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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 Alfv\'en waves along the magnetic field lines. The position of the moons is therefore "Alfv\'enically" 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 Alfv\'en waves to reach the Jovian ionosphere varies accordingly. Using spectral images of the Main Alfv\'en Wing auroral spots collected by Juno-UVS during the first forty-three orbits, this work provides the first empirical model of the Io, Europa and Ganymede equatorial lead angles for the northern and southern hemispheres. Alfv\'en 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 15RJ . Finally, we shows how the added knowledge of the lead angle can improve the interpretation of the moon-induced decametric emissions., Comment: 20 pages, 8 figures, Accepted for publication in Journal of Geophysical Research: Space Physics on 20 April 2023

The upper mesosphere and lower thermosphere of Mars (70–150 km) is of high interest because it is a region affected by climatological/meteorological events in the lower atmosphere and external solar forcing. However, only a few measurements are available at this altitude range. OI 557.7 nm dayglow emission has been detected at these altitudes by the limb observations with Nadir and Occultation for Mars Discovery (NOMAD) aboard the ExoMars Trace Gas Orbiter (TGO). We develop an inversion method to retrieve density and temperature at these altitudes from the OI 557.7 nm dayglow limb profiles. We demonstrate that the atmospheric density around 90 and 140 km and temperature around 80 km during the daytime can be retrieved from the TGO/NOMAD limb measurements. The retrieved densities show a large seasonal variation both around 90 and 140 km and reach maximum values around perihelion period. This can be explained by temperature variation in the lower atmosphere driven by the dust content and Sun-Mars distance. Temperature around 80 km is higher than predicted by general circulation models, which is tentatively consistent with the warm atmospheric layer recently discovered in nighttime. The temperature retrieval relies on the temperature dependence of the quenching coefficient of 1S oxygen by CO2. Further validation of this coefficient in the range of the Mars upper atmosphere is needed for the verification of the retrieved high temperature. © 2022. American Geophysical Union. All Rights Reserved., The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB-BIRA), assisted by Co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS), and the United Kingdom (The Open University). This project acknowledges funding by the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 4000121493), by the Spanish Ministry of Science and Innovation (MCIU) and by European funds under grants PGC2018-101836-B-I00 and ESP2017-87143-R (MINECO/FEDER), as well as by UK Space Agency through grants ST/V002295/1, ST/V005332/1 and ST/S00145X/1 and Italian Space Agency through Grant 2018-2-HH.0. This work was supported by the Belgian Fonds de la Recherche Scientifique – FNRS under Grant No. 30442502 (ET_HOME). The IAA/CSIC team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the “Center of Excellence Severo Ochoa” award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709). US investigator was supported by the National Aeronautics and Space Administration. F.G.-G. is funded by the Spanish Ministerio de Ciencia, Innovación y Universidades, the Agencia Estatal de Investigación and EC FEDER funds under project RTI2018-100920-J-I00.

International audience; 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.

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 ~ 80RJ to 60RJ during 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.

JIRAM is the imaging spectrometer on board the NASA Juno mission. Data collected since August 2016 on both Jupiter Northern and Southern aurora have an unprecedent spatial resolution. Moreover, JIRAM scanning mirror allows observations of the same area at serveral adiacent time frames. In this work we focus on the spatial and temporal variability of the Southern aurora. JIRAM data of the L imager channel have been averaged in bins of 2.5°LAT × 2°LON and variations of the signal have been investigated for 17h50m < time < 19h45m, 27 August 2016. Time frames have been carefully selected in order to avoid possibile instrumental residuals in the signal (Mura et al., 2017). We find that on the South Pole, for -87.5°

The NASA Juno mission has been investigating Jupiter's atmosphere since August 2016, providing unprecedented insight on the atmosphere of the planet. The Jupiter Infrared Auroral Mapper (JIRAM) experiment, on board Juno, performed spectroscopic observations of the H3+ emissions both in the auroral regions (Dinelli et al., 2017; Adriani et al.,2017; Mura et al., 2017) and at mid latitudes. In the present work, we concentrate on the observations acquired by the JIRAM spectrometer during the first Jupiter passage on 26-27 August 2016, when the spacecraft was at about 500,000-1,200,000 km from the planet. During a portion of the observations, the slit of the spectrometer was sampling Jupiter's limb in the latitude range from 30 to 60 deg for both hemispheres. In the 3-4 ?m spectral region, the limb spectra show the typical features of the H3+ emission, usually used to retrieve concentration and temperature of this species in the auroral region. In this work we exploit this spectral region to provide new insight on the H3+ vertical distribution and, more generally, on thecomposition of the atmosphere of Jupiter. The spatial resolution of the limb observations of Jupiter, ranging between 50 and 130 km, is favourable for investigating the vertical distribution of H3+. The vertical profiles of the integrated H3+ intensity will be presented along with the preliminary results of the retrieval of the H3+ vertical volume mixing ratio (VMR) distribution and compared with predictions from available atmospheric models of the planet (Achilleos et al. 1998). Possible variability of the altitude distribution of the peak emission with respect to latitude and longitude will also be discussed.

The main auroral emission at Jupiter generally appears as a quasi-closed curtain centered around the magnetic pole. This auroral feature, which accounts for approximately half of the total power emitted by the aurorae in the ultraviolet range, is related to corotation enforcement currents in the middle magnetosphere. Early models for these currents assumed axisymmetry, but significant local time variability is obvious on any image of the Jovian aurorae. Here we use far-UV images from the Hubble Space Telescope to further characterize these variations on a statistical basis. We show that the dusk side sector is ~ 3 times brighter than the dawn side in the southern hemisphere and ~ 1.1 brighter in the northern hemisphere, where the magnetic anomaly complicates the interpretation of the measurements. We suggest that such an asymmetry between the dawn and the dusk sectors could be the result of a partial ring current in the nightside magnetosphere.

The aurorae at Jupiter are made up of many different features associated with a variety of generation mechanisms. The main auroral emission, also known as the main oval, is the most prominent of them as it accounts for approximately half of the total power emitted by the aurorae in the ultraviolet range. The energy of the precipitating electrons is a crucial parameter to characterize the processes at play which give rise to these auroral emissions, and the altitude of the emissions directly depends on this energy. Here we make use of far-UV (FUV) images acquired with the Advanced Camera for Surveys on board the Hubble Space Telescope and spectra acquired with the Space Telescope Imaging Spectrograph to measure the vertical profile of the main emissions. The altitude of the brightness peak as seen above the limb is ~ 400 km, which is significantly higher than the 250 km measured in the post-dusk sector by Galileo in the visible domain. However, a detailed analysis of the effect of hydrocarbon absorption, including both simulations and FUV spectral observations, indicates that FUV apparent vertical profiles should be considered with caution, as these observations are not incompatible with an emission peak located at 250 km. The analysis also calls for spectral observations to be carried out with an optimized geometry in order to remove observational ambiguities.

The first simultaneous observations of fields and plasmas in Saturn's high-latitude magnetosphere and UV images of the conjugate auroral oval were obtained by the Cassini spacecraft and the Hubble Space Telescope (HST) in January 2007. These data have shown that the southern auroral oval near noon maps to the dayside cusp boundary between open and closed field lines, associated with a major layer of upward-directed field-aligned current (Bunce et al., 2008). The results thus support earlier theoretical discussion and quantitative modelling of magnetosphere-ionosphere coupling at Saturn (Cowley et al., 2004), that suggests the oval is produced by electron acceleration in the field-aligned current layer required by rotational flow shear between strongly sub-corotating flow on open field lines and near-corotating flow on closed field lines. Here we quantitatively compare these modelling results (the "CBO" model) with the Cassini-HST data set. The comparison shows good qualitative agreement between model and data, the principal difference being that the model currents are too small by factors of about five, as determined from the magnetic perturbations observed by Cassini. This is suggested to be principally indicative of a more highly conducting summer southern ionosphere than was assumed in the CBO model. A revised model is therefore proposed in which the height-integrated ionospheric Pedersen conductivity is increased by a factor of four from 1 to 4 mho, together with more minor adjustments to the co-latitude of the boundary, the flow shear across it, the width of the current layer, and the properties of the source electrons. It is shown that the revised model agrees well with the combined Cassini-HST data, requiring downward acceleration of outer magnetosphere electrons through a ~10 kV potential in the current layer at the open-closed field line boundary to produce an auroral oval of ~1° width with UV emission intensities of a few tens of kR.

A selection of twenty-two Hubble Space Telescope images of Saturn's ultraviolet auroras obtained during 1997–2004 has been analysed to determine the median location and width of the auroral oval, and their variability. Limitations of coverage restrict the analysis to the southern hemisphere, and to local times from the post-midnight sector to just past dusk, via dawn and noon. It is found that the overall median location of the poleward and equatorward boundaries of the oval with respect to the southern pole are at ~14° and ~16° co-latitude, respectively, with a median latitudinal width of ~2°. These median values vary only modestly with local time around the oval, though the poleward boundary moves closer to the pole near noon (~12.5°) such that the oval is wider in that sector (median width ~3.5°) than it is at both dawn and dusk (~1.5°). It is also shown that the position of the auroral boundaries at Saturn are extremely variable, the poleward boundary being located between 2° and 20° co-latitude, and the equatorward boundary between 6° and 23°, this variability contrasting sharply with the essentially fixed location of the main oval at Jupiter. Comparison with Voyager plasma angular velocity data mapped magnetically from the equatorial magnetosphere into the southern ionosphere indicates that the dayside aurora lie poleward of the main upward-directed field-aligned current region associated with corotation enforcement, which maps to ~20°–24° co-latitude, while agreeing reasonably with the position of the open-closed field line boundary based on estimates of the open flux in Saturn's tail, located between ~11° and ~15°. In this case, the variability in location can be understood in terms of changes in the open flux present in the system, the changes implied by the Saturn data then matching those observed at Earth as fractions of the total planetary flux. We infer that the broad (few degrees) diffuse auroral emissions and sub-corotating auroral patches observed in the dayside sector at Saturn result from precipitation from hot plasma sub-corotating in the outer magnetosphere in a layer a few Saturn radii wide adjacent to the magnetopause, probably having been injected either by Dungey-cycle or Vasyliunas-cycle dynamics on the nightside.

ESA's Venus Express probe has been in orbit since April 2006. Eight research papers in this issue present new results from the mission, covering the atmosphere, polar features, interactions with the solar wind and the controversial matter of venusian lightning. Hakan Svedham et al. open the section with a review of the similarities and (mostly) differences between Venus and its 'twin', the Earth. Andrew Ingersoll considers the latest results, and also how the project teams plan to make the most of the probe's remaining six years of life. Venus' mesosphere is a transition region between the retrograde super rotation at the top of the thick clouds and the solar-antisolar circulation in the thermosphere. The mesospheric distributions of HF, HCl, H2O and HDO are reported, and an unexpected extensive layer of warm air at altitudes 90–120 km on the nightside is found. Venus has thick clouds of H2SO4 aerosol particles extending from altitudes of 40 to 60 km. The 60–100 km region (the mesosphere) is a transition region between the 4 day retrograde superrotation at the top of the thick clouds and the solar–antisolar circulation in the thermosphere (above 100 km), which has upwelling over the subsolar point and transport to the nightside1,2. The mesosphere has a light haze of variable optical thickness, with CO, SO2, HCl, HF, H2O and HDO as the most important minor gaseous constituents, but the vertical distribution of the haze and molecules is poorly known because previous descent probes began their measurements at or below 60 km. Here we report the detection of an extensive layer of warm air at altitudes 90–120 km on the night side that we interpret as the result of adiabatic heating during air subsidence. Such a strong temperature inversion was not expected, because the night side of Venus was otherwise so cold that it was named the ‘cryosphere’ above 100 km. We also measured the mesospheric distributions of HF, HCl, H2O and HDO. HCl is less abundant than reported 40 years ago3. HDO/H2O is enhanced by a factor of ∼2.5 with respect to the lower atmosphere, and there is a general depletion of H2O around 80–90 km for which we have no explanation.