References of "Gérard, Jean-Claude"
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See detailSaturn's polar auroral emissions
Radioti, Aikaterini ULg; Gérard, Jean-Claude ULg; Grodent, Denis ULg et al

Conference (2010)

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See detailUVIS observations of the FUV OI and CO 4P Venus dayglow during the Cassini flyby
Hubert, Benoît ULg; Gérard, Jean-Claude ULg; Gustin, Jacques ULg et al

in Icarus (2010), 207

We analyze FUV spatially-resolved dayglow spectra obtained at 0.37 nm resolution by the UVIS instrument during the Cassini flyby of Venus. We use a least-squares fit method to determine the brightness of ... [more ▼]

We analyze FUV spatially-resolved dayglow spectra obtained at 0.37 nm resolution by the UVIS instrument during the Cassini flyby of Venus. We use a least-squares fit method to determine the brightness of the OI emissions at 130.4 and OI 135.6 nm, and of the bands of the CO fourth positive system which are dominated by fluorescence scattering. We compare the brightness observed along the UVIS foot track of the two OI multiplets with that deduced from a model of the excitation of these emissions by photoelectron impact on O atoms and resonance scattering of the solar 130.4 nm emission. The large optical thickness 130.4 nm emission is accounted for using a radiative transfer model. The airglow intensities are calculated along the foot track and found to agree with the observed 130.4 nm brightness within ˜10%. The modeled OI 135.6 nm brightness is also well reproduced by the model. The oxygen density profile of the VTS3 model is found to be consistent with the observations. We find that self-absorption of the (0, v″) bands of the fourth positive emission of CO is important and we derive a CO vertical column of about 6.4 × 10[SUP]15[/SUP] cm[SUP]‑2[/SUP] in close agreement with the value provided by the VTS3 empirical atmospheric model. [less ▲]

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See detailCharacteristics of Saturn’s FUV airglow from limb-viewing spectra obtained with Cassini-UVIS
Gustin, Jacques ULg; Stewart, Ian; Gérard, Jean-Claude ULg et al

in Icarus: International Journal of Solar System Studies (2010), 210

This study reports the analysis of far ultraviolet (FUV) limb spectra of the airglow of Saturn in the 1150–1850 Å spectral window, obtained with the Ultraviolet Imaging Spectrograph (UVIS) onboard Cassini ... [more ▼]

This study reports the analysis of far ultraviolet (FUV) limb spectra of the airglow of Saturn in the 1150–1850 Å spectral window, obtained with the Ultraviolet Imaging Spectrograph (UVIS) onboard Cassini, spanning altitudes from 1200 to 4000 km. The FUV limb emission consists of three main contributions: (1) H Ly-a peaking at 1100 km with a brightness of 0.8 kilo-Rayleighs (kR), (2) reflected sunlight longward of 1550 Å which maximizes at 950 km with 16.5 kR and (3) H2 bands in the 1150–1650 Å bandwidth, peaking at 1050 km reaching a maximum of 3.9 kR. A vertical profile of the local H2 volume emission rate has been derived using the hydrocarbon density profiles from a model of the Saturn equatorial atmosphere. It is well matched by a Chapman function, characterized by a maximum value of 3.5 photons cm-3 s-1 in the 800–1650 Å UV bandwidth, peaking at 1020 km. Comparisons between the observed spectra and a first-order synthetic airglow H2 model in the 1150–1650 Å bandwidth show that the spectral shape of the H2 bands is accounted for by solar fluorescence and photoelectron excitation. The best fits are obtained with a combination of H2 fluorescence lines and 20 eV electron impact spectra, the latter contributing 68% of the total H2 airglow emission. [less ▲]

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See detailVariation of Saturn's UV aurora with SKR phase
Nichols, J. D.; Cecconi, B.; Clarke, J. T. et al

in Geophysical Research Letters (2010), 37

It is well known that a wide range of kronian magnetospheric phenomena, including the Saturn kilometric radiation (SKR), exhibit oscillations near the planetary rotation period. However, although the SKR ... [more ▼]

It is well known that a wide range of kronian magnetospheric phenomena, including the Saturn kilometric radiation (SKR), exhibit oscillations near the planetary rotation period. However, although the SKR is believed to be generated by unstable auroral electrons, no connection has been established to date between diurnal SKR modulations and UV auroral power. We use an empirical SKR phase determined from Cassini observations to order the 'quiet time' total emitted UV auroral power as observed by the Hubble Space Telescope in programs during the interval 2005-2009. Our results indicate that both the northern and southern UV powers are dependent on SKR phase, varying diurnally by factors of similar to 3. We also show that the UV variation originates principally from the morning half of the oval, consistent with previous observations of the SKR sources. Citation: Nichols, J. D., B. Cecconi, J. T. Clarke, S. W. H. Cowley, J.-C. Gerard, A. Grocott, D. Grodent, L. Lamy, and P. Zarka (2010), Variation of Saturn's UV aurora with SKR phase, Geophys. Res. Lett., 37, L15102, doi: 10.1029/2010GL044057. [less ▲]

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See detailLocation and spatial shape of electron beams in Io's wake
Jacobsen, S.; Saur, J.; Neubauer, F. M. et al

in Journal of Geophysical Research. Solid Earth (2010), 115

The Galileo spacecraft observed energetic field-aligned electron beams very close to Io during several flybys. We apply a three-dimensional magnetohydrodynamic (MHD) model of the far-field Io-Jupiter ... [more ▼]

The Galileo spacecraft observed energetic field-aligned electron beams very close to Io during several flybys. We apply a three-dimensional magnetohydrodynamic (MHD) model of the far-field Io-Jupiter interaction to simulate for the first time the location and spatial shape of field-aligned electron beams. Io continuously generates MHD waves by disturbing the Jovian magnetoplasm. Currents carried by Alfven waves propagate predominantly along the magnetic field lines. As the number of charge carriers decreases along the travel path, electrons are accelerated toward Jupiter. These energetic electrons precipitate into the Jovian ionosphere, visible as prominent Io footprint emission. Electrons are also accelerated toward Io and form the equatorial beams observed by the Galileo spacecraft. Unlike the beam formation, the position and spatial structure of these beams have not been addressed in detail before. We use a 3-D MHD model with initial conditions corresponding to the individual Galileo flyby and determine the spatial morphology of the beams in Io's orbital plane. Our results for the beam locations are in good agreement with the Galileo Energetic Particle Detector observations. We find that the ratio of the one-way travel time of the Alfven wave from Io to Jupiter and the convection time of the plasma past the obstacle controls the location of the beam. This leads to the conclusion that at other satellites with other plasma environments, the electrons might not be close to the satellite but can be shifted significantly downstream along its plasma wake. Thus, the future search for electron beams near a satellite should be further extended to the wake region. [less ▲]

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See detailThe Spatial Morphology of Equatorial Electron Beams Near Io
Jacobsen, S.; Saur, J. S.; Neubauer, F. M. et al

Conference (2009, December 14)

The Galileo spacecraft observed energetic field-aligned electron beams very close to Io during several flybys. We apply a three-dimensional magnetohydrodynamic (MHD) model of the far-field Io-Jupiter ... [more ▼]

The Galileo spacecraft observed energetic field-aligned electron beams very close to Io during several flybys. We apply a three-dimensional magnetohydrodynamic (MHD) model of the far-field Io-Jupiter interaction to simulate for the first time the location and spatial shape of field-aligned electron beams. Io continuously generates MHD waves by disturbing the Jovian magnetoplasma. Currents carried by Alfvén waves propagate predominantly along the magnetic field lines. As the number of charge carriers decreases along the travelpath, electrons are accelerated towards Jupiter. These energetic electrons precipitate into the Jovian ionosphere, visible as prominent Io footprint (IFP) emission. Electrons are also accelerated towards Io and form the equatorial beams observed by the Galileo spacecraft. Unlike the beam formation, the position and spatial structure of these beams has not been addressed in detail before. We use a 3D MHD model with initial conditions corresponding to the individual Galileo flyby and determine the spatial morphology of the beams in Io's orbital plane. Our results are in good agreement with the Galileo observations. We find that the ratio of the one-way traveltime of the Alfvén wave from Io to Jupiter and the convection time of the plasma past the obstacle controls the location of the beams. This leads to the conclusion that at other satellites with other plasma environments, e.g. Ganymede, Callisto, Europa and Enceladus, the electron beams might not be close to the satellite, but can be shifted significantly downstream along its plasma wake. Thus, the future search for field-aligned electron populations near a satellite should be further extended to the wake region. [less ▲]

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See detailBelgian-Italian connections: decoding signals from planets
Gérard, Jean-Claude ULg

Conference given outside the academic context (2009)

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See detailSaturn Auroral Movies from Cassini UVIS
Pryor, W. R.; Stewart, I.; Esposito, L. W. et al

in American Geophysical Union, Fall Meeting 2009 (2009, December 01)

Cassini's Ultraviolet Imaging Spectrograph (UVIS) continues to obtain Saturn auroral data. Two long slit spectral channels are used to obtain EUV data from 56.3-118.2 nm and FUV data from 111.5-191.3 nm ... [more ▼]

Cassini's Ultraviolet Imaging Spectrograph (UVIS) continues to obtain Saturn auroral data. Two long slit spectral channels are used to obtain EUV data from 56.3-118.2 nm and FUV data from 111.5-191.3 nm. 64 spatial pixels along each slit are combined with slit motion to construct spectral images of Saturn. Auroral emissions are seen from electron-excited molecular and atomic hydrogen. In 2007-2009 UVIS obtained data with the spacecraft well out of Saturn's ring plane, permitting UVIS to obtain a number of short movies of the rotating auroral structures. Selected movies will be presented with geometric overlays and in polar projections. In some movies a cusp-like feature is present near noon inside the oval. One movie from 2008 day 201 shows parallel linear features on the day side almost at right angles to the main auroral oval that appear, then lengthen, separate in the middle, and then fade away. Other movies show similar cusp-related structures that resemble the letter "Q" where a dynamical feature at right angles to the oval moves away from the cusp region. The 2008 day 201 movie also shows one bright "polar flare" inside the oval with a spectrally distinct signature indicating the presence of higher energy electrons. A few of the most recent images were obtained at sufficiently close range that 2 spacecraft slews were needed to completely cover the oval. These images provide almost 100 pixels of information across the oval and clearly show multiple arcs of emission on the main oval and scattered emissions inside the oval. Several frames show emissions associated with the footprint of the Enceladus field line. We will discuss these features, their locations, and possible interpretations. [less ▲]

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See detailRecurrent energization of plasma in the midnight-to-dawn quadrant of Saturn's magnetosphere, and its relationship to auroral UV and radio emissions
Mitchell, D. G.; Krimigis, S. M.; Paranicas, C. et al

in Planetary and Space Science (2009), 57

We demonstrate that under some magnetospheric conditions protons and oxygen ions are accelerated once per Saturn magnetosphere rotation, at a preferred local time between midnight and dawn. Although ... [more ▼]

We demonstrate that under some magnetospheric conditions protons and oxygen ions are accelerated once per Saturn magnetosphere rotation, at a preferred local time between midnight and dawn. Although enhancements in energetic neutral atom (ENA) emission may in general occur at any local time and at any time in a Saturn rotation, those enhancements that exhibit a recurrence at a period very close to Saturn's rotation period usually recur in the same magnetospheric location. We suggest that these events result from current sheet acceleration in the 15-20 Rs range, probably associated with reconnection and plasmoid formation in Saturn's magnetotail. Simultaneous auroral observations by the Hubble Space Telescope (HST) and the Cassini Ultraviolet Imaging Spectrometer (UVIS) suggest a close correlation between these dynamical magnetospheric events and dawn-side transient auroral brightenings. Likewise, many of the recurrent ENA enhancements coincide closely with bursts of Saturn kilometric radiation, again pointing to possible linkage with high latitude auroral processes. We argue that the rotating azimuthal asymmetry of the ring current pressure revealed in the ENA images creates an associated rotating field aligned current system linking to the ionosphere and driving the correlated auroral processes. [less ▲]

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See detailThe Mars ultraviolet dayglow variability: SPICAM observations and model comparison
Gérard, Jean-Claude ULg; Cox, Cédric ULg; Bougher, S. W. et al

Conference (2009, September 16)

Limb profiles of the CO Cameron and CO2+ doublet airglow have been observed over different conditions (latitude, season, SZA, F10.7). They have been individually modelled using currently accepted cross ... [more ▼]

Limb profiles of the CO Cameron and CO2+ doublet airglow have been observed over different conditions (latitude, season, SZA, F10.7). They have been individually modelled using currently accepted cross sections, and outputs from the MTGCM. They have been shown to co-vary, with a ICO/ICO2+ ratio of about 4.7, less than the modelled ratio.The peak brightness varies linearly with the F10.7 solar flux proxy, in a way compatible with the Mariner 6 and 7 observations. The intensity of both emissions is overestimated by the model (but large uncertainties exist in excitation cross sections). An increase of the altitude of both emissions has been observed during the 2005 summer season. It is a consequence of the dust load, followed by an increased thermospheric CO2 density observed with SPICAV during the same period. This density enhancement is partly predicted by GCM models. [less ▲]

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See detailThe very busy auroral footprint of Ganymede
Grodent, Denis ULg; Bonfond, Bertrand ULg; Radioti, Aikaterini ULg et al

Conference (2009, September 13)

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See detailConcurrent observations of the ultraviolet nitric oxide and infrared O[SUB]2[/SUB] nightglow emissions with Venus Express
Gérard, Jean-Claude ULg; Cox, Cédric ULg; Soret, Lauriane ULg et al

in Journal of Geophysical Research. Planets (2009), 114

Two prominent features of the Venus nightside airglow are the nitric oxide delta and gamma bands produced by radiative association of O and N atoms in the lower thermosphere and the O[SUB]2[/SUB] infrared ... [more ▼]

Two prominent features of the Venus nightside airglow are the nitric oxide delta and gamma bands produced by radiative association of O and N atoms in the lower thermosphere and the O[SUB]2[/SUB] infrared emission generated by three-body recombination of oxygen atoms in the upper mesosphere. The O[SUB]2[/SUB] airglow has been observed from the ground, during the Cassini flyby, and with VIRTIS on board Venus Express. It now appears that the global structure of the two emissions shows some similarities, but the statistical location of the region of strongest emission is not coincident. The Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus (SPICAV) ultraviolet spectrograph has collected a large number of spectra of the Venus nitric oxide nightside airglow. Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) images have been obtained at the limb and in the nadir-viewing mode and have provided new information on the horizontal and vertical distribution of the emission. We present the first concurrent observations of the two emissions observed with Venus Express. We show that nadir observations generally indicate a low degree of correlation between the two emissions observed quasi-simultaneously at a common location. A statistical study of limb profiles indicates that the altitude and the brightness of the two airglow layers generally do not covary. We suggest that this lack of correlation is explained by the presence of strong horizontal winds in the mesosphere-thermosphere transition region. They carry the downflowing atoms over large distances in such a way that regions of enhanced NO emission generally do not coincide with zones of bright O[SUB]2[/SUB] airglow. [less ▲]

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See detailAn Overview Of Spicav/soir Results On The Atmosphere Of Venus From Venus Express Mission
Bertaux, Jean-Loup; Vandaele, A.; Korablev, O. et al

in AAS/Division for Planetary Sciences Meeting Abstracts (2009, September 01)

SPICAV/SOIR is a suite of three spectrometers in the UV and IR range flying on ESA Venus Express orbiter, dedicated to the study of the atmosphere of Venus : UV (110-320 nm), Vis-Nir (0.65-1.65 µm), and ... [more ▼]

SPICAV/SOIR is a suite of three spectrometers in the UV and IR range flying on ESA Venus Express orbiter, dedicated to the study of the atmosphere of Venus : UV (110-320 nm), Vis-Nir (0.65-1.65 µm), and mid IR (2.3-4.4 µm). The UV spectrometer discovered a high altitude layer of SO2 ( 85-105 km), apparently correlated with the density of haze particles. Ozone is detected for the first time in the atmosphere of Venus. The night side γ and δ bands of NO intensities are maximal at 2 am (influence of super-rotation), while the O2 emission mapped simultaneously by Virtis peaking at 95 km altitude ( 10 km below NO emission) is centered at midnight, a puzzle for general circulation models. The hot hydrogen component of the exosphere, extending at more than 30,000 km, is variable. The SPICAV VIS-IR sensor (0.7-1.7 μm, resolution 0.5-1.2 nm) employs a pioneering technology: acousto-optical tunable filter (AOTF). Day side observations indicate a variable latitude distribution of cloud top altitude (decreasing toward the pole) and water vapor mixing ratio. The SOIR spectrometer is a new Solar Occultation IR spectrometer in the range λ=2.2-4.3 µm, with a spectral resolution λ/Πλ>20,000, the highest ever flown in a planetary mission. This new concept includes a combination of an echelle grating and an AOTF crystal to sort out one order at a time. Vertical profiles of CO, HDO, H2O, HCl, SO[SUB]2[/SUB], CO[SUB]2[/SUB] isotopes and temperature are regularly retrieved, as well as aerosols. The CO mixing ratio (80-130 km), a tracer of atmospheric exchange from thermosphere to lower atmosphere, is 10 times less than the VIRA model prescription, showing also some time variability. [less ▲]

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See detailJupiter's ultraviolet polar auroral emissions
Coumans, Valérie ULg; Bonfond, Bertrand ULg; Grodent, Denis ULg et al

Conference (2009, September)

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See detailLatitudinal - local time distribution of the O2 and OH infrared nightglows and O density in the Venus lower thermosphere
Soret, Lauriane ULg; Gérard, Jean-Claude ULg; Saglam, Adem et al

Conference (2009, September)

Atomic oxygen has been measured in situ only above 145 km on both the day and the night sides of Venus. Limb observations obtained with the Venus Infrared Thermal Imaging Spectrometer (VIRTIS) on board ... [more ▼]

Atomic oxygen has been measured in situ only above 145 km on both the day and the night sides of Venus. Limb observations obtained with the Venus Infrared Thermal Imaging Spectrometer (VIRTIS) on board Venus Express show that the O2 infrared nightglow peaks at ~97 km [1, 2], with a mean intensity value of about 1 MR. Yet, the distribution is largely inhomogeneous, with an enhanced region of ~3 MR statistically located near the midnight meridian at low latitude [3]. The oxygen density can be mapped using the O2 airglow and CO2 density vertical distributions [4]. The O2 vol-ume emission rates are obtained with an Abel inversion of the O2 limb profiles using CO2 vertical distributions taken from the Venus International Reference Atmosphere (VIRA) model. The results show that the O density peak varies in altitude with a mean value of 105 km. It ranges from 1.0x1010 to 14.5x1011 cm-3, with a mean value of 2.2x1011 cm-3. The zonally averaged peak altitude appears to be constant while its amplitude decreases with latitude. Another approach uses the O2 volume emission rates obtained with an Abel inversion of the O2 limb profiles. In-deed, it is then possible to vertically integrate these profiles to simulate nadir observations. The resulting map gives values between 0 and 2.8 MR (with a mean value of 0.6 MR) in the north hemisphere. A statistical map created with actual nadir observations shows intensities ranging from 0 to 2.1 MR, with a mean of 0.5 MR in the south hemisphere. A combination of the two types of observations could cover Venus entire nightside. Statistical mapping of the OH Meinel emission has also been performed using limb profiles. A strong correlation with the O2 emission is revealed. The average altitude of the emission peak is ~95.3 km for the OH(1-0) band and the average intensity is 0.4 MR [5]. [less ▲]

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See detailRecurrent Energization of Plasma in the Midnight-to-Dawn Quadrant of Saturn's Magnetosphere, and its Relationship to Auroral UV and Radio Emissions
Mitchell, D.; Krimigis, S.; Paranicas, C. et al

Poster (2009, August 11)

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See detailAurora at Earth, Jupiter and Saturn: a comparative view
Gérard, Jean-Claude ULg

Conference (2009, August)

The interaction of the solar wind with the magnetospheres of the giant planets is different from that with the terrestrial magnetosphere in several respects bearing consequences on the morphology and ... [more ▼]

The interaction of the solar wind with the magnetospheres of the giant planets is different from that with the terrestrial magnetosphere in several respects bearing consequences on the morphology and dynamics of the aurora: - the magnetospheres of the outer planets are dominated by the planetary rotation that can provide much of the energy for the processes acting within these magnetospheres. - the circulation of plasma may be driven by mass loading of the giant magnetospheres by moons and rings rather than by reconnection with the IMF. - the magnitude and orientation of the interplanetary magnetic field plays a key role in controlling the Earth’s auroral dynamics whereas the solar wind dynamic pressure appears as a key factor in triggering auroral intensification on Saturn. - The combination of imaging and spectral techniques indicates the characteristic energy of the auroral electrons is comparable on Earth and Saturn, but higher energies are associated with the Jovian aurora. As a consequence, Jupiter’s aurora appears relatively shielded from solar wind influences, except inside the auroral oval where transient bright emission has been observed following sudden compressions of the magnetosphere. The size and brightness of the main oval shows only a week dependence on solar wind conditions. By contrast, Saturn’s aurora is quite responsive to solar wind perturbations, as was observed during campaigns of concurrent observations between HST and Cassini. Auroral intensification events that are reminiscent of shock-induced Earth aurora appear to be triggered by sudden increases of the solar dynamic pressure more than by changes in the IMF orientation. However, simultaneous HST-Cassini observations indicate that localised auroral brightenings and ENA acceleration events may be observed under quiet solar wind conditions. The occurrence of substorms is a key characteristics of the Earth’s auroral dynamics. Localized auroral enhancements have also been observed on Jupiter and Saturn. They have been associated to reconnection events causing events similar to substorms. [less ▲]

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