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Discrete aurora are sporadic emissions of light originating in Mars upper atmosphere. We report nadir imaging observations from MAVEN's Imaging UltraViolet Spectrograph which identify the conditions which trigger electron precipitation causing these events. Prior studies have shown that discrete aurora events in the strong crustal magnetic field region in the southern hemisphere are the brightest and most repeatable compared to events occurring outside the region. Our new data set offers a more complete and accurate characterization of aurora in this area. The region of strongest crustal fields is composed of two distinct magnetic regions, with magnetic fields in opposite directions; discrete aurora events trigger in one region after dusk and in the other before dawn. Magnetic reconnection in these two adjacent regions with the draped interplanetary field may open the crustal fields in these regions during opposing local times. Particle precipitation can then cause discrete aurora at the observed times and locations. Plain Language Summary: Mars has a surprising variety of types of aurora, all different from Earth's "northern lights." Mars lacks the familiar high‐latitude aurora because it no longer has the same kind of global magnetic field Earth does. This study examines "discrete aurora" events that occur in region in Mars southern hemisphere that has retained some of the ancient magnetic field. It takes the form of long arcades of magnetic loops that resemble a set of arches. As Mars rotates, these arcades are carried around the planet as the solar wind and its imbedded magnetic field are carried past the planet. We show that when conditions are favorable, the magnetic field locked in the solar wind can interact and "reconnect" with Mars magnetic loops, allowing energetic particles to spiral down the field lines into the atmosphere to cause discrete aurora. Key Points: New data confirm that Mars discrete aurora events occur most frequently near strong crustal fields and vary with local timeAuroral events in adjacent regions with opposite magnetic polarity occur before or after midnight depending on local magnetic field directionMagnetic reconnection between crustal fields and the draped interplanetary field appears to control regional and local time behavior [ABSTRACT FROM AUTHOR]

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 Trace Gas Orbiter has been orbiting Mars since 2016 with the Nadir and Occultation for MArs Discovery (NOMAD) UltraViolet and Visible Spectrometer (UVIS) instrument on board. Focusing on limb observations recorded in the ultraviolet (UV) part of the NOMAD/UVIS spectra, we describe here the CO Cameron bands, CO2+ ${\mathrm{C}\mathrm{O}}_{2}^{+}$ UV doublet and Fox‐Duffendach‐Barker (FDB) bands and [OI] UV emissions. Averaged limb profiles are presented, showing that the strongest brightness and the highest peak altitudes are reached near perihelion. Ratios between the UV emissions are also estimated and compared with previous observations from Mariner and Mars Express. NOMAD/UVIS is the first instrument able to simultaneously acquire data both in the UV and the visible in the Mars atmosphere so that the oxygen green line at 557.7 nm and its UV counterpart at 297.2 nm, both originating from the same O(1S) upper state level, may be directly compared. A mean ratio of 15.8 is derived, in close agreement with ab initio calculations. The spectral composition of the CO2+ ${\mathrm{C}\mathrm{O}}_{2}^{+}$ FDB system that has not been observed entirely since the Mariner missions in the 1970s is analyzed. According to the spectral composition of the FDB bands, we show that this emission is produced at ∼70% by photoionization of CO2 (which populates the shorter wavelengths of the spectrum) and ∼30% by resonance scattering of solar radiation (which populates longer wavelengths). No evidence of a change with altitude in the CO2+ ${\mathrm{C}\mathrm{O}}_{2}^{+}$ FDB spectral composition is observed in the NOMAD/UVIS spectra. Plain Language Summary: Dayglow is an atmospheric emission occurring on the illuminated side of a planet. This phenomenon occurs when atmospheric atoms or molecules interact with incoming solar radiation that produce excited atoms and molecules. When these atoms and molecules de‐excite radiatively, they release photons at specific wavelengths and altitudes, generating airglow. Several space missions since the 1970s observed the Mars ultraviolet (UV) dayglow. The Nadir and Occultation for MArs Discovery (NOMAD)/UltraViolet and Visible Spectrometer (UVIS) instrument, currently orbiting Mars since 2016, is providing new insights on this phenomenon. In this study, we describe the results of the observations of the Martian UV dayglow by NOMAD/UVIS. We show that the strongest emissions occur at higher altitudes, when the planet is closest to the Sun. We also estimate the intensity ratios between the emissions and compare our results with previous observations from Mariner and Mars Express. The CO2+ ${\mathrm{C}\mathrm{O}}_{2}^{+}$ emission between 300 and 400 nm is also detected for the first time since the Mariner missions in the 1970s. The production mechanisms can be deduced by analyzing the spectral distribution of this emission. We conclude that photoionization of CO2 represents ∼70% of its production, while resonance scattering of solar radiation accounts for ∼30%. This proportion was predicted by models, but this is the first time that observations validate these simulations. This statement is true at all the observed altitudes. Key Points: The Mars ultraviolet (UV) dayglow emissions from CO Cameron, CO2+ ${\mathrm{C}\mathrm{O}}_{2}^{+}$ ultraviolet doublet, [OI] UV and CO2+ ${\mathrm{C}\mathrm{O}}_{2}^{+}$ Fox‐Duffendach‐Barker (FDB) are analyzed using Nadir and Occultation for MArs Discovery/UltraViolet and Visible Spectrometer limb spectraThe controversial 557.7/297.2 nm intensity ratio derived from those simultaneous measurements is highly reliably determinedThe full wavelength range of the CO2+ ${\mathrm{C}\mathrm{O}}_{2}^{+}$ FDB band system is detected for the first time since Mariner [ABSTRACT FROM AUTHOR]

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 NH3 at 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. Plain Language Summary: 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. Key Points: 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 [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]

The Nadir and Occultation for MArs Discovery (NOMAD)/UV‐visible (UVIS) spectrometer on the ExoMars Trace Gas Orbiter provided observations of ozone (O3) and water vapor in the global dust storm of 2018. Here we show in detail, using advanced data filtering and chemical modeling, how Martian O3 in the middle atmosphere was destroyed during the dust storm. In data taken exactly 1 year later when no dust storm occurred, the normal situation had been reestablished. The model simulates how water vapor is transported to high altitudes and latitudes in the storm, where it photolyzes to form odd hydrogen species that catalyze O3. O3 destruction is simulated at all latitudes and up to 100 km, except near the surface where it increases. The simulations also predict a strong increase in the photochemical production of atomic hydrogen in the middle atmosphere, consistent with the enhanced hydrogen escape observed in the upper atmosphere during global dust storms. Plain Language Summary: Global dust storms are rare but impactful events on Mars, occurring about once in a decade. Previous investigations found how water vapor is redistributed throughout the entire atmosphere in a dust storm. Photolysis of water vapor by sunlight produces highly reactive species that destroy ozone (O3). Here we present O3 measurements taken by the NOMAD/UVIS instrument on the ExoMars Trace Gas Orbiter in the 2018 global dust storm. After advanced data filtering, they demonstrate how O3 in the middle atmosphere was much reduced compared to one Mars year later when no dust storm occurred. 3D atmospheric model simulations of atmospheric chemistry in the global dust storm confirm this planet‐wide O3 destruction, and help to understand the involved processes. The simulations also predict a strong increase in production of atomic hydrogen in the middle atmosphere, that can explain the observed increased hydrogen atmospheric escape during global dust storms. Key Points: NOMAD ozone (O3) data filtering during the 2018 global dust storm shows strong O3 destruction compared to one year later with no dust storm3D simulations of atmospheric chemistry in the 2018 global dust storm are presented to understand impact on odd hydrogen and odd oxygenThe model confirms middle‐atmospheric O3 destruction in the dust storm and predicts increased photochemical production of hydrogen [ABSTRACT FROM AUTHOR]

Observations of planetary auroras form a new area of planetary exploration from space, especially for nonmagnetic planets since various kinds of auroras like Discrete, Proton and Diffuse auroras have been observed at Mars. We review the latest results of Martian auroras obtained by the instruments (1) SPICAM (Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars) aboard Mars Express (MEX) and (2) IUVS (the Imaging Ultraviolet Spectrograph) on MAVEN (the Mars Atmosphere and Volatile Evolution mission). The MARSIS instrument (the Mars Advanced Radar for the Subsurface and Ionosphere Sounding) on MEX, in addition, exhibited strong ionizations in some electron density profiles, thus providing further evidence for the existence of Martian auroras. We review these MARSIS observations as well. In addition, we review various models of Martian auroras. [ABSTRACT FROM AUTHOR]