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See detailH3+ characteristics in the Jupiter atmosphere as observed at limb with Juno/JIRAM
Migliorini, Alessandra; Dinelli, B.M.; Moriconi, M.L. et al

in Icarus (2019), 329

NASA’s Juno spacecraft has been orbiting Jupiter since August 2016, providing unprecedented insights into the giant planet’s atmosphere. The Jupiter Infrared Auroral Mapper (JIRAM) experiment on board ... [more ▼]

NASA’s Juno spacecraft has been orbiting Jupiter since August 2016, providing unprecedented insights into the giant planet’s atmosphere. The Jupiter Infrared Auroral Mapper (JIRAM) experiment on board Juno has made spectroscopic observations of the trihydrogen cation (H3+) emissions in both northern and southern auroral regions (Dinelli et al. 2017; Adriani et al. 2017; Mura et al. 2017) and at mid-to-low latitudes (this paper). Observations targeting the limb of the planet from 60° North to 60° South latitudes were acquired with JIRAM’s spectrometer in August 2016 and March 2017. We use these observations to characterize, for the first time, the vertical distribution of the H3+ emissions as a function of latitude across Jupiter’s sunlit face dayside. H3+ emission features in the 3-4 μm spectral band were used to retrieve the H3+ volume mixing ratio (VMR) and atmospheric temperatures as a function of altitude. The H3+ density profile has a quasi-symmetric distribution with latitude, decreasing from 5×105 cm-3 at 500 km altitude above the 1-bar level to 2×105 cm-3 at 650 km (column densities of 3.5×1013 cm-2 to 1.4×1013 cm-2, assuming a 700 km column depth; altitudes are referenced to 1-bar pressure level). The H3+ VMR is higher in the Southern hemisphere than in the North with values at 500 km altitude of ~4×10-4 ppmv at 40°N and ~8×10-4 ppmv at 40°S. Retrieved temperatures increase almost monotonically with increasing altitude, hovering around 400 K at 300 km and greater than 900 K at about 700 km. [less ▲]

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See detailLyman-α emission in the Martian proton aurora: Line profile and role of horizontal induced magnetic field
Gérard, Jean-Claude ULiege; Hubert, Benoît ULiege; Ritter, Birgit ULiege et al

in Icarus (2019), 321

Enhancements of the dayside Lyman-α emission by as much as ∼50% have been observed between 120 and 130 km in the lower Martian thermosphere from the Mars Express and MAVEN satellites, usually following ... [more ▼]

Enhancements of the dayside Lyman-α emission by as much as ∼50% have been observed between 120 and 130 km in the lower Martian thermosphere from the Mars Express and MAVEN satellites, usually following solar events such as coronal mass ejections and corotating interaction regions. They have been assumed to be optical signatures of proton aurora related to an increase in the solar wind proton flux hitting Mars’ bow shock. We present model simulations of the Lyman-α line profiles at different altitudes. These are partly guided by in situ measurements of the energy spectrum of protons in the magnetosheath region by the SWIA instrument on board the MAVEN spacecraft. We show that the auroral Lyman-α line profile is significantly broader than the hydrogen core of the planetary thermal H atom. Consequently, most of the auroral emission is produced outside the optically thick hydrogen core and creates the observed intensity enhancement. Simulations with incident energetic hydrogen atoms (H ENAs) produce a somewhat broader line profile. Monte Carlo calculations are made separately for incident solar wind protons and H ENAs produced by charge exchange in the hydrogen corona. Absorption by CO2 along the line of sight significantly affects the intensity distribution in the lower thermosphere. The calculated altitude of the peak emission for both types of incident particles is consistent with the observed characteristics of the proton aurora. We show that the presence of a horizontal induced magnetic field somewhat increases the line width and decreases the altitude of the emission peak as a consequence of the magnetic barrier effect on proton precipitation. The brightness of the Lyman-α emission also drops as a result of increased magnetic shielding of the protons. [less ▲]

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See detailNew polarimetric and spectroscopic evidence of anomalous enrichment in spinel-bearing calcium-aluminium-rich inclusions among L-type asteroids
Devogele, Maxime ULiege; Tanga, P.; Cellino, A. et al

in Icarus (2018), 304

Asteroids can be classified into several groups based on their spectral reflectance. Among these groups, the one belonging to the L-class in the taxonomic classification based on visible and near-infrared ... [more ▼]

Asteroids can be classified into several groups based on their spectral reflectance. Among these groups, the one belonging to the L-class in the taxonomic classification based on visible and near-infrared spectra exhibit several peculiar properties. First, their near-infrared spectrum is characterized by a strong absorption band interpreted as the diagnostic of a high content of the FeO bearing spinel mineral. This mineral is one of the main constituents of Calcium-Aluminum-rich Inclusions (CAI) the oldest mineral compounds found in the solar system. In polarimetry, they possess an uncommonly large value of the inversion angle incompatible with all known asteroid belonging to other taxonomical classes. Asteroids found to possess such a high inversion angle are commonly called Barbarians based on the first asteroid on which this property was first identified, (234) Barbara. In this paper we present the results of an extensive campaign of polarimetric and spectroscopic observations of L-class objects. We have derived phase-polarization curves for a sample of 7 Barbarians, finding a variety of inversion angles ranging between 25 and 30°. Spectral reflectance data exhibit variations in terms of spectral slope and absorption features in the near-infrared. We analyzed these data using a Hapke model to obtain some inferences about the relative abundance of CAI and other mineral compounds. By combining spectroscopic and polarimetric results, we find evidence that the polarimetric inversion angle is directly correlated with the presence of CAI, and the peculiar polarimetric properties of Barbarians are primarily a consequence of their anomalous composition. © 2017. [less ▲]

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See detailTemperature estimation from hydroxyl airglow emission in the Venus night side mesosphere
Migliorini, Alessandra; Snels, M.; Gérard, Jean-Claude ULiege et al

in Icarus (2018), 300

The temperature of the night side of Venus at about 95 km has been determined by using spectral features of the hydroxyl airglow emission around 3 μm, recorded from July 2006 to July 2008 by VIRTIS ... [more ▼]

The temperature of the night side of Venus at about 95 km has been determined by using spectral features of the hydroxyl airglow emission around 3 μm, recorded from July 2006 to July 2008 by VIRTIS onboard Venus Express. The retrieved temperatures vary from 145.5 to about 198.1 K with an average value of 176.3 ± 14.3 K and are in good agreement with previous ground-based and space observations. The variability with respect to latitude and local time has been studied, showing a minimum of temperature at equatorial latitudes, while temperature values increase toward mid latitudes with a local maximum at about 35°N. The present work provides an independent contribution to the temperature estimation in the transition region between the Venus upper mesosphere and the lower thermosphere, by using the OH emission as a thermometer, following the technique previously applied to the high-resolution O[SUB]2[/SUB](a[SUP]1[/SUP]Δ[SUB]g[/SUB]) airglow emissions observed from ground. [less ▲]

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See detailConcurrent ultraviolet and infrared observations of the north Jovian aurora during Juno's first perijove
Gérard, Jean-Claude ULiege; Mura, A.; Bonfond, Bertrand ULiege et al

in Icarus (2018), 312

The UltraViolet Spectrograph (UVS) and the Jupiter InfraRed Auroral Mapper (JIRAM) observed the north polar aurora before the first perijove of the Juno orbit (PJ1) on 27 August 2016. The UVS bandpass ... [more ▼]

The UltraViolet Spectrograph (UVS) and the Jupiter InfraRed Auroral Mapper (JIRAM) observed the north polar aurora before the first perijove of the Juno orbit (PJ1) on 27 August 2016. The UVS bandpass corresponds to the H2 Lyman and Werner bands that are directly excited by collisions of auroral electrons with molecular hydrogen. The spectral window of the JIRAM L-band imager includes some of the brightest H3+ thermal features between 3.3 and 3.6 µm. A series of spatial scans obtained with JIRAM every 30 s is used to build up five quasi-global images, each covering ∼12 min. of observations. JIRAM's best spatial resolution was on the order of 50 km/pixel during this time frame, while UVS has a resolution of about 750 km. Most of the observed large-scale auroral features are similar in the two spectral regions, but important differences are also observed in their morphology and relative intensity. Only a part of the UV-IR differences stems from the higher spatial resolution of JIRAM, as some of them are still present following smoothing of the JIRAM images at the UVS resolution. For example, the JIRAM images show persistent narrow arc structures in the 100°–180° SIII longitude sector at dusk not resolved in the ultraviolet, but consistent with the structure of in situ electron precipitation measured two hours later. The comparison between the H2 intensity and the H3+ radiance measured along two radial cuts from the center of the main emission illustrates the complex relation between the electron energy input, their characteristic energy and the H3+ emission. Low values of the H3+ intensity relative to the H2 brightness are observed in regions of high FUV color ratio corresponding to harder electron precipitation. The rapid loss of H3+ ions reacting with methane near and below the homopause appears to play a significant role in the control of the relative brightness of the two emissions. Cooling of the auroral thermosphere by H3+ radiation is spatially variable relative to the direct particle heating resulting from the precipitated electron flux. [less ▲]

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See detailSimilarity of the Jovian satellite footprints: spots multiplicity and dynamics
Bonfond, Bertrand ULiege; Grodent, Denis ULiege; Badman, S. V. et al

in Icarus (2017), 292(2017), 208217

In the magnetospheres of Jupiter and Saturn, the intense interaction of the satellites Io, Europa, Ganymede and Enceladus with their surrounding plasma environment leaves a signature in the aurora of the ... [more ▼]

In the magnetospheres of Jupiter and Saturn, the intense interaction of the satellites Io, Europa, Ganymede and Enceladus with their surrounding plasma environment leaves a signature in the aurora of the planet. Called satellite footprints, these auroral features appear either as a single spot (Europa and Enceladus) or as multiple spots (Io and Ganymede). Moreover, they can be followed by extended trailing tails in the case of Io and Europa, while no tail has been reported for Ganymede and Enceladus, yet. Here we show that all Jovian footprints can be made of several spots. Furthermore, the footprints all experience brightness variations on timescale of 2-3 minutes. We also demonstrate that the satellite location relative to the plasma sheet is not the only driver for the footprint brightness, but that the plasma environment and the magnetic field strength also play a role. These new findings demonstrate that the Europa and Ganymede footprints are very similar to the Io footprint. As a consequence, the processes expected to take place at Io, such as the bi-directional electron acceleration by Alfvén waves or the partial reflection of these waves on plasma density gradients, can most likely be extended to the other footprints, suggesting that they are indeed universal processes. [less ▲]

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See detailThe thermal structure of the Venus atmosphere: Intercomparison of Venus Express and ground based observations of vertical temperature and density profiles✰
Limaye, Sanjay S.; Lebonnois, Sebastien; Mahieux, Arnaud et al

in Icarus (2017), 294

The Venus International Reference Atmosphere (VIRA) model contains tabulated values of temperature and number densities obtained by the experiments on the Venera entry probes, Pioneer Venus Orbiter and ... [more ▼]

The Venus International Reference Atmosphere (VIRA) model contains tabulated values of temperature and number densities obtained by the experiments on the Venera entry probes, Pioneer Venus Orbiter and multi-probe missions in the 1980s. The instruments on the recent Venus Express orbiter mission generated a significant amount of new observational data on the vertical and horizontal structure of the Venus atmosphere from 40 km to about 180 km altitude from April 2006 to November 2014. Many ground based experiments have provided data on the upper atmosphere (90-130 km) temperature structure since the publication of VIRA in 1985. The "Thermal Structure of the Venus Atmosphere" Team was supported by the International Space Studies Institute (ISSI), Bern, Switzerland, from 2013 to 2015 in order to combine and compare the ground-based observations and the VEx observations of the thermal structure as a first step towards generating an updated VIRA model. Results of this comparison are presented in five latitude bins and three local time bins by assuming hemispheric symmetry. The intercomparison of the ground-based and VEx results provides for the first time a consistent picture of the temperature and density structure in the 40 km-180 km altitude range. The Venus Express observations have considerably increased our knowledge of the Venus atmospheric thermal structure above ∼40 km and provided new information above 100 km. There are, however, still observational gaps in latitude and local time above certain regions. Considerable variability in the temperatures and densities is seen above 100 km but certain features appear to be systematically present, such as a succession of warm and cool layers. Preliminary modeling studies support the existence of such layers in agreement with a global scale circulation. The intercomparison focuses on average profiles but some VEx experiments provide sufficient global coverage to identify solar thermal tidal components. The differences between the VEx temperature profiles and the VIRA below 0.1 mbar/95 km are small. There is, however, a clear discrepancy at high latitudes in the 10-30 mbar (70-80 km) range. The VEx observations will also allow the improvement of the empirical models (VTS3 by Hedin et al., 1983 and VIRA by Keating et al., 1985) above 0.03 mbar/100 km, in particular the 100-150 km region where a sufficient observational coverage was previously missing. The next steps in order to define the updated VIRA temperature structure up to 150 km altitude are (1) define the grid on which this database may be provided, (2) fill what is possible with the results of the data intercomparison, and (3) fill the observational gaps. An interpolation between the datasets may be performed by using available General Circulation Models as guidelines. An improved spatial coverage of observations is still necessary at all altitudes, in latitude-longitude and at all local solar times for a complete description of the atmospheric thermal structure, in particular on the dayside above 100 km. New in-situ observations in the atmosphere below 40 km are missing, an altitude region that cannot be accessed by occultation experiments. All these questions need to be addressed by future missions. [less ▲]

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See detailThe Martian diffuse aurora: a model of ultraviolet and visible emissions
Gérard, Jean-Claude ULiege; Soret, Lauriane ULiege; Shematovich, V.I. et al

in Icarus (2017), 288

A new type of Martian aurora, characterized by an extended spatial distribution, an altitude lower than the discrete aurora and electron precipitation up to 200 keV has been observed following solar ... [more ▼]

A new type of Martian aurora, characterized by an extended spatial distribution, an altitude lower than the discrete aurora and electron precipitation up to 200 keV has been observed following solar activity on several occasions from the MAVEN spacecraft. We describe the results of Monte Carlo simulations of the production of several ultraviolet and violet auroral emissions for initial electron energies extending from 0.25 to 200 keV. These include the CO2+ ultraviolet doublet (UVD) at 288.3 and 289.6 nm and the Fox–Duffendack–Barker (FDB) bands, CO Cameron and Fourth Positive bands, OI 130.4 and 297.2 nm and CI 156.1 nm and 165.7 nm multiplets. We calculate the nadir and limb production rates of several of these emissions for a unit precipitated energy flux. Our results indicate that electrons in the range 50-200 keV produce maximum CO2+ UVD emission below 75 km, in agreement with the MAVEN observations. We calculate the efficiency of photon production per unit precipitated electron power. The strongest emissions are the CO2+ FDB, UVD and CO Cameron bands and the oxygen mission at 297.2 nm. The metastable a 3Π state which radiates the Cameron bands is deactivated by collisions below about 110 km. As a consequence, we show that the Cameron band emission is expected to peak at a higher altitude than the CO2+ UVD and FDB bands. Collisional quenching also causes the intensity ratio of the CO2+ UVD to CO Cameron bands to increase below ∼100 km in the energetic diffuse aurora. [less ▲]

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See detailInfluence of the crustal magnetic field on the Mars aurora electron flux and UV brightness
Bisikalo, D. V.; Shematovich, V. I.; Gérard, Jean-Claude ULiege et al

in Icarus (2017), 282

Observations with the SPICAM instrument on board Mars Express have shown the occasional presence of localized ultraviolet nightside emissions associated with enhanced energetic electron fluxes. These ... [more ▼]

Observations with the SPICAM instrument on board Mars Express have shown the occasional presence of localized ultraviolet nightside emissions associated with enhanced energetic electron fluxes. These features generally occur in regions with significant radial crustal magnetic field. We use a Monte-Carlo electron transport model to investigate the role of the magnetic field on the downward and upward electron fluxes, the brightness and the emitted power of auroral emissions. Simulations based on an ASPERA-3 measured auroral electron precipitation indicate that magnetic mirroring leads to an intensification of the energy flux carried by upward moving electrons- from about 20% in the absence of crustal magnetic field up to 33-78% when magnetic field is included depending on magnetic field topology. Conservation of the particle flux in a flux tube implies that the presence of the B-field does not appreciably modify the emission rate profiles for an initially isotropic pitch angle distribution. However, we find that crustal magnetic field results in increase of the upward electron flux, and, consequently, in reduction of the total auroral brightness for given energy flux of precipitating electrons. [less ▲]

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See detailSPICAM on Mars Express: A 10 year in-depth survey of the Martian atmosphere
Montmessin, F.; Korablev, O.; Lefèvre, F. et al

in Icarus (2017), 297

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See detailStatistical study of Saturn's auroral electron properties with Cassini/UVIS FUV spectral images
Gustin, Jacques ULiege; Grodent, Denis ULiege; Radioti, Aikaterini ULiege et al

in Icarus (2016)

About 2000 FUV spectra of different regions of Saturn's aurora, obtained with Cassini/UVIS from December 2007 to October 2014 have been examined. Two methods have been employed to determine the mean ... [more ▼]

About 2000 FUV spectra of different regions of Saturn's aurora, obtained with Cassini/UVIS from December 2007 to October 2014 have been examined. Two methods have been employed to determine the mean energy 〈E〉 of the precipitating electrons. The first is based on the absorption of the auroral emission by hydrocarbons and the second uses the ratio between the brightness of the Lyman-α line and the H2 total UV emission (Lyα/H2), which is directly related to 〈E〉 via a radiative transfer formalism. In addition, two atmospheric models obtained recently from UVIS polar occultations have been employed for the first time. It is found that the atmospheric model related to North observations near 70° latitude provides the results most consistent with constraints previously published. On a global point of view, the two methods provide comparable results, with 〈E〉 mostly in the 7–17 keV range with the hydrocarbon method and 〈E〉 in the 1–11 keV range with the Lyα/H2 method. Since hydrocarbons have been detected on ∼20% of the auroral spectra, the Lyα/H2 technique is more effective to describe the primary auroral electrons, as it is applicable to all spectra and allows an access to the lowest range of energies (≤5 keV), unreachable by the hydrocarbon method. The distribution of 〈E〉 is found fully compatible with independent HST/ACS constraints (emission peak in the 840–1450 km range) and FUSE findings (emission peaking at pressure level ≤0.2 µbar). In addition, 〈E〉 exhibits enhancements in the 3 LT–10 LT sector, consistent with SKR intensity measurements. An energy flux–electron energy diagram built from all the data points strongly suggests that acceleration by field-aligned potentials as described by Knight's theory is a main mechanism responsible for electron precipitation creating the aurora. Assuming a fixed electron temperature of 0.1 keV, a best-fit equatorial electron source population density of 3 × 103 m−3 is derived, which matches very well to the plasma properties observed with Cassini MAG and CAPS/ELS instruments. However, several auroral regions are characterized by relatively high 〈E〉 and low energy flux, suggesting that additional processes such as plasma injections or magnetic reconnections must be accounted for to explain the emission in these regions. The Lyα/H2 ratio technique can be used to build maps of 〈E〉 from single spectral images. As expected, preliminary results show that the spatial distribution of 〈E〉 is not uniform, as seen on Jupiter. Our study reveals that a fraction of the aurora is due to very low energy electrons (<1 keV). Even in this case, comparisons between observed and modeled spectra show that 100 eV is a suitable value to represent the average energy of the secondary electrons. [less ▲]

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See detailThe compositional evolution of C/2012 S1 (ISON) from ground-based high-resolution infrared spectroscopy as part of a worldwide observing campaign
Dello Russo, N.; Vervack, R. J.; Kawakita, H. et al

in Icarus (2016), 266

Volatile production rates, relative abundances, rotational temperatures, and spatial distributions in the coma were measured in C/2012 S1 (ISON) using long-slit high-dispersion (λ/Δλ ∼ 2.5 × 10[SUP]4[/SUP ... [more ▼]

Volatile production rates, relative abundances, rotational temperatures, and spatial distributions in the coma were measured in C/2012 S1 (ISON) using long-slit high-dispersion (λ/Δλ ∼ 2.5 × 10[SUP]4[/SUP]) infrared spectroscopy as part of a worldwide observing campaign. Spectra were obtained on UT 2013 October 26 and 28 with NIRSPEC at the W.M. Keck Observatory, and UT 2013 November 19 and 20 with CSHELL at the NASA IRTF. H[SUB]2[/SUB]O was detected on all dates, with production rates increasing markedly from (8.7 ± 1.5) × 10[SUP]27[/SUP] molecules s[SUP]-1[/SUP] on October 26 (R[SUB]h[/SUB] = 1.12 AU) to (3.7 ± 0.4) × 10[SUP]29[/SUP] molecules s[SUP]-1[/SUP] on November 20 (R[SUB]h[/SUB] = 0.43 AU). Short-term variability of H[SUB]2[/SUB]O production is also seen as observations on November 19 show an increase in H[SUB]2[/SUB]O production rate of nearly a factor of two over a period of about 6 h. C[SUB]2[/SUB]H[SUB]6[/SUB], CH[SUB]3[/SUB]OH and CH[SUB]4[/SUB] abundances in ISON are slightly depleted relative to H[SUB]2[/SUB]O when compared to mean values for comets measured at infrared wavelengths. On the November dates, C[SUB]2[/SUB]H[SUB]2[/SUB], HCN and OCS abundances relative to H[SUB]2[/SUB]O appear to be within the range of mean values, whereas H[SUB]2[/SUB]CO and NH[SUB]3[/SUB] were significantly enhanced. There is evidence that the abundances with respect to H[SUB]2[/SUB]O increased for some species but not others between October 28 (R[SUB]h[/SUB] = 1.07 AU) and November 19 (R[SUB]h[/SUB] = 0.46 AU). The high mixing ratios of H[SUB]2[/SUB]CO/CH[SUB]3[/SUB]OH and C[SUB]2[/SUB]H[SUB]2[/SUB]/C[SUB]2[/SUB]H[SUB]6[/SUB] on November 19, and changes in the mixing ratios of some species with respect to H[SUB]2[/SUB]O between October 28 to November 19, indicates compositional changes that may be the result of a transition from sampling radiation-processed outer layers in this dynamically new comet to sampling more pristine natal material as the outer processed layer was increasingly eroded and the thermal wave propagated into the nucleus as the comet approached perihelion for the first time. On November 19 and 20, the spatial distribution for dust appears asymmetric and enhanced in the antisolar direction, whereas spatial distributions for volatiles (excepting CN) appear symmetric with their peaks slightly offset in the sunward direction compared to the dust. Spatial distributions for H[SUB]2[/SUB]O, HCN, C[SUB]2[/SUB]H[SUB]6[/SUB], C[SUB]2[/SUB]H[SUB]2[/SUB], and H[SUB]2[/SUB]CO on November 19 show no definitive evidence for significant contributions from extended sources; however, broader spatial distributions for NH[SUB]3[/SUB] and OCS may be consistent with extended sources for these species. Abundances of HCN and C[SUB]2[/SUB]H[SUB]2[/SUB] on November 19 and 20 are insufficient to account for reported abundances of CN and C[SUB]2[/SUB] in ISON near this time. Differences in HCN and CN spatial distributions are also consistent with HCN as only a minor source of CN in ISON on November 19 as the spatial distribution of CN in the coma suggests a dominant distributed source that is correlated with dust and not volatile release. The spatial distributions for NH[SUB]3[/SUB] and NH[SUB]2[/SUB] are similar, suggesting that NH[SUB]3[/SUB] is the primary source of NH[SUB]2[/SUB] with no evidence of a significant dust source of NH[SUB]2[/SUB]; however, the higher production rates derived for NH[SUB]3[/SUB] compared to NH[SUB]2[/SUB] on November 19 and 20 remain unexplained. This suggests a more complete analysis that treats NH[SUB]2[/SUB] as a distributed source and accounts for its emission mechanism is needed for future work. [less ▲]

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See detailRecurrent pulsations in Saturn’s high latitude magnetosphere
Mitchell, D.G.; Carbary, J.F.; Bunce, E.J. et al

in Icarus (2016)

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See detailSPICAM observations and modeling of Mars aurorae
Soret, Lauriane ULiege; Gérard, Jean-Claude ULiege; Libert, Ludivine ULiege et al

in Icarus (2016), 264

Martian aurorae have been detected with the SPICAM instrument on board Mars Express both in the nadir and the limb viewing modes. In this study, we focus on three limb observations to determine both the ... [more ▼]

Martian aurorae have been detected with the SPICAM instrument on board Mars Express both in the nadir and the limb viewing modes. In this study, we focus on three limb observations to determine both the altitudes and the intensities of the auroral emissions. The CO (a3P–X1R) Cameron bands between 190 and 270 nm, the CO Fourth Positive system (CO 4P) between 135 and 170 nm, the CO2+ doublet at 289 nm, the OI at 297.2 nm and the 130.4 nm OI triplet emissions have been identified in the spectra and in the time variations of the signals. The intensities of these auroral emissions have been quantified and the altitude of the strongest emission of the CO Cameron bands has been estimated to be 137 ± 27 km. The locations of these auroral events have also been determined and correspond to the statistical boundary of open-closed magnetic field lines, in cusp-like structures. The observed altitudes of the auroral emissions are reproduced by a Monte-Carlo model of electron transport in the Martian thermosphere for mono-energetic electrons between 40 and 200 eV. No correlation between electron fluxes measured in the upper thermosphere and nadir auroral intensity has been found. Here, we simulate auroral emissions observed both at the limb and at the nadir using electron energy spectra simultaneously measured with the ASPERA-3/ELS instrument. The simulated altitudes are in very good agreement with the observations. We find that predicted vertically integrated intensities for the various auroral emissions are overestimated, probably as a consequence of the inclination and curvature of the magnetic field line threading the aurora. However, the relative brightness of the CO and CO2+ emissions is in good agreement with the observations. [less ▲]

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See detailSaturn’s auroral morphology and field-aligned currents during a solar wind compression
Badman, S.V.; Provan, G.; Bunce, E.J. et al

in Icarus (2016)

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See detailSimultaneous multi-scale and multi-instrument observations of Saturn's aurorae during the 2013 observing campaign
Melin, H.; Badman, S.; Stallard, T. et al

in Icarus (2016)

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See detailCharacteristics of north jovian aurora from STIS FUV spectral images
Gustin, Jacques ULiege; Grodent, Denis ULiege; Ray, Licia et al

in Icarus (2016)

We analyzed two observations obtained in Jan. 2013, consisting of spatial scans of the jovian north ultraviolet aurora with the HST Space Telescope Imaging Spectrograph (STIS) in the spectroscopic mode ... [more ▼]

We analyzed two observations obtained in Jan. 2013, consisting of spatial scans of the jovian north ultraviolet aurora with the HST Space Telescope Imaging Spectrograph (STIS) in the spectroscopic mode. The color ratio (CR) method, which relates the wavelength-dependent absorption of the FUV spectra to the mean energy of the precipitating electrons, allowed us to determine important characteristics of the entire auroral region. The results show that the spatial distribution of the precipitating electron energy is far from uniform. The morning main emission arc is associated with mean energies of around 265 keV, the afternoon main emission (kink region) has energies near 105 keV, while the ‘flare’ emissions poleward of the main oval are characterized by electrons in the 50–85 keV range. A small scale structure observed in the discontinuity region is related to electrons of 232 keV and the Ganymede footprint shows energies of 157 keV. Interestingly, each specific region shows very similar behavior for the two separate observations. The Io footprint shows a weak but undeniable hydrocarbon absorption, which is not consistent with altitudes of the Io emission profiles (∼900 km relative to the 1 bar level) determined from HST-ACS observations. An upward shift of the hydrocarbon homopause of at least 100 km is required to reconcile the high altitude of the emission and hydrocarbon absorption. The relationship between the energy fluxes and the electron energies has been compared to curves obtained from Knight’s theory of field-aligned currents. Assuming a fixed electron temperature of 2.5 keV, an electron source population density of ∼800 m−3 and ∼2400 m−3 is obtained for the morning main emission and kink regions, respectively. Magnetospheric electron densities are lowered for the morning main emission (∼600 m−3) if the relativistic version of Knight’s theory is applied. Lyman and Werner H2 emission profiles, resulting from secondary electrons produced by precipitation of heavy ions in the 1–2 MeV/u range, have been applied to our model. The low CR obtained from this emission suggests that heavy ions, presumably the main source of the X-ray aurora, do not significantly contribute to typical UV high latitude emission. [less ▲]

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