rate as opposed to the abundance of constituents, revealing where circulation patterns concentrate N and O and enhance recombination. Emissions are brightest around the winter poles, with equatorial regions brightening around the equinoxes. These changes offer clear evidence of circulation patterns transitioning from a single cross‐equatorial cell operating during solstice periods to more symmetric equator‐to‐poles circulation around the equinoxes. Prominent atmospheric tides intensify the emissions at different longitudes, latitude ranges, and seasons. We find a strong eastward‐propagating diurnal tide (DE2) near the equator during the equinoxes, with a remarkably bright spot narrowly confined near (0°, 0°). Wave features at the opposite winter poles are dissimilar, reflecting different circulation patterns at perihelion versus aphelion. LMD‐MGCM simulations agree with the patterns of most observed phenomena, confirming that the model captures the dominant physical processes. At the south winter pole, however, the model fails to match a strong wave‐1 spiral feature. Observed brightnesses exceed model predictions by a factor of 1.9 globally, probably due to an underestimation of the dayside production of N and O atoms. Further study of discrepancies between the model and observations offers opportunities to improve our understanding of chemical and transport processes controlling the emission. Plain Language Summary: The MAVEN spacecraft orbiting Mars has obtained a new type of imaging data which reveals the effects of global‐scale winds and waves in the upper atmosphere. Ultraviolet emissions confirm the expected change of wind patterns with season and pinpoint an unusually bright spot on Mars' nightside. Computer model runs successfully match many of the observed features and allow an examination of the underlying physical causes. Differences between the model and observations offer insights on how to improve the model. Key Points: The MAVEN spacecraft orbiting Mars has obtained a new type of nightside imaging data which reveals the effects of global‐scale winds and waves in the upper atmosphereUltraviolet nightglow emissions confirm the expected change of circulation patterns with season and pinpoint an unusually bright spot on Mars' nightsideA Mars general circulation model successfully matches many of the observed features and allows an examination of the underlying physical causes [ABSTRACT FROM AUTHOR]

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 H2 emission 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

The National Aeronautics and Space Administration Ionospheric Connection Explorer (ICON) mission will study the close relationship between the ionosphere, the atmospheric weather, and space weather using in situ and remote sensing instruments proving plasma density, temperature, ion drift velocity, and thermospheric wind velocity over the equatorial region. In particular, the far ultraviolet (FUV) instrument will image the terrestrial limb in two wavelength channels. During nighttime, only the channel characterizing the bright 135.6‐nm emission of atomic oxygen will be used. The purpose of this study is to simulate FUV nightglow measurements under quiet as well as disturbed ionospheric conditions. Classical medium‐scale traveling ionospheric disturbances (MSTIDs), which are understood as the ionospheric signature of atmospheric gravity waves, are one of the main sources of ionospheric variability. Here, we simulate their potential appearance in the FUV instrument data. The simulation model produces FUV images used as input to identify and characterize MSTIDs. MSTID propagation parameters can be retrieved under specific geometrical configurations between the FUV lines of sight and propagation direction of the MSTID, which differs depending on the limb or sublimb observing geometry. The largest MSTID signature is expected during equinoxes under solar maximum periods, for MSTID periods of less than 30 min. The weak brightness of the 135.6‐nm multiplet under solar minimum conditions is the main limitation to the MSTID detection on the nightside. Future MSTID detection algorithms would have to cope with very low signal‐to‐noise ratio, in particular during solstices and under solar minimum conditions. Key Points: Simulations of MSTID signature in the nighttime OI‐135.6 nm brightness for ICON‐FUV instrument are performedICON‐FUV will be able to resolve and detect MSTIDs under specific observing geometriesThe weakness of the nightglow prevents MSTID detection during low background conditions [ABSTRACT FROM AUTHOR]

We analyze two Martian years of dayglow measurements of the CO Cameron bands and the CO2+ ultraviolet doublet (UVD) at 298–299 nm with the Imaging UltraViolet Spectrograph on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter. We show that the altitude and the brightness of the two emissions peaks are strongly correlated, although data were collected over a wide range of latitudes and seasons. Averaged limb profiles are presented and compared with numerical simulations based on updated calculations of the production of the CO (a3Π) and the CO2+ (B 2Σ) states. The model simulations use the solar flux directly measured on board MAVEN with the Extreme Ultraviolet Monitor and the neutral densities provided by the Mars Climate Database version 5.3, adapted to the conditions of the observations. We show that the altitude and the shape of the sample limb profiles are well reproduced using the Mars Climate Database neutral atmosphere. The simulated peak intensities of the CO2+ UVD and Cameron bands are in good agreement considering the uncertainties on the excitation cross sections and the calibration of the Imaging Ultraviolet Spectrograph (IUVS) and Extreme Ultraviolet Monitor instruments. No significant adjustment of the electron impact cross section on CO2 to produce the a3Π state is needed. Seasonal‐latitudinal maps of the Cameron and UVD peak altitude observed during two Martian years show variations as large as 23 km. Model simulations of the amplitude of these changes are in fair agreement with the observations except during the southern summer dust period (Ls = 270–320°) when the calculated rise of the dayglow layer is underestimated. Key Points: The peak altitudes of the CO2+ UVD and CO Cameron dayglow are strongly correlated and indicators of the changing overlying CO2 columnThe altitude and the shape of the dayglow limb profiles are well modeled outside dust storm periods using the MCD neutral atmosphereThe upward shift of the peak altitudes of both emissions by up to 20 km during the dust storm season is underestimated by the model [ABSTRACT FROM AUTHOR]

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. Key Points: 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 [ABSTRACT FROM AUTHOR]

We present an overview of two Martian Years (MYs) oxygen dayglow limb observations of the ultraviolet (UV) emissions at 130.4 and 135.6 nm. The data have been collected with the Imaging Ultraviolet Spectrograph (IUVS) instrument on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. We use solar flux measurements of EUVM on board MAVEN to remove the solar‐induced variation and show the variations of the maximum limb brightness and altitude with season, solar zenith angle, and latitude, which reflects the strong variability of the Martian atmosphere. The 130.4‐ and 135.6‐nm peak brightness and altitudes are strongly correlated and behave similarly. Both emissions are modeled for selected data using Monte Carlo codes to calculate emissions arising from electron impact on O and CO2. Additional radiative transfer calculations are made to analyze the optically thick 130.4‐nm emission. Model atmospheres from the Mars Climate Database serve as input. Both simulated limb profiles are in good agreement with the observations despite some deviations. We furthermore show that the observed 130.4‐nm brightness is dominated by resonance scattering of the solar multiplet with a contribution (15–20%) by electron impact on O. Over 95% of the excitation at 135.6 nm arises from electron impact on O. Simulations indicate that the limb brightness is dependent on the oxygen and CO2 content, while the peak emission altitude is mainly driven by the CO2 content because of absorption processes. We deduce [O]/[CO2] mixing ratios of 3.1% and 3.0% at 130 km for data sets collected at LS = 350° in Martian years 32 and 33. Key Points: We analyze two MYs of UV dayglow limb observations of O at 130.4 and135.6 nm; results at 135.6 nm are the first published since Mariner 6/7Solar flux cannot fully explain the brightness variations; strong correlation of the emissions confirms current photochemistry understandingEmissions mainly arise from direct excitation of O; limb brightness and peak altitude are strongly modulated by CO2 absorption [ABSTRACT FROM AUTHOR]

We have measured the 30 and 100 eV far ultraviolet (FUV) emission cross sections of the optically allowed Fourth Positive Group (4PG) band system (A1Π → X1Σ+) of CO and the optically forbidden O (5So → 3P) 135.6 nm atomic transition by electron‐impact‐induced‐fluorescence of CO and CO2. We present a model excitation cross section from threshold to high energy for the A1Π state, including cascade by electron impact on CO. The A1Π state is perturbed by triplet states leading to an extended FUV glow from electron excitation of CO. We derive a model FUV spectrum of the 4PG band system from dissociative excitation of CO2, an important process observed on Mars and Venus. Our unique experimental setup consists of a large vacuum chamber housing an electron gun system and the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission Imaging Ultraviolet Spectrograph optical engineering unit, operating in the FUV (110–170 nm). The determination of the total O i (5So) at 135.6 nm emission cross section is accomplished by measuring the cylindrical glow pattern of the metastable emission from electron impact by imaging the glow intensity about the electron beam from nominally zero to ~400 mm distance from the electron beam. The study of the glow pattern of O i (135.6 nm) from dissociative excitation of CO and CO2 indicates that the O i (5So) state has a kinetic energy of ~1 eV by modeling the radial glow pattern with the published lifetime of 180 μs for the O i (5So) state. Plain Language Summary: Both Mars and Venus have upper atmospheres that are similar in composition: mostly CO2, CO, and N2 are dominant molecular gases, with nearly identical UV spectra. The modeling studies of atmospheric UV emissions cannot presently be accurately conducted to the same accuracy as the planetary UV measurements, which avail themselves with state‐of‐the‐art calibrated spectrographs. This dichotomy in accuracy between planetary observation and model occurs because the atomic and molecular emission cross sections with uncertainties of certain transitions are greater than 100%. Furthermore, the analysis is complicated by the spectral blending of the various emissions of the low‐resolution spectral spaceborne instruments. We present in this paper a UV laboratory instrument unique in the world at the University of Colorado that can measure for the first time the excitation mechanisms with accurate emission cross sections of both allowed and optically forbidden transitions that are occurring in a planetary atmosphere. Key Points: We measured 30 and 100 eV emission cross sections of Fourth Positive band system of CO and O i (135.6 nm) from electron impact on CO and CO2We conducted a laboratory experiment measuring single‐scattering electron‐impact‐induced‐fluorescence FUV spectra from excitation of CO and CO2Fragment kinetic energy measurement for O i (5So) atoms found to be around ~1 eV from both CO and CO2 molecular dissociation [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

The goal of this paper is to understand the processes by which solar wind electrons are energized in the Martian magnetosphere and how this compares to processes at Venus and Earth. Each is unique in the source of its magnetic field topology and how this influences electron energization. To achieve this goal, 24 million spectra spanning 13 years have been examined using the electron spectrometer from the Mars Express spacecraft between about 12,000 km and about 250 km altitude, and from all latitudes and local times. The top 10 largest differential energy flux at energies above the differential energy flux peak have been found: seven spectra from the magnetosheath near noon, three from the dark tail (the largest two from the middle and ionospheric edge of the magnetosheath). Spectral comparisons show a decade range in the peak of the electron distributions; however, all distributions show a similar energy maximum dictated by solar wind/planet interaction. Similarly derived, the largest Venus spectrum occurred near the magnetosheath bow shock and had the same shape as the most intense Mars inner magnetosheath spectrum. The Mars and Venus dayside spectra compared to the Mars nightside spectrum that included an enhanced optical signal attributed to discrete "auroral" precipitation show a similar shape. These spectra are also compared to a selected auroral zone electron spectra from the Earth. The Mars and Venus results suggest that there is no more energy needed to generate electrons forming the nightside precipitation than is gained during the solar wind/planet interaction. Key Points: Top 10 orbit‐selected electron energy spectra from Mars are identified from MEx ASPERA‐3 ELS between 2004 and 2016The Mars and Venus dayside electron spectra are similar, and a discrete "auroral" spectrum from the nightside tail of Mars is also similarNo more energy than that supplied by the solar wind‐planet interaction is required to produce discrete auroral spectrum from the nightside tail of Mars [ABSTRACT FROM AUTHOR]

Abstract: We present model results of the interaction of proton and hydrogen atom precipitation with the Martian atmosphere. We use a kinetic Monte Carlo model developed earlier for the analysis of the Analyzer of Space Plasmas and Energetic Atoms (ASPERA‐3) Mars Express data. With the availability of Mars Atmosphere and Volatile Evolution Mission in situ measurements, not only the flux of protons incident on the atmosphere but also their degradation along the orbit may now be described. The comparison of the simulations with data collected with the Solar Wind Ion Analyzer shows that the Monte Carlo model reproduces some of the measured features. The results of comparison between simulations and measurements of the proton fluxes at low altitudes make it possible to infer the efficiency of charge exchange between solar wind and the extended hydrogen corona if the value of the magnetic field is measured simultaneously. We also find that the induced magnetic field plays a very important role in the formation of the backscattered flux and strongly controls its magnitude. At the same time, discrepancies between the modeled and the measured energy spectra of the backscattered protons are pointed out. We suggest that some of the physical processes controlling the upward flux are not fully understood or that the data processing of the measured backscattered proton flux should be improved. [ABSTRACT FROM AUTHOR]

Abstract: 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 O2 atmosphere, 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 O2 atmosphere 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. [ABSTRACT FROM AUTHOR]

Abstract: A large set of observations of Jupiter's ultraviolet aurora was collected with the Hubble Space Telescope concurrently with the NASA‐Juno mission, during an eight‐month period, from 30 November 2016 to 18 July 2017. These Hubble observations cover Juno orbits 3 to 7 during which Juno in situ and remote sensing instruments, as well as other observatories, obtained a wealth of unprecedented information on Jupiter's magnetosphere and the connection with its auroral ionosphere. Jupiter's ultraviolet aurora is known to vary rapidly, with timescales ranging from seconds to one Jovian rotation. The main objective of the present study is to provide a simplified description of the global ultraviolet auroral morphology that can be used for comparison with other quantities, such as those obtained with Juno. This represents an entirely new approach from which logical connections between different morphologies may be inferred. For that purpose, we define three auroral subregions in which we evaluate the auroral emitted power as a function of time. In parallel, we define six auroral morphology families that allow us to quantify the variations of the spatial distribution of the auroral emission. These variations are associated with changes in the state of the Jovian magnetosphere, possibly influenced by Io and the Io plasma torus and by the conditions prevailing in the upstream interplanetary medium. This study shows that the auroral morphology evolved differently during the five ~2 week periods bracketing the times of Juno perijove (PJ03 to PJ07), suggesting that during these periods, the Jovian magnetosphere adopted various states. [ABSTRACT FROM AUTHOR]

Abstract: 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]