References of "Gérard, Jean-Claude"
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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 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 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 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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See detailThe seasonality of the CO2 exchange between the atmosphere and the land biosphere: A study with a global mechanistic vegetation model
Nemry, B.; François, Louis ULg; Warnant, Pierre ULg et al

in Journal of Geophysical Research (1996), 101(D3), 7111-7125

Two simulations of the seasonal variation of the global atmospheric CO2 distribution are obtained by combining an atmospheric transport model, two parameterizations of soil heterotrophic respiration (SHR ... [more ▼]

Two simulations of the seasonal variation of the global atmospheric CO2 distribution are obtained by combining an atmospheric transport model, two parameterizations of soil heterotrophic respiration (SHR), and a mechanistic model of carbon assimilation in the biosphere (CARAIB) that estimates the net primary production (NPP) of continental vegetation. The steady state hypothesis of the biosphere allows the spatial distribution and the global content of the soil carbon to be expressed as a function of the root fractions of soil respiration under forested and herbaceous vegetation covers. The sensitivity of the modeled CO2 signal to the wind field does not exceed the observed interannual variability. The influence of the various vegetation zones is quantified by the Fourier analysis of the modeled atmospheric signal. In the northern hemisphere, the temperate ecosystems dominate the seasonal atmospheric signal of the extratropical latitudes. The ecosystems of the tropical northern zone determine the local signal, while the southern tropical ecosystems influence largely the signal in the whole southern hemisphere. The results give credence to the mechanistic modeling of NPP since the simulated atmospheric signal is comparable with that obtained with normalized difference vegetation index (NDVI) based diagnostic models coupled with a parameterization of SHR fitted to optimize the atmospheric signal. [less ▲]

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See detailHigh rotational excitation of NO infrared thermospheric airglow: A signature of superthermal nitrogen atoms?
Hubert, Benoît ULg; Gérard, Jean-Claude ULg; Shematovich, Valeri I et al

in Geophysical Research Letters (1996), 23

The reaction between superthermal N([SUP]4[/SUP]S) atoms produced by exothermic processes and O[SUB]2[/SUB] has been proposed to explain observations of highly rotationally excited nitric oxide in the ... [more ▼]

The reaction between superthermal N([SUP]4[/SUP]S) atoms produced by exothermic processes and O[SUB]2[/SUB] has been proposed to explain observations of highly rotationally excited nitric oxide in the sunlit thermosphere. We examine the importance of this mechanism using a detailed calculation of the fast N([SUP]4[/SUP]S) atoms energy distribution. It is shown that the hot thermal N atoms are able to produce rotationally excited NO in the upper thermosphere through the reaction of O[SUB]2[/SUB] with N([SUP]4[/SUP]S). By contrast, near the NO peak at 110 km, the Maxwellian nitrogen atoms produce substantially less rovibrationally excited NO than the superthermal component. Consequently, the non Maxwellian N([SUP]4[/SUP]S) atoms show a clear spectral signature in the (1-0) and (2-1) bandheads at this altitude. The calculated rovibrationally excited NO concentration at 140 km is shown to be consistent with the value derived from the analysis of infrared airglow spectra. [less ▲]

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See detailCO2 atmosphérique et simulation de ses variations d'origine naturelle et anthropique
Gérard, Jean-Claude ULg; François, Louis ULg

in Revue de la Science et des Techniques (1996), 14

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See detailSeasonal and interannual influences of the terrestrial ecosystems on atmospheric CO2: a model study
François, Louis ULg; Nemry, Bernard; Warnant, Pierre ULg et al

in Physics & Chemistry of the Earth - Parts A/B/C (1996), 21

The prognostic CARAIB (Carbon Assimilation In the Biosphere) model has been used in conjunction with the Max-Planck Institut TM2 atmospheric transport model to calculate the atmospheric CO2 fluctuations ... [more ▼]

The prognostic CARAIB (Carbon Assimilation In the Biosphere) model has been used in conjunction with the Max-Planck Institut TM2 atmospheric transport model to calculate the atmospheric CO2 fluctuations at the global scale. Two applications are briefly described. In the first one, the seasonal CO2 variation is calculated and a Fourier analysis is performed to determine the relative contributions of the various vegetation types. It is found that the seasonal signal is dominated by the grasslands and needle leaf forests in the northern boreal and temperate zones. In the southern hemisphere, tropical deciduous forests and grasslands make the primary contribution. In the second application, the net primary productivity (NPP), soil heterotrophic respiration (SHR) and net ecosystem productivity (NEP) are calculated for years 1987 and 1988 with the model driven by observed climatic variables. Preliminary results indicate that the NEP variations between these two years are strongly dominated by tropical ecosystems. However, it is shown that the results are strongly dependent on the dataset used for the 1987-88 temperature record, raising the question of reliability of sudl modelling studies of the interannual variability of the biosphere. 01997 Elsevier Science Ltd [less ▲]

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See detailA new distribution of vegetation types and its inclusion in a global biosphere model
Warnand; François, Louis ULg; Nemry, B. et al

Poster (1995, August)

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