References of "Gérard, Jean-Claude"
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See detailCARAIB - A global model of terrestrial biological productivity
Warnant, Pierre ULg; François, Louis ULg; Strivay, David ULg et al

in Global Biogeochemical Cycles (1994), 8(3), 255-270

CARAIB, a mechanistic model of carbon assimilation in the biosphere estimates the net primary productivity (NPP) of the continental vegetation on a grid of 1 degrees x 1 degrees in latitude and longitude ... [more ▼]

CARAIB, a mechanistic model of carbon assimilation in the biosphere estimates the net primary productivity (NPP) of the continental vegetation on a grid of 1 degrees x 1 degrees in latitude and longitude. The model considers the annual and diurnal cycles. It is based on the coupling of the three following submodels; a leaf assimilation model including estimates of stomatal conductance and leaf respiration, a canopy model describing principally the radiative transfer through the foliage, and a wood respiration model. Present-day climate and vegetation characteristics allow the discrimination between ecotypes. In particular, specific information on vegetation distribution and properties is successfully used at four levels; the leaf physiological level, the plant level, the ecosystem level, and the global level. The productivity determined by the CARAIB model is compared with local measurements and empirical estimates showing a good agreement with a global value of 65 Gt C yr(-1). The sensitivity of the model to the diurnal cycle and to the abundance of C-4 species is also tested. The productivity slightly decreases (10%) when the diurnal cycle of the temperature is neglected. By contrast, neglecting the diurnal cycle of solar irradiance produces unrealistically high values of NPP. Even if the importance of this increase would presumably be reduced by the coupling of CARAIB with a nutrient cycle model, this test emphasizes the key role of the diurnal cycle in a mechanistic model of the NPP. Uncertainties on the abundance and spatial distribution of C-4 plants may cause errors in the NPP estimates, however, as demonstrated by two sensitivity tests, these errors are certainly lower than 10% at the global scale as shown by two tests. [less ▲]

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See detailSuperthermal particules in planetary atmospheres
Shematovich, V. I.; Bisikalo, D. V.; Gérard, Jean-Claude ULg

in Non-stable processes in the Univers (1994)

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See detailHIGH-RESOLUTION SPECTRA OF JUPITER NORTHERN AURORAL ULTRAVIOLET EMISSION WITH THE HUBBLE-SPACE-TELESCOPE
TRAFTON, L. M.; Gérard, Jean-Claude ULg; Munhoven, Guy ULg et al

in Astrophysical Journal (1994), 421(2), 816-827

The first spectroscopic observations of planetary aurora with the HST are reported. These include spectral regions centered on the H-2 Lyman and Werner bands of a region of Jupiter's northern aurora. The ... [more ▼]

The first spectroscopic observations of planetary aurora with the HST are reported. These include spectral regions centered on the H-2 Lyman and Werner bands of a region of Jupiter's northern aurora. The observations were made with the Goddard High Resolution Spectrograph (GHRS) using the Large Science Aperture as part of a campaign to study Jupiter at the time of the Ulysses flyby. The individual rotational-vibrational bands are resolved and the observed emissions are essentially all from H-2. A rotational-vibrational temperature for H-2 of 530 +/- 100 K is derived, a value significantly less than the 850-1100 K reported for Jovian H-3(+) in the near-infrared but consistent with the temperature reported for fundamental-band quadrupole H-2 emission. Comparison with the Faint Object Camera (FOC) images shows that the observed region was not one of the hot spots of the aurora. The results are interpreted in terms of electron impact excitation of H-2 from secondary particles generated by primaries precipitating into Jupiter's atmosphere from the magnetosphere. In the region of the aurora observed, the homopause level is found to be significantly hotter but not necessarily higher than observed at nonauroral latitudes. The equatorial H-2 dayglow spectrum was also detected; its intensity was 3.2 kR or 13% of the strength of the observed auroral emission. [less ▲]

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See detailMorphology and time variation of the Jovian Far UV aurora: Hubble Space Telescope observations
Gérard, Jean-Claude ULg; Dols, Vincent; Paresce, Francesco et al

in Journal of Geophysical Research (1993), 98

High spatial resolution images of the north polar region of Jupiter have been obtained with the Faint Object Camera (FOC) on board the Hubble Space Telescope (HST). The first set of two images collected ... [more ▼]

High spatial resolution images of the north polar region of Jupiter have been obtained with the Faint Object Camera (FOC) on board the Hubble Space Telescope (HST). The first set of two images collected 87 min apart in February 1992 shows a bright (approximately or equal to 180 kR) emission superimposed on the background in rotation with the planet. Both Ly alpha images show common regions of enhanced emission but differences are also observed, possibly due to temporal variations. The second group of images obtained on June 23 and 26, 1992 isolates a spectral region near 153 nm dominated by the H2 Lyman bands and continuum. Both pictures exhibit a narrow arc structure fitting the L = 30 magnetotail field line footprint in the morning sector and a broader diffuse aurora in the afternoon. They show no indication of an evening twilight enhancement. Although the central meridian longitudes were similar, significant differences are seen in the two exposures, especially in the region of diffuse emission, and interpreted as signatures of temporal variations. The total power radiated in the H2 bands is approximately or equal to 2 x 10[SUP]12[/SUP] W, in agreement with previous UV spectrometer observations. The high local H2 emission rates (approximately 450 kR) imply a particle precipitation carrying an energy flux of about 5 x 10[SUP]-2[/SUP] W/sq m. [less ▲]

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See detailEffect of hot N(4S) atoms on the NO solar cycle variation in the lower thermosphere
Gérard, Jean-Claude ULg; Shematovich, V. I.; Bisikalo, D. V.

in Journal of Geophysical Research (1993), 98

The variation of the nitric oxide peak density near 110 km with solar activity is calculated using a photochemical diffusive model of thermospheric odd nitrogen. This model includes the reaction of ... [more ▼]

The variation of the nitric oxide peak density near 110 km with solar activity is calculated using a photochemical diffusive model of thermospheric odd nitrogen. This model includes the reaction of translationally excited ('hot') nitrogen atoms with O2 as a source of nitric oxide, in addition to the classical photochemistry. It is confirmed that the dissociation of N2 by energetic photoelectrons due to the ionization of atmospheric constituents by solar soft X-rays is an important source of atomic nitrogen which controls the observed NO maximum near 110 km. The consideration of the hot N(4S) source increases the NO peak density by 45 to 60 percent dependent on the solar activity level considered. The calculated NO peak density increases by a factor of about 3.5 from low to high solar activity conditions, in agreement with the Solar Mesosphere Explorer satellite observations. The absolute concentrations calculated in the model with an N(2D) effective yield of 54 percent from N2 electron impact dissociation are midway between the two sets of solar cycle NO variation measurements currently available. [less ▲]

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See detailHigh-latitude ion transport and energetic explorer (HI-LITE): a mission to investigate ion outflow from the high-latitude ionosphere
Smith, Mark F; Herrero, Federico A; Hesse, Michael et al

in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series (1993, July 01)

The proposed HI-LITE Explorer will investigate the global ion outflow from the high-latitude ionosphere, its relationship to auroral features, and the consequences of this outflow on magnetospheric ... [more ▼]

The proposed HI-LITE Explorer will investigate the global ion outflow from the high-latitude ionosphere, its relationship to auroral features, and the consequences of this outflow on magnetospheric processes. The unique nature of the HI-LITE Explorer images will allow temporal and spatial features of the global ion outflow to be determined. The mission's scientific motivation comes from the fundamental role high-latitude ionospheric ions play in the dynamics of the solar wind driven magnetospheric-ionospheric system. These outflows are a major source of plasma for the magnetosphere and it is believed they play an important role in the triggering of substorms. In addition this paper describes the HI-LITE spacecraft and instruments. [less ▲]

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See detailModeling the Emergent Profiles and Intensities of Lyman Alpha and H[SUB]2[/SUB] Bands Induced by Protons Precipitation in the Jovian Atmosphere
Rego, D.; Prangé; Benjaffel, L. et al

in Bulletin of the American Astronomical Society (1993, June 01)

Not Available

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See detailA first look at the ASSI ultraviolet results
Chakrabarti, S.; Gladstone, G. R.; Tobiska, W. K. et al

in Advances in Space Research (1993), 13

The Airglow and Solar Spectrometer Instrument (ASSI) on the San Marco D satellite has obtained near-simultaneous measurements of solar irradiances and airglow emissions in the 200-7000 A spectral region ... [more ▼]

The Airglow and Solar Spectrometer Instrument (ASSI) on the San Marco D satellite has obtained near-simultaneous measurements of solar irradiances and airglow emissions in the 200-7000 A spectral region. The satellite was placed in an equatorial, elliptical orbit on 25 March 1988, which permitted observations of airglow emissions in the 280-600-km altitude range at various local times. The instrument complement on the satellite provides an opportunity both for self-consistent examination of the excitation mechanisms of various airglow features and for constraining model parameters. An overview of the data obtained by ASSI is presented along with preliminary modeling results of the UV airglow. [less ▲]

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See detailCoupled modelling of the global chemical-climatic changes due to human activities
Gérard, Jean-Claude ULg; François, Louis ULg; Delire, C. et al

in Proceedings of the global change symposium (1993)

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See detailThe thermospheric odd nitrogen photochemistry - Role of non thermal N(4S) atoms
Shematovich, V. I.; Bisikalo, D. V.; Gérard, Jean-Claude ULg

in Annales Geophysicae [= ANGEO] (1992), 10

A nonequilibrium model to calculate the Maxwellian 'hot' N(4S) atom population using a Monte Carlo method is developed, and the NO vertical distribution for minimum solar cycle conditions is calculated ... [more ▼]

A nonequilibrium model to calculate the Maxwellian 'hot' N(4S) atom population using a Monte Carlo method is developed, and the NO vertical distribution for minimum solar cycle conditions is calculated. It is shown that the energetic photoelectrons produced by solar soft X-rays produce translationally 'hot' N(4S) atoms near 110 km which contribute significantly to the formation of the E-region NO peak observed at this altitude. Consideration of this new source of nitric oxide reconciles the need for an effective N(2D) quantum yield from N2 dissociation exceeding 50 percent derived from odd nitrogen models with the limits put on this value by laboratory and theoretical constraints. The altitude of the modeled NO peak is found to critically depend on the adopted N(2D) yield, and good agreement with the observed peak altitude and density is obtained using the quantum yield derived from theoretical considerations. [less ▲]

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See detailUltraviolet imaging of the Jovian aurora with the Hubble Space Telescope
Dols, V.; Gérard, Jean-Claude ULg; Paresce, F. et al

in Geophysical Research Letters (1992), 19

We present here for the first time a Lyman-alpha image of the north polar region of Jupiter obtained with the Faint Object Camera on board the Hubble Space Telescope a few hours after the encounter of the ... [more ▼]

We present here for the first time a Lyman-alpha image of the north polar region of Jupiter obtained with the Faint Object Camera on board the Hubble Space Telescope a few hours after the encounter of the ULYSSES spacecraft with Jupiter. The presence of high latitude regions of enhanced emission is clearly observed. A comparison with the location of the 'UVS oval', the Io (L = 6) and high-latitude field-line footprints shows that the best agreement is obtained with the L not less than 15 footprint and the UVS oval which are close to each other for the particular longitudinal sector observed. These two L-shells correspond to two possible sources of precipitation: particles originating respectively from the region of the plasma torus of Io in a distorted magnetic field or particles from the distant magnetosphere by analogy with the terrestrial aurora. [less ▲]

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See detailSolar system observations with HST
Paresce, Francesco; Gérard, Jean-Claude ULg

in Environment Observation and Climate Modelling Through International Space Projects (1992)

Imaging and spectrographic measurements of solar system objects with the Hubble Space Telescope (HST) are reviewed from a scientific and technical standpoint. Special emphasis is placed on observations of ... [more ▼]

Imaging and spectrographic measurements of solar system objects with the Hubble Space Telescope (HST) are reviewed from a scientific and technical standpoint. Special emphasis is placed on observations of Jupiter and Io during the Ulysses encounter by the Faint Object Camera and HRS. The bottom line is that, although the mirror aberration limits HST's extragalactic performance, the potential for a deep systematic probe of planetary conditions over a wide range of observing conditions is still practically intact and will get better as the bugs are ironed out and the COSTAR (Corrective Optics Space Telescope Axial Replacement) fix is implemented. [less ▲]

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See detailObservations of the H and H[SUB]2[/SUB] Ultraviolet Jovian Aurora with the HST Faint Object Camera
Gérard, Jean-Claude ULg; Dols, V.; Paresce, F. et al

in Bulletin of the American Astronomical Society (1992, June 01)

Not Available

Detailed reference viewed: 4 (2 ULg)
See detailA Model of Lyman ë± and H[SUB]2[/SUB] Bands Excitation by Protons Precipitation in the Jovian Atmosphere
Rego, D.; Prangé; Gérard, Jean-Claude ULg

in Bulletin of the American Astronomical Society (1992, June 01)

Not Available

Detailed reference viewed: 4 (2 ULg)
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See detailThe climate induced variation of the continental biosphere: A model simulation of the last glacial maximum
Friedlingstein, P.; Delire, C.; Müler, J. F. et al

in Geophysical Research Letters (1992), 19

A simplified three-dimensional global climate model was used to simulate the surface temperature and precipitation distributions for the Last Glacial Maximum (LGM), 18000 years ago. These fields were ... [more ▼]

A simplified three-dimensional global climate model was used to simulate the surface temperature and precipitation distributions for the Last Glacial Maximum (LGM), 18000 years ago. These fields were applied to a bioclimatic scheme which parameterizes the distribution of eight vegetation types as a function of biotemperature and annual precipitation. The model predicts a decrease, for LGM compared to present, in forested area balanced by an increase in desert and tundra extent, in agreement with a reconstruction of the distribution of vegetation based on paleodata. However, the estimated biospheric carbon content (phytomass and soil carbon) at LGM is less reduced than in the reconstructed one. Possible reasons for this discrepancy are discussed. [less ▲]

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See detailThermospheric odd nitrogen
Gérard, Jean-Claude ULg

in Planetary and Space Science (1992), 40

The photochemistry of NO, N(2D) and N(4S) and results of recent space measurements of their density distribution are discussed. In particular, the role of the reaction between metastable N(2D) atoms and ... [more ▼]

The photochemistry of NO, N(2D) and N(4S) and results of recent space measurements of their density distribution are discussed. In particular, the role of the reaction between metastable N(2D) atoms and O2 as a source of O(1D) is discussed in the light of laboratory and aeronomical observations. Global satellite measurements are compared with results of 2D and 3D models including transport. The possibility of explaining the odd nitrogen observations gathered in the Venusian and Martian thermospheres with the current understanding of the terrestrial models adapted to CO2-rich atmospheres is examined. It is concluded that understanding of the processes governing the distribution of odd nitrogen in terrestrial planets is generally satisfactory, although several aspects require further quantitative investigation. [less ▲]

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See detailHigh Resolution Near UV Observations of Jupiter's Satellite Io with HST
Paresce, F.; Sartoretti, P.; Dols, V. et al

in Benvenuti, Piero; Schreier, Ethan J. (Eds.) Science with the Hubble Space Telescope, (1992)

Detailed reference viewed: 4 (0 ULg)
See detailObservations and physical properties of small solar system bodies
Gérard, Jean-Claude ULg; Brahic, André; Surdej, Jean ULg

Book published by Université de Liège (1992)

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See detailA sensitivity study of the role of continental area and location on Paleozoic climate
Hauglustaine, Didier A.; Gérard, Jean-Claude ULg

in Global and Planetary Change (1992), 97

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See detailThe faint young sun climatic paradox: A simulation with an interactive seasonal climate-sea ice model
Gérard, Jean-Claude ULg; Hauglustaine, Didier A.; François, Louis ULg

in Global and Planetary Change (1992), 97(3), 133-150

A seasonal one-and-a-half-dimensional (1 1/2-D) energy-balance climate model including a detailed sea ice calculation and an interactive albedo formulation has been developed and is used to investigate ... [more ▼]

A seasonal one-and-a-half-dimensional (1 1/2-D) energy-balance climate model including a detailed sea ice calculation and an interactive albedo formulation has been developed and is used to investigate the faint young sun climatic paradox. This model is shown to reproduce the present climate and sea ice observations. In spite of its greater complexity, its behavior is globally similar to simple energy-balance models with highly parameterized ice-albedo feedback used in previous studies of this question. It is found that when the solar luminosity drops by more than about 5% below its present value, the ice albedo feedback causes a global irreversible glaciation. Several sensitivity experiments show that the value of the critical solar constant and associated global surface temperature are only little sensitive to the set of model parameters describing the ice and snow albedo and meridional heat transport. In contrast, the absence or polar location of the continental mass introduce a nearly 10% decrease of the critical luminosity. The minimum level of atmospheric CO2 needed to prevent a global glaciation through enhanced greenhouse warming is calculated as a function of the solar luminosity. A 30% drop in solar output requires a 2 x 10(4)-fold increase in atmospheric CO2, an unacceptably large value. However, in the absence of continents, a carbon dioxide partial pressure of 2000 times the present level is found to be sufficient to stabilize the climate. The effects of a reduced continental area, paleogeographic changes and higher CO2 greenhouse effect combine to ensure a larger stability of the non-frozen configuration. Their cumulated and interactive effects may be able to solve the young sun paradox. [less ▲]

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