References of "Gérard, Jean-Claude"
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See detailJupiter's conjugate ultraviolet aurora
Gérard, Jean-Claude ULg; Grodent, Denis ULg; Radioti, Aikaterini ULg et al

Conference (2013, July)

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See detailThe time evolution of O2(a1Δ) individual observations acquired by VIRTIS-M on board Venus Express
Soret, Lauriane ULg; Gérard, Jean-Claude ULg; Piccioni, Giuseppe et al

Poster (2013, June 10)

The O2(a1Δ) nightglow emission at 1.27 µm may be used as a tracer of the Venus upper mesosphere dynamics. This emission has been observed with VIRTIS-M-IR on board Venus Express. Previous studies showed ... [more ▼]

The O2(a1Δ) nightglow emission at 1.27 µm may be used as a tracer of the Venus upper mesosphere dynamics. This emission has been observed with VIRTIS-M-IR on board Venus Express. Previous studies showed that the emission maximum is statistically located close to the antisolar point at ∼96 km. This airglow results from the production of oxygen atoms on the Venus dayside by photodissociation and electron impact dissociation of CO2 and CO, which are then transported to the nightside by the subsolar to antisolar general circulation, where they recombine to create metastable O2(a1Δ) molecules. Their radiative deexcitation produces the O2(a1Δ) nightglow with a maximum near the antisolar point. However, VIRTIS individual observations indicate that the O2(a1Δ) nightglow emission is highly variable, both in intensity and location. Individual observations acquired every hour during a short period of time can also be grouped sequentially. Bright emission patches can thus be tracked and both their displacement and intensity variations can be analyzed. The peak intensity can vary from 1 to 6 megaRayleighs. We show that the emission peak moves with a mean value of ~80 m s-1, in good agreement with an earlier study by Hueso et al. (2008). The velocity vector in intensity and direction is evaluated approximately every 40 min. These displacements are highly variable, but some dynamical characteristics can be deduced from the observations. These results will be compared with other results of velocity determination in the upper mesosphere. [less ▲]

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See detailDynamics of the auroral bifurcations at Saturn and their role in magnetopause reconnection
Gérard, Jean-Claude ULg; Radioti, Aikaterini ULg; Grodent, Denis ULg

Conference (2013, June)

We summarize recent results obtained with the UVIS instrument on board Cassini. They demonstrate that auroral signatures of magnetic field reconnection events in the flanks of the magnetopause are ... [more ▼]

We summarize recent results obtained with the UVIS instrument on board Cassini. They demonstrate that auroral signatures of magnetic field reconnection events in the flanks of the magnetopause are observed in the UV images collected by UVIS. [less ▲]

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See detailVenus night side measurements of winds at 115 km altitude from NO bright patches tracking.
Bertaux, J.-L.; Gérard, Jean-Claude ULg; Stiepen, Arnaud ULg et al

Conference (2013, June)

N and O atoms produced by photo-dissociation of CO2 and N2 on the day side of Venus are transported to the night side in the thermospheric circulation. When the air parcel is descending, the recombination ... [more ▼]

N and O atoms produced by photo-dissociation of CO2 and N2 on the day side of Venus are transported to the night side in the thermospheric circulation. When the air parcel is descending, the recombination N+O→ NO produces the famous γ and δ bands of NO emission. Pioneer Venus (1978) suggested that the statistical center of the emission is off from the anti-solar point, about one- two hours in Local time after midnight. This is confirmed from SPICAV/VEX results, and the explanation generally accepted is the influence of retrograde super rotation. However, the emission takes place at 115 km, while VIRTIS/VEX, with maps of O2 emission (peak altitude 95 km) in the night side of Venus (recombination of O+O coming from the day side), has shown that the maximum of emission is statistically centered on the antisolar point. Therefore, there is no influence of super-rotation at 95 km. One way to explain this paradox is that the cause of the super rotation is different at 115 km and in the lower atmosphere. Alternately, some gravity waves could propagate from below, crossing the altitude 95 km with minimal interaction, and breaking around 115, depositing their momentum. Another consideration is that the altitude of N2 photo-dissociation is higher in the thermosphere than CO2, therefore the thermospheric circulation pattern may be different for the transport of N atoms, and O atoms. We have started building maps of the NO emission by moving around the spacecraft along its orbit on the night side. The idea is that NO emission is concentrated generally in rather well defined patches of light. Therefore, by comparing maps taken at 1 hour or 24 hr interval, we can make a “bright patch tracking”, and derive directly the velocity of the moving air parcel containing N and O (we are aware that a part of the motion could be due to a phase shift of a gravity wave, if it has some influence on the NO emission). Preliminary results from this exercise with Venus Express will be presented and discussed. [less ▲]

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See detailTwenty years of Hubble observations of Jupiter's and Saturn's UV aurora
Gérard, Jean-Claude ULg

Scientific conference (2013, May 27)

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See detailThe aurora of giant planets: 20 years of observations with the Hubble Space Telescope
Gérard, Jean-Claude ULg

Scientific conference (2013, May)

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See detailStatistical analysis of Saturn's UV auroral outer emission
Grodent, Denis ULg; Radioti, Aikaterini ULg; Schippers, Patricia et al

Conference (2013, March 15)

Recent observations of Saturn’s aurora with the UVIS spectrograph on-board Cassini not only confirm the presence of a quasi-permanent partial ring of emission equatorward of the main auroral oval, but ... [more ▼]

Recent observations of Saturn’s aurora with the UVIS spectrograph on-board Cassini not only confirm the presence of a quasi-permanent partial ring of emission equatorward of the main auroral oval, but they also increase the number of positive cases and allow for a statistical analysis of the characteristics of this outer emission. This faint but distinct auroral feature appears at both hemispheres in the nightside sector. It magnetically maps to relatively large distances in the nightside magnetosphere, on the order of 9 RS. It was initially thought that pitch angle scattering of electrons into the loss cone by whistler waves would be responsible for the outer auroral emission. Rough estimates suggested that a suprathermal electron population observed with Cassini in the nightside sector between 7 and 10 RS might power this process. However, a new analysis of 7 years of Cassini electron plasma data indicates the presence of layers of upward and downward field aligned currents. They appear to be part of a large-scale current system involving dayside-nightside asymmetries as well as trans- hemispheric variations. This system comprises a net upward current layer, carried by warm electrons, limited to the nightside sector which may as well generate the outer UV auroral emission. The growing dataset of UVIS spectro- images is used to find any such asymmetries in the outer auroral emission. [less ▲]

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See detailOxygen nightglow emissions of Venus: Vertical distribution and collisional quenching
Gérard, Jean-Claude ULg; Soret, Lauriane ULg; Migliorini, Alessandra et al

in Icarus (2013)

We compare the altitude of three O2 night airglow emissions observed at the limb of Venus by the VIRTIS spectral imager with the values predicted by a model accounting for the different radiative ... [more ▼]

We compare the altitude of three O2 night airglow emissions observed at the limb of Venus by the VIRTIS spectral imager with the values predicted by a model accounting for the different radiative lifetimes and collisional deactivation of the upper O2 states. The O and CO2 density profiles are based on remote sensing observations from the Venus Express spacecraft. Effective production efficiencies of the involved O2 metastable states and quenching coefficients by oxygen and carbon dioxide are adjusted to provide the best match with the measured emission limb profiles. We find values in general good agreement with earlier studies for the c1Σ-u state which gives rise to the Herzberg II bands. In particular, we confirm the low net yield of the c state production and the importance of its deactivation by CO2, for which we derive a quenching coefficient of 3x10-16 cm-3 s-1. The ∼4.5 km higher altitude of the Chamberlain band emission also recently detected by VIRTIS and the ratio of the Herzberg II/Chamberlain bands observed with Venera are well reproduced. To reach agreement, we use a 12% yield for the A’3Δu production following O atom association and quenching coefficients by O and CO2 of 1.3x10-11 cm-3 s-1 and 4.5x10-13 cm-3 s-1 respectively. We conclude that the different peak altitudes of the IR Atmospheric, Herzberg II and the Chamberlain bands reflect the relative importance of radiative relaxation and collisional quenching by O and CO2. [less ▲]

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See detailThe characteristics of the O2 Herzberg II and Chamberlain bands observed with VIRTIS/Venus Express
Migliorini, A.; Piccioni, G.; Gérard, Jean-Claude ULg et al

in Icarus (2013)

The oxygen Venus nightglow emissions in the visible spectral range have been known since the early observations from the Venera spacecraft. Recent observations with the VIRTIS instrument on board Venus ... [more ▼]

The oxygen Venus nightglow emissions in the visible spectral range have been known since the early observations from the Venera spacecraft. Recent observations with the VIRTIS instrument on board Venus Express allowed us to re-examine the Herzberg II system of O2 and to further study its vertical distribution, in particular the (0–m00 with m00 = 7–13) bands. The present work describes the vertical profile of the observed bands and relative intensities from limb observation data. The wavelength-integrated intensities of the Herzberg II bands, with m00 = 7–11, are inferred from the recorded spectra. The resulting values lie in the range of 84–116 kR at the altitudes of maximum intensity, which are found to lie in the range of 93–98 km. Three bands of the Chamberlain system, centered at 560 nm, 605 nm, and 657 nm have been identified as well. Their emission peak is located at about 100 km, 4 km higher than the Herzberg II bands. For the first time, the O2 nightglow emissions were investigated simultaneously in the visible and in the IR spectral range, showing a good agreement between the peak position for the Herzberg II and the O2ða1Dg—X3R g Þ bands. An airglow model, proposed by Gérard et al. (Gérard, J.C., Soret, L., Migliorini, A., Piccioni, G. [2012]. Icarus.) starting from realistic O and CO2 vertical distributions derived from Venus-Express observations, allows reproduction of the observed profiles for the three O2 systems. [less ▲]

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See detailRemote sensing of the energy of auroral electrons in Saturn’s atmosphere: Hubble and Cassini spectral observations
Gérard, Jean-Claude ULg; Gustin, Jacques ULg; Pryor, Wayne et al

in Icarus (2013), 223

Saturn’s north ultraviolet aurora has been successfully observed twice between March and May 2011 with the STIS long-slit spectrograph on board the Hubble Space Telescope. Spatially resolved spectra at ... [more ▼]

Saturn’s north ultraviolet aurora has been successfully observed twice between March and May 2011 with the STIS long-slit spectrograph on board the Hubble Space Telescope. Spatially resolved spectra at ∼12 Å spectral resolution have been collected at different local times from dawn to dusk to determine the amount of hydrocarbon absorption. For this purpose, the HST telescope slewed across the auroral oval from mid-latitudes up to beyond the limb while collecting spectral data in the timetag mode. Spectral images of the north ultraviolet aurora were obtained within minutes and hours with the UVIS spectrograph on board Cassini. Several daytime sectors and one nightside location were observed and showed signatures of weak absorption by methane present in (or above) the layer of the auroral emission. No absorption from other hydrocarbons (e.g. C2H2) has been detected. For the absorbed spectra, the overlying slant CH4 column varies from 3x1015 to 2x1016 cm-2, but no clear dependence on local time is identified. A Monte Carlo electron transport model is used to calculate the vertical distribution of the H2 emission and to relate the observed spectra to the energy of the primary auroral electrons. Assuming electron precipitation with a Maxwellian energy distribution into a standard model atmosphere, we find that the mean energy ranges from less than 3 to ∼10 keV. These results are compared with previous determinations of the energy of Saturn’s aurora based on ultraviolet spectra and limb images. We conclude that the energies derived from spectral methods indicate a wide range of electron energies while the nightside limb images suggest that the auroral precipitation is consistently soft. We emphasize the need for more realistic model atmospheres with temperature and hydrocarbon distributions appropriate to high-latitude conditions. [less ▲]

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See detailAuroral signatures of multiple magnetopause reconnection at Saturn
Radioti, Aikaterini ULg; Grodent, Denis ULg; Gérard, Jean-Claude ULg et al

in Geophysical Research Letters (2013)

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See detailUnraveling electron acceleration mechanisms in Ganymede's space environment through N-S conjugate imagery of Jupiter's aurora
Grodent, Denis ULg; Bonfond, Bertrand ULg; Gérard, Jean-Claude ULg et al

E-print/Working paper (2013)

There is strong scientific interest in Ganymede (Jupiter's third Galilean moon) and its surrounding environment, which stems from the likely presence of a liquid water ocean underneath its icy crust and ... [more ▼]

There is strong scientific interest in Ganymede (Jupiter's third Galilean moon) and its surrounding environment, which stems from the likely presence of a liquid water ocean underneath its icy crust and from its internally driven magnetic field. The interaction of the latter with Jupiter's magnetospheric plasma and its magnetic field gives rise to a unique situation in our solar system implying a mini-magnetosphere embedded within a giant-magnetosphere. This interaction generates Ganymede's ultraviolet auroral footprint in Jupiter's atmosphere. We propose to investigate the strong auroral connection between Jupiter and Ganymede and the variable characteristics of Ganymede's magnetosphere with an innovative approach, taking advantage of the large scale north-south asymmetries of Jupiter's magnetic field. The results obtained for Ganymede will be compared with the case of small injected hot plasma bubbles observed by the Galileo spacecraft and whose size and location are similar to those of Ganymede's magnetosphere. HST is currently the sole instrument capable of obtaining this information which pins down the proposed mechanisms linking the source and sink regions of auroral particles in the giant planets' magnetospheres. [less ▲]

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See detailVenus nitric oxide nightglow mapping from SPICAV nadir observations.
Stiepen, Arnaud ULg; Gérard, Jean-Claude ULg; Dumont, Maïté ULg et al

in Icarus (2013)

Nitric oxide δ (190-240 nm) and γ (255-270 nm) emissions on the Venus nightside have been observed with Venus Express SPICAV instrument operated in the nadir mode. These ultraviolet emissions arise from ... [more ▼]

Nitric oxide δ (190-240 nm) and γ (255-270 nm) emissions on the Venus nightside have been observed with Venus Express SPICAV instrument operated in the nadir mode. These ultraviolet emissions arise from the desexcitation of excited NO molecules created by radiative recombination of O(3P) and N(4S) atoms. These atoms are produced on the dayside of the planet through photodissociation of CO2 and N2 molecules and are transported to the nightside by the global subsolar to antisolar circulation. We analyze a wide dataset of nadir observations obtained since 2006 to determine the statistical distribution of the NO nightglow and its variability. Individual observations show a great deal of variability and may exhibit multiple maxima along latitudinal cuts. We compare this global map with the results obtained during the Pioneer-Venus mission and with the recent O2(a1Δg) nightglow map. The NO airglow distribution shows a statistical bright region extending from 01:00 and 03:30 local time and 25°N to 10°S, very similar to the Pioneer result obtained 35 years earlier during maximum solar activity conditions. The shift from the antisolar point and the difference with the O2 airglow indicate that superrotating zonal winds are statistically weak near 97 km, but play an important role in the lower thermosphere. We compare these results with other evidence for superrotation in the thermosphere and point out possible sources of momentum transfer. [less ▲]

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