References of "Gérard, Jean-Claude"
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See detailJovian Auroral Lyalpha Self-Reversals: A Window on Jupiter's Auroral Electrojet?
Gladstone, G. R.; Waite, J H, Jr; Gérard, Jean-Claude ULg

in Bulletin of the American Astronomical Society (1998, September 01)

High-resolution GHRS profiles of Lyalpha lines emitted from Jupiter's auroral regions were presented by Prang{e} et al. (Astrophys. J., 484, L169--L173, 1997). Their data show asymmetric self-reversed ... [more ▼]

High-resolution GHRS profiles of Lyalpha lines emitted from Jupiter's auroral regions were presented by Prang{e} et al. (Astrophys. J., 484, L169--L173, 1997). Their data show asymmetric self-reversed line profiles, with the blue or red peak brighter depending on the target location in Jupiter's northern auroral region. The measured asymmetries are equivalent to Doppler velocities towards and away from the observer of several km/s. As suggested by Sommeria et al. (Icarus, 119, 2--24, 1995), electrojet velocities of ~ 10--20 km/s may be present at Jupiter. Here we investigate the possibility that the observed wavelength shifts of the auroral Lyalpha line are a result of multiple scattering by H atoms carried along in Jupiter's auroral electrojet. If this explanation is found to be viable, then HST/STIS mapping of the velocity shifts in the Lyalpha line may represent (as with ground-based high-resolution observations of jovian auroral H_3(+) emission lines) a means for determining the dynamics of Jupiter's upper atmosphere and ionosphere. [less ▲]

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See detailHST-STIS Observations of Jupiter's Aurora
Clarke, J. T.; Ajello, J.; Ballester, G. E. et al

in Bulletin of the American Astronomical Society (1998, September 01)

HST observations of the UV emissions from Jupiter's aurora have been obtained with the new Space Telescope Imaging Spectrograph (STIS) since July 1997. UV images of the aurora are now possible with an ... [more ▼]

HST observations of the UV emissions from Jupiter's aurora have been obtained with the new Space Telescope Imaging Spectrograph (STIS) since July 1997. UV images of the aurora are now possible with an order of magnitude higher sensitivity than possible with earlier cameras, and improved angular resolution from shorter exposures. Images have been obtained at 4 epochs since Sept. 1997, and reveal several new features of the auroral emission pattern. These include i) faint auroral emissions extending roughly 60 deg. in the wake or plasma flow direction beyond Io's magnetic footprint, ii) systematic motions of the main auroral oval both toward the pole and toward the equator at different local times and longitudes, and iii) clear identification of an auroral emission feature with Ganymede's magnetic footprint, from the relative motion of the feature remaining underneath Ganymede in a time series of images. Preliminary interpretations of these features will be presented. Spectra have also been obtained of auroral emission features including the northern and southern main ovals, the diffuse emission poleward of the main oval, and the northern and southern Io footprints. These spectra will be presented with estimates of the overlying hydrocarbon columns, the energy of the exciting electrons, the rotational temperature of the emitting hydrogen, and the overlying column of atomic hydrogen. This work was supported by NASA under grants GO-6743.01-95A and GO-7308.01-96A to the University of Michigan. [less ▲]

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See detailFUV spectroscopy of the H_2 emission in the Jovian aurora: model update and results
Gérard, Jean-Claude ULg; Dols, V.; Colignon, David ULg et al

in Bulletin of the American Astronomical Society (1998, September 01)

The Jovian aurora exhibits bright emissions mainly due the bright Lyman-alpha line and to radiation from excited singlet states to the ground electronic state extending from 800 to 1600 Angstroms. Above ... [more ▼]

The Jovian aurora exhibits bright emissions mainly due the bright Lyman-alpha line and to radiation from excited singlet states to the ground electronic state extending from 800 to 1600 Angstroms. Above 1200 Angstroms, the molecular spectrum is dominated by the Lyman (B-X) bands and continuum and the Werner (C-X) bands. These transitions have been observed with the Goddard High Resolution Spectrometer (GHRS) and the Space Telescope Imaging Spectrometer (STIS) at various spectral and spatial resolutions. To analyze these spectra, a model synthetic auroral spectrum has been constructed and applied to the analysis of the ro-vibrational temperature of H_2 and a search for non-H_2 emission features. It has been recently updated to include the latest singlet state excitation and cascading cross sections, self-absorption in optically thick lines and to account for the energy distribution of the secondary electrons which cause additional excitation. We illustrate these effects in a few study cases and apply the model to high resolution (0.5 Angstroms) GHRS spectra and low resolution STIS spectra of the entire H_2 spectrum at wavelengths longer than Lyman-alpha. We determine the methane column overlying the auroral emission peak and find that a better fit is obtained with additional absorption by acetylene. [less ▲]

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See detailObservation of short and long timescale variability of the jovian UV aurora
Grodent, Denis ULg; Gérard, Jean-Claude ULg; Dols, V. et al

in Bulletin of the American Astronomical Society (1998, September 01)

Three sets of Hubble Space Telescope far-ultraviolet images of Jupiter's North pole aurora have been obtained with the Wide Field and Planetary Camera 2 in June 1996, May 1997 and August 1997. The ... [more ▼]

Three sets of Hubble Space Telescope far-ultraviolet images of Jupiter's North pole aurora have been obtained with the Wide Field and Planetary Camera 2 in June 1996, May 1997 and August 1997. The exposure time was limited to 160 seconds in order to reveal short timescale auroral features that are normally averaged during longer exposures and blurred by the rapid jovian rotation. The 3 sets of images show the aurora in a comparable (quiet) activity level, exhibiting long term persistent features such as : (i) a stable thin morning arc, (ii) a morning-afternoon emission dichotomy, (iii) a minimum of low latitude emission around CML=175 deg, (iv) bright localized afternoon structures. The equatorward boundary of the arc closely follows but is not coincident with the footprint of the 20 RJ magnetic field line given by the VIP4 model. These stable structures contrast with rapidly changing features like small size spots blasting in one single image, bright regions connecting trans-auroral structures, and the inner diffuse emission. The question of temporal variability and spatial extent of the auroral features is of major importance in understanding the origin and acceleration mechanisms of the auroral particles exciting the jovian UV aurora. In particular, short timescale processes can be related to field aligned currents generating acceleration structures and discrete aurora. [less ▲]

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See detailHST spectra of the Jovian ultraviolet aurora: Search for heavy ion precipitation
Trafton, L. M.; Dols, V.; Gérard, Jean-Claude ULg et al

in Astrophysical Journal (1998), 507(2), 955-967

Ultraviolet spectra using Hubble Space Telescope sampled between 1250 and 1680 Angstrom, at spectral resolution less than or equal to 0.57 Angstrom are reported for characteristically bright regions of ... [more ▼]

Ultraviolet spectra using Hubble Space Telescope sampled between 1250 and 1680 Angstrom, at spectral resolution less than or equal to 0.57 Angstrom are reported for characteristically bright regions of Jupiter's morning and afternoon northern aurora. Several observed spectra exhibit sharply enhanced resolution. We interpret this as bright auroral emission foreshortened on the morning limb with a maximum intensity at least as high as 2000 kR. We have searched for evidence that the primary precipitating particles exciting the aurora include the heavy ions known to exist in Jupiter's plasma torus and magnetosphere. We have also searched for such ambient heavy ions and neutrals at rest in the auroral ionosphere, the end products of previous precipitation, excited by the auroral cascade. We argue that primary emission would be characterized by a dramatically Doppler-broadened (similar to 10-15 Angstrom) and redshifted line profile resulting from the cascade process and the angle between the line of sight and the magnetic field lines in the atmosphere. In contrast, ambient emission would be distinguished by narrow emission lines. We have modeled the theoretical sulfur and oxygen line shapes for ion precipitation and conclude that electron precipitation is responsible for most of the H-2 emissions. O ions contributed <13% of the precipitating energy flux, and S ions contributed < 50%. This dominance suggests that field-aligned magnetospheric currents are more important than energetization of energetic ions and subsequent scattering by plasma waves as a mechanism for generating the Jovian aurora. We set an upper limit over our spectra of 35-43 R to the emission from ambient oxygen and sulfur ions and their neutrals, except that for the S II 1256 triplet, the upper limit for the nominally brightest line, at 1260 Angstrom, is 74 R. Hence, we find no evidence for the accumulation of sulfur in the auroral ionosphere. A single narrow emission line from an unidentified ambient specie near 1254 Angstrom may be detected at the 4 sigma level, introducing the possibility of complex auroral aeronomy. Differences were observed in the auroral spectral hydrocarbon absorption at different locations, which cannot be interpreted without ambiguity between auroral and atmospheric structural causes. We have found that the brighter emission in an auroral sector consistently shows more spectral hydrocarbon absorption than the dimmer emission. We suggest two alternative physical explanations for this phenomenon. [less ▲]

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See detailThe longitudinal variation of the color ratio of the Jovian ultraviolet aurora: a geometric effect?
Gérard, Jean-Claude ULg; Grodent, Denis ULg; Dols, V. et al

in Geophysical Research Letters (1998), 25(10), 1601-1604

A three-dimensional model is used to assess the role of the viewing geometry on the auroral color ratio. The simulations show that both an auroral are with a geometry deduced from images obtained with the ... [more ▼]

A three-dimensional model is used to assess the role of the viewing geometry on the auroral color ratio. The simulations show that both an auroral are with a geometry deduced from images obtained with the Hubble Space Telescope (HST) and a uniform polar cap emission produce no modulation or a minimum absorption when the longitude of the Jovian central meridian (CML) is close to 200 degrees. This result is in contrast with the statistical measurements made with the International Ultraviolet Explorer (IUE) spectrograph that the hydrocarbon optical depth above the auroral emission maximizes for CMLs about 180 degrees. In the frame of this simplified model, we examine a possible way to reconcile the model with the IUE data. An intrinsic longitudinal dependence of the column of methane above the level of the auroral emission is introduced in the simulation. It may result from a combination of a vigorous upwelling in sectors of strong acid stable precipitation and/or a longitudinal dependence of the characteristic energy of the auroral particles. [less ▲]

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See detailHST Observations of Jupiter's Aurora Simultaneous with GALILEO Measurements
Clarke, J. T.; Ballester, G.; Trauger, J. et al

in Bulletin of the American Astronomical Society (1997, July 01)

An HST observing program is underway to obtain images and spectra of Jupiter's ultraviolet aurora during key events in the GALILEO orbiter mission, including remote observations of the nightside aurora ... [more ▼]

An HST observing program is underway to obtain images and spectra of Jupiter's ultraviolet aurora during key events in the GALILEO orbiter mission, including remote observations of the nightside aurora and measurements of the magnetic field and charged particle environments. We have obtained WFPC 2 images and GHRS spectra of Jupiter's aurora overlapping with GALILEO measurements during GALILEO orbits G1 (June 1996), G2 (Sept. 1996), G7 (April 1997), and G8 (May 1997), and at the time of writing we are scheduling STIS spectra for summer 1997. The WFPC 2 images appear similar to earlier reported auroral images, with the main oval at the same location observed over the last 2 years, rapidly variable emission poleward of the main oval, and the Io footprint with a similar longitudinal offset from the local magnetic field. Spectra were obtained of auroral emission features including the northern and southern main ovals, the diffuse emission poleward of the main oval, and the northern and southern Io footprints. These spectra will be presented with estimates of the overlying hydrocarbon columns, the energy of the exciting electrons, the rotational temperature of the emitting hydrogen, and the overlying column of atomic hydrogen. This work was supported by NASA under contract JPL 959122 and grants GO-5828.01-94A and GO-6743.01-95A to the University of Michigan. [less ▲]

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See detailObservations of short time scale variability of the Jovian UV aurora and simulation of morphological patterns
Gérard, Jean-Claude ULg; Grodent, Denis ULg; Dols, V. et al

in Bulletin of the American Astronomical Society (1997, July 01)

A database of far ultraviolet auroral images collected with the Faint Object Camera and Wide Field Planetary Camera (WFPC2) on board the Hubble Space Telescope has been constructed over the last five ... [more ▼]

A database of far ultraviolet auroral images collected with the Faint Object Camera and Wide Field Planetary Camera (WFPC2) on board the Hubble Space Telescope has been constructed over the last five years. Some morphological features are recurrent but significant time variations are also observed. A set of WFPC2 images obtained in May 1997 shows that, within a relatively stable general morphology, variations occur inside the polar cap in 4 minutes or less, implying short timescale acceleration processes. A model simulating Earth view of auroral arcs and diffuse emissions in the north polar region has been developed. Simple geometric cases are described to illustrate the dependence on the altitude, atmospheric scale height and central meridian planetary longitude of an idealized auroral morphology seen from Earth orbit. The numerical simulation makes it possible to assess the importance of limb brightening and the contribution from high altitude auroral emission located behind the planetary limb. As an application, four images obtained with WFPC2 are used to determine the characteristics of their auroral (discrete and diffuse) structures. The apparent brightness distribution along the arcs may only be reproduced if intrinsic longitudinal (or local time) variations are introduced, in addition to the path length effects of the viewing geometry. [less ▲]

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See detailComment on ``A model for meteoric magnesium in the ionosphere'' by W. J. McNeil, S. T. Lai, and E. Murad
Fesen, C. G.; Gérard, Jean-Claude ULg; Hays, P. B. et al

in Journal of Geophysical Research (1997), 102

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See detailAn updated model of the hot nitrogen atom kinetics and thermospheric nitric oxide
Gérard, Jean-Claude ULg; Bisikalo, D. V.; Shematovich, V. I. et al

in Journal of Geophysical Research (1997), 102

New observations and reanalysis of previous measurements suggest an upward revision of the measured thermospheric nitric oxide density. Our previous model of NO production by fast N([SUP]4[/SUP]S) atom ... [more ▼]

New observations and reanalysis of previous measurements suggest an upward revision of the measured thermospheric nitric oxide density. Our previous model of NO production by fast N([SUP]4[/SUP]S) atom collisions with O[SUB]2[/SUB] has been updated. It includes the effect of soft solar X rays, Auger electrons, and a detailed treatment of N[SUB]2[/SUB] dissociative ionization channels. In addition, new calculations indicate that the transfer of translational energy in N+N[SUB]2[/SUB] collisions is less efficient than in the hard sphere approximation. This result leads to reevaluation of the energy dependent relaxation cross section and to an upward revision of the reacting efficiency of collisions of N with O[SUB]2[/SUB] to form nitric oxide. The calculated peak NO density increases by a factor of ~2 when the effect of superthermal nitrogen atoms is included. The model response of the N([SUP]4[/SUP]S) energy distribution function and NO density to solar cycle variations is presented. The NO density at 110 km changes from 5.4×10[SUP]7[/SUP] to 1.3×10[SUP]8[/SUP]cm[SUP]-3[/SUP] when the solar F[SUB]10.7[/SUB] index varies from 70 to 245, but its response depends on the magnitude of the soft X ray increase with solar activity. [less ▲]

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See detailNonequilibrium processes in the planetary and cometary atmospheres : theory and applications
Marov, M.Ya.; Shematovich, V. I.; Bisikalo, D. V. et al

Book published by Kluwer Academic Publ (1997)

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See detailSimulation of the morphology of the Jovian UV north aurora observed with the Hubble Space Telescope
Grodent, Denis ULg; Gladstone, G. R.; Gérard, Jean-Claude ULg et al

in Icarus: International Journal of Solar System Studies (1997), 128(2), 306-321

A model simulating Earth views of UV auroral arcs and diffuse emissions in the Jovian north polar region is described. It assumes a Chapman profile of vertical brightness distribution and various ... [more ▼]

A model simulating Earth views of UV auroral arcs and diffuse emissions in the Jovian north polar region is described. It assumes a Chapman profile of vertical brightness distribution and various horizontal structures, Simple geometric cases are described to illustrate the dependence on the altitude, atmospheric scale height, and central meridian planetary longitude (CML) of an idealized amoral morphology seen from Earth orbit, The numerical simulation makes it possible to assess the importance of limb brightening and the contribution from high altitude auroral emission located behind the planetary limb. As an application of the simulation model, four images obtained with the Wide Field and Planetary Camera 2 (WFPC2) on board the Hubble Space Telescope are used to determine the characteristics of their auroral (discrete and diffuse) structures, The apparent brightness distribution along the arcs can only be reproduced if intrinsic longitudinal (or local time) variations are introduced, in addition to the path length effects of the viewing geometry, A composite average auroral distribution is built by mapping 10 WFPC2 images from the same dataset, It illustrates the dichotomy frequently observed between a narrow single structure are at System III longitudes larger than 180 degrees (or morning sector) and the multiple are and broad diffuse emission at longitudes less than 180 degrees (or afternoon sector), It is shown that the equatorial auroral emission boundary is located between the 6R(J) and the 30R(J) magnetic field line footprints of the Goddard Space Flight Center (GSFC) O-6 model. (C) 1997 Academic Press. [less ▲]

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See detailA global model of the biosphere : validation and applications to present and past climatic conditions
François, Louis ULg; Gérard, Jean-Claude ULg; Nemry, Bernard et al

in Sciences Géologiques. Bulletin (1997), 50(1-4), 89-107

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See detailChanges in the Chemical composition of the atmosphere
Adams, F.; Colin, R. G.; De muer, D. et al

Report (1997)

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See detailThe distribution of hot hydrogen atoms produced by electron and proton precipitation in the Jovian aurora
Bisikalo, D. V.; Shematovich, V. I.; Gérard, Jean-Claude ULg et al

in Journal of Geophysical Research (1996), 101

The energy distribution functions of nonthermal thermospheric hydrogen atoms are calculated for electron and proton precipitation in the Jovian aurora. A numerical model taking into account the production ... [more ▼]

The energy distribution functions of nonthermal thermospheric hydrogen atoms are calculated for electron and proton precipitation in the Jovian aurora. A numerical model taking into account the production, elastic and inelastic relaxation and transport processes for hot H atoms is developed. This model is based on a Monte Carlo solution of the nonlinear Boltzmann equation for hot H atoms produced by electron and proton impact on H and H[SUB]2[/SUB] and exothermic chemical reactions. The distribution functions show a much higher energy tail for proton than electron precipitation. It is shown that the steady state flux of hot atoms (E>=2 eV) is essentially isotropic. The peak and column hot H densities are about 3×10[SUP]5[/SUP] cm[SUP]-3[/SUP] and 1×10[SUP]14[/SUP] cm[SUP]-3[/SUP] for a 100 erg cm[SUP]-2[/SUP]s[SUP]-1[/SUP] precipitation combining hard (22 keV) and soft (0.2 keV) electrons mixed with a 10 erg cm[SUP]-2[/SUP]s[SUP]-1[/SUP] flux of soft (0.3 keV) protons. These column densities, coupled with the wide range of hot H atom energies, may play an important role in the formation of the Lyman alpha line profile. Multiple scattering in the wings of the Ly alpha line by the fast H atoms is shown to partly account for the broad Ly alpha profile observed in the Jovian aurora with the Hubble space telescope. [less ▲]

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See detailCARAIB: A GLOBAL MECHANISTIC VEGETATION MODEL TO STUDY THE CO2 EXCHANGE BETWEEN THE ATMOSPHERE AND THE CONTINENTAL BIOSPHERE
Gérard, Jean-Claude ULg; François, Louis ULg; Nemry, Beranrd et al

Conference (1996, September)

The TM2 atmospheric transport model, a parameterization of soil heterotrophic respiration and a mechanistic model of carbon assimilation in the biosphere (CARAIB) are combined to estimate the net primary ... [more ▼]

The TM2 atmospheric transport model, a parameterization of soil heterotrophic respiration and a mechanistic model of carbon assimilation in the biosphere (CARAIB) are combined to estimate the net primary productivity of the continental vegetation. The model includes the determination of the leaf area index (LAI) of the various vegetation covers, the CO2 assimilation by the leaves, and the respiration of standing biomass. It also includes a soil hydrological model forced with monthly mean air temperature, precipitation and solar radiation fields and using a stochastic generation of daily weather within each month. It is shown that the seasonal fluctuations observed at various monitoring stations can be well reproduced, thus validating the mechanistic approach adopted to calculate the global net primary productivity. A Fourier analysis of the modelled atmospheric signal is performed to quantify the relative importance of the vegetation in the different latitude zones and the contribution of the various vegetation types. In the southern hemisphere, the calculated phase and amplitude of the CO2 signal are not so well reproduced since atmosphere-ocean exchanges should also be considered in addition to the biospheric source. The latest updates and simulations performed with the model will be presented and discussed. [less ▲]

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See detailCharacterization and Numerical Simulation of the Jovian UV Aurora Observed with the HST Cameras
Grodent, Denis ULg; Dols, V.; Gérard, Jean-Claude ULg et al

in Science with the Hubble Space Telescope - II (1996)

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See detailThe equatorial boundary of the ultraviolet Jovian north aurora observed with multispectral Hubble Space Telescope images
Grodent, Denis ULg; Dols, V.; Gérard, Jean-Claude ULg et al

in Journal of Geophysical Research (1996), 101(E1), 2163-2168

Multispectral observations of the far ultraviolet Jovian aurora with the faint object camera (FOC) on board the Hubble Space Telescope are reported. They are used to describe and compare the morphology ... [more ▼]

Multispectral observations of the far ultraviolet Jovian aurora with the faint object camera (FOC) on board the Hubble Space Telescope are reported. They are used to describe and compare the morphology and the mean brightness of the H-2 Lyman and Werner bands observed at 153, 125, and 130 nm. It is shown that most of the emissions are confined inside the 6 R(J) O-6-GSFC auroral oval and fill a large fraction of the polar cap. During the similar to 50 hours time span of the observations following a very strong aurora, no bright are was observed along the oval mapping the 30 R(J) O-6 oval. Hydrogen emissions at 125 and 130 nm are observed down to 50 degrees N and somewhat below, a region not accessible to previous FOC observations at longer wavelengths. Temporal variations are also observed on timescales of hours to days. [less ▲]

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