References of "Gérard, Jean-Claude"
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See detailModelling short-term CO2 fluxes and long-term tree growth in temperate forests with ASPECTS
Rasse, Daniel P.; François, Louis ULg; Aubinet, Marc ULg et al

in Ecological Modelling (2001), 141(1-3), 35-52

The net ecosystem exchange (NEE) Of CO2 between temperate forests and the atmosphere governs both carbon removal from the atmosphere and forest growth. In recent years, many experiments have been ... [more ▼]

The net ecosystem exchange (NEE) Of CO2 between temperate forests and the atmosphere governs both carbon removal from the atmosphere and forest growth. In recent years, many experiments have been conducted to determine temperate forest NEE. These data have been used by forest modellers to better understand the processes that govern CO, fluxes, and estimate the evolution of these fluxes under changing environmental conditions. Nevertheless, it is not clear whether models capable of handling short-term processes, which are mostly source-driven, can provide an accurate estimate of long-term forest growth, which is potentially more influenced by sink- and phenology-related processes. To analyse the interactions between short- and long-term processes, we developed the ASPECTS model, which predicts long-term forest growth by integrating, over time, hourly NEE estimates. Validation data consisting of measurements of NEE by eddy-covariance and forest carbon reservoir estimates were obtained from mixed deciduous and evergreen experimental forests located in Belgium. ASPECTS accurately estimated both: (1) the NEE fluxes for several years of data; and (2) the amount of carbon contained in stems, branches, leaves, fine and coarse roots. Our simulations demonstrated that: (1) NEE measurements in Belgian forests are compatible with forest growth over the course of the 20th century, and (2) that forest history and long-term processes need to be considered for accurate simulation of short-term CO2 fluxes. [less ▲]

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See detailA self-consistent model of the Jovian auroral thermal structure
Grodent, Denis ULg; Waite, J. Hunter; Gérard, Jean-Claude ULg

in Journal of Geophysical Research (2001), 106(A7), 12933-12952

A one-dimensional (1-D) model coupling a two-stream electron transport model of energy deposition with a 1-D thermal conduction model has been developed. It is applied to investigate the links between ... [more ▼]

A one-dimensional (1-D) model coupling a two-stream electron transport model of energy deposition with a 1-D thermal conduction model has been developed. It is applied to investigate the links between auroral heat input and the vertical temperature of Jupiter's upper atmosphere. Two energy distributions meant to reproduce the emissions of a diffuse and a discrete aurora are used to evaluate the importance of the energy spectrum of the incident electrons for the thermal balance of Jupiter's auroral thermosphere. The values of observable quantities such as the altitude of the H-2 emission peak, thermal infrared (LR), ultraviolet (UV) emissions, and temperatures associated with various optical signatures are used to constrain the parameters of these distributions. It is shown that the high-energy component of these energy distributions heats a region of the homosphere between 10(-4) and 10(-6) bar and mainly controls the H-2 temperature and the far-UV (FUV) emission. A 3-keV soft electron component is necessary to heat the region directly above the homopause, between 10(-6) and 10(-9) bar. It has a large influence on the H-2 and H-3(+) temperatures and on the H-3(+) near-IR(NIR) emission. It is used in conjunction with a weak 100 eV component which is responsible for heating the thermosphere, from 10(-9) to 10(-12) bar and exerts a control on the exospheric temperature. The calculated temperatures, UV, and IR emissions suggest that the model probably misses a nonparticle heat source in the 10(-5) bar region, that is expected to balance the strong hydrocarbon cooling. Sensitivity tests are performed to evaluate the importance of the parameters of the energy distributions. They show that the FUV color ratio increases with the characteristic energy (or high-energy cutoff) of the high-energy component, while the H-2 rovibrational temperature varies inversely. A trade-off is therefore necessary for these two parameters to simultaneously meet their observational constraints. Further tests demonstrate the essential thermostatic role played by H-3(+), which regulates the net heating in the thermosphere. An increased eddy diffusion reproduces the effect of a possible auroral upwelling of methane but gives rise to an H-2 temperature smaller than the observed value. [less ▲]

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See detailThe Proton and Electron Aurora as Seen by Image-FUV and FAST
Gérard, Jean-Claude ULg; Hubert, Benoît ULg; Habraken, Serge ULg et al

Conference (2000, December)

The Far Ultraviolet (FUV) instrument on IMAGE images the aurora in three different wavelength regions. The Wideband Imaging Camera (WIC) observes the molecular Lyman-Birge-Hopfield (LBH) and atomic ... [more ▼]

The Far Ultraviolet (FUV) instrument on IMAGE images the aurora in three different wavelength regions. The Wideband Imaging Camera (WIC) observes the molecular Lyman-Birge-Hopfield (LBH) and atomic Nitrogen emissions at 140-190 nm. The two channels of the Spectrographic Imager (SI) observe the Doppler shifted Lyman Alpha emission of precipitating protons at 121.8 nm (SI 12) and the mostly electron produced emission from oxygen OI at 135.6 nm (SI13). The major advantage of FUV compared to previously flown UV imagers is the simultaneous operation of all three imaging systems without the need for filter changes and the uncertainty of temporal changes of the aurora between exposures. The FAST satellite passes every two hours through FUV images during apogee operations. This enables a comparison between the remotely imaged particle precipitation and the in-situ measurements along the FAST track. The detailed analysis of images from all three systems together with a full simulation of auroral emissions based on in-situ measurements by FAST confirms the laboratory calibrations of FUV. The spatial resolution and image quality of WIC permits the observation of a good correspondence between in-situ and remote measurements of precipitation boundaries. The clear separation between the lower latitude proton precipitation and the more structured higher latitude electron precipitation is obtained with both SI-channels. [less ▲]

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See detailStellar calibration of the WIC and SI imagers and the GEO photometers on IMAGE/FUV
Gladstone, G. R.; Mende, S. B.; Frey, H. U. et al

Poster (2000, December)

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See detailA dawn auroral storm on Jupiter: measurement of complex hydocarbons
Clarke, J. T.; Gladstone, R.; Pryor, W. et al

Conference (2000, December)

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See detailHST/STIS Observations of a Dawn Auroral Storm on Jupiter
Clarke, J. T.; Gladstone, R.; Pryor, W. et al

in Bulletin of the American Astronomical Society (2000, October 01)

The HST/STIS recorded a detailed time series of images and spectra of the UV emissions from a dawn auroral storm on Jupiter on 21 Sept. 1999. The images show complex and evolving fine structure in the ... [more ▼]

The HST/STIS recorded a detailed time series of images and spectra of the UV emissions from a dawn auroral storm on Jupiter on 21 Sept. 1999. The images show complex and evolving fine structure in the storm, while the emission center remained along the main oval and near dawn in magnetic local time. We serendipitously recorded low resolution UV spectra of the north-south spatial distribution of the auroral emissions. These spectra show far stronger hydrocarbon absorptions than observed in any previous auroral spectra. This indicates an unusually deep penetration of the incident primary particles with respect to the neutral atmosphere, and correspondingly high energy of the primary particles. We can thus use these spectra to identify many complex hydrocarbons in Jupiter's auroral atmosphere which are not normally measured in auroral spectra. These results will be presented along with model fits to the spectra derived both from fitting the observed hydrocarbon absorption features and from comparison with a photochemical model for the expected composition of the auroral atmosphere. This research has been supported by grant GO-8171.01-97A from the Space Telescope Science Institute to the University of Michigan. [less ▲]

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See detailObservations of substorm auroras from the IMAGE spacecraft
Frey, H. U.; Mende, S. B.; Lampton, M. et al

Conference (2000, October)

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See detailFar ultraviolet imaging of the aurora: new directions
Mende, S. B.; Frey, H. U.; Lampton, M. et al

Conference (2000, October)

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See detailAller plus haut, voir de plus près
Gérard, Jean-Claude ULg

Article for general public (2000)

Carte blanche concernant la vision globale de l'environnement terrestre et son évolution.

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See detailA model of the Lyman-alpha line profile in the proton aurora
Gérard, Jean-Claude ULg; Hubert, Benoît ULg; Bisikalo, Dimitry V et al

in Journal of Geophysical Research (2000), 105

The Lyman-alpha auroral emission is characterized by a broad line profile whose shape depends on the energy and pitch angle distributions of the initial proton beam, whereas its total brightness reflects ... [more ▼]

The Lyman-alpha auroral emission is characterized by a broad line profile whose shape depends on the energy and pitch angle distributions of the initial proton beam, whereas its total brightness reflects the proton energy flux precipitated into the auroral upper atmosphere. Global remote sensing of the proton aurora through its ultraviolet signature makes it is increasingly important to relate the characteristics of the Lyman-alpha emission to the physical properties of the precipitated proton flux. We present a numerical model of proton and hydrogen flux transport and kinetics based on the direct simulation Monte Carlo method. In this approach, all elastic and inelastic processes are stochastically simulated as well as is the production of Lyman-alpha photons with the associated Doppler velocity component. The model also includes collisional, geomagnetic, and geometric spreading of the proton-hydrogen beam. We show that consideration of the stochastic character of the H atom velocity redistribution after collisions produces line profiles different from those obtained in the strictly forward or mean scattering angle approximations previously used in proton transport codes. In particular, the predicted fraction of photons due to backscattered particles is considerably larger when stochastic collision scattering is considered than in the strictly forward or mean scattering angle approximations. In contrast to the median wavelength, the position of the peak in the line profile shows a weak inverse dependence on the proton energy. The efficiency of the Lyman-alpha photon production per unit incident energy flux significantly drops as the mean proton energy increases. The line profile and the amount of blue-shifted (for downward viewing) emission depends in a complex way on the initial energy and pitch angle distribution of the protons. The line profiles expected for the noon cusp and midnight proton aurora are shown to be significantly different. [less ▲]

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See detailFar ultraviolet imaging from the IMAGE spacecraft. 1. System design
Mende, S. B.; Heetderks, H.; Frey, H. U. et al

in Space Science Reviews (2000), 91

Direct imaging of the magnetosphere by the IMAGE spacecraft will be supplemented by observation of the global aurora, the footprint of magnetospheric regions. To assure the simultaneity of these ... [more ▼]

Direct imaging of the magnetosphere by the IMAGE spacecraft will be supplemented by observation of the global aurora, the footprint of magnetospheric regions. To assure the simultaneity of these observations and the measurement of the magnetospheric background neutral gas density, the IMAGE satellite instrument complement includes three Far Ultraviolet (FUV) instruments. In the wavelength region 120-190 nm, a downward-viewing auroral imager is only minimally contaminated by sunlight, scattered from clouds and ground, and radiance of the aurora observed in a nadir viewing geometry can be observed in the presence of the high-latitude dayglow. The Wideband Imaging Camera (WIC) will provide broad band ultraviolet images of the aurora for maximum spatial and temporal resolution by imaging the LBH N_2 bands of the aurora. The Spectrographic Imager (SI), a monochromatic imager, will image different types of aurora, filtered by wavelength. By measuring the Doppler-shifted Ly-alpha, the proton-induced component of the aurora will be imaged separately. Finally, the GEO instrument will observe the distribution of the geocoronal emission, which is a measure of the neutral background density source for charge exchange in the magnetosphere. The FUV instrument complement looks radially outward from the rotating IMAGE satellite and, therefore, it spends only a short time observing the aurora and the Earth during each spin. Detailed descriptions of the WIC, SI, GEO, and their individual performance validations are discussed in companion papers. This paper summarizes the system requirements and system design approach taken to satisfy the science requirements. One primary requirement is to maximize photon collection efficiency and use efficiently the short time available for exposures. The FUV auroral imagers WIC and SI both have wide fields of view and take data continuously as the auroral region proceeds through the field of view. To minimize data volume, multiple images are taken and electronically co-added by suitably shifting each image to compensate for the spacecraft rotation. In order to minimize resolution loss, the images have to be distortion-corrected in real time for both WIC and SI prior to co-adding. The distortion correction is accomplished using high speed look up tables that are pre-generated by least square fitting to polynomial functions by the on-orbit processor. The instruments were calibrated individually while on stationery platforms, mostly in vacuum chambers as described in the companion papers. Extensive ground-based testing was performed with visible and near UV simulators mounted on a rotating platform to estimate their on-orbit performance. The predicted instrument system performance is summarized and some of the preliminary data formats are shown. [less ▲]

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See detailFar ultraviolet imaging from the IMAGE spacecraft. 3. Spectral imaging of Lyman-alpha and OI 135.6 nm
Mende, S. B.; Heetderks, H.; Frey, H. U. et al

in Space Science Reviews (2000), 91

Two FUV Spectral imaging instruments, the Spectrographic Imager (SI) and the Geocorona Photometer (GEO) provide IMAGE with simultaneous global maps of the hydrogen (121.8 nm) and oxygen 135.6 nm ... [more ▼]

Two FUV Spectral imaging instruments, the Spectrographic Imager (SI) and the Geocorona Photometer (GEO) provide IMAGE with simultaneous global maps of the hydrogen (121.8 nm) and oxygen 135.6 nm components of the terrestrial aurora and with observations of the three dimensional distribution of neutral hydrogen in the magnetosphere (121.6 nm). The SI is a novel instrument type, in which spectral separation and imaging functions are independent of each other. In this instrument, two-dimensional images are produced on two detectors, and the images are spectrally filtered by a spectrograph part of the instrument. One of the two detectors images the Doppler-shifted Lyman-alpha while rejecting the geocoronal `cold' Ly-alpha, and another detector images the OI 135.6 nm emission. The spectrograph is an all-reflective Wadsworth configuration in which a grill arrangement is used to block most of the cold, un-Doppler-shifted geocoronal emission at 121.567 nm. The SI calibration established that the upper limit of transmission at cold geocoronal Ly-alpha is less than 2%. The measured light collecting efficiency was 0.01 and 0.008 cm^2 at 121.8 and at 135.6 nm, respectively. This is consistent with the size of the input aperture, the optical transmission, and the photocathode efficiency. The expected sensitivity is 1.8x10^-2 and 1.3x10^-2 counts per Rayleigh per pixel for each 5 s viewing exposure per satellite revolution (120 s). The measured spatial resolution is better than the 128x128 pixel matrix over the 15 degx15 deg field of view in both wavelength channels. The SI detectors are photon counting devices using the cross delay line principle. In each detector a triple stack microchannel plate (MCP) amplifies the photo-electronic charge which is then deposited on a specially configured anode array. The position of the photon event is measured by digitizing the time delay between the pulses detected at each end of the anode structures. This scheme is intrinsically faster than systems that use charge division and it has a further advantage that it saturates more gradually at high count rates. The geocoronal Ly-alpha is measured by a three-channel photometer system (GEO) which is a separate instrument. Each photometer has a built in MgF_2 lens to restrict the field of view to one degree and a ceramic electron multiplier with a KBr photocathode. One of the tubes is pointing radially outward perpendicular to the axis of satellite rotation. The optic of the other two subtend 60 deg with the rotation axis. These instruments take data continuously at 3 samples per second and rely on the combination of satellite rotation and orbital motion to scan the hydrogen cloud surrounding the earth. The detective efficiencies (effective quantum efficiency including windows) of the three tubes at Ly-alpha are between 6 and 10%. [less ▲]

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See detailMultispectral observations of Jupiter's Aurora
Waite, J. H.; Grodent, Denis ULg; Mauk, B. M. et al

in Advances in Space Research (2000), 26(10), 1453-1475

Remote sensing of Jupiter's aurora from x-ray to radio wavelengths has revealed much about the nature of the jovian aurora and about the impact of ionosphere-magnetosphere coupling on the upper atmosphere ... [more ▼]

Remote sensing of Jupiter's aurora from x-ray to radio wavelengths has revealed much about the nature of the jovian aurora and about the impact of ionosphere-magnetosphere coupling on the upper atmosphere of Jupiter, As indicated by the combination of x-ray and ultraviolet observations, both energetic heavy ions and electrons energized in the outer magnetosphere contribute to auroral excitation. Imaging with the Hubble Space Telescope in the ultraviolet and with the InfraRed Telescope Facility at infrared wavelengths shows several distinct regions of interaction: 1) a dusk sector where turbulent auroral patterns extend well into the polar cap; 2) a morning sector generally characterized by a single spatially confined auroral are originating in the outer or middle magnetosphere of Jupiter; 3) diffuse emissions associated with the Io plasma - spectroscopy has provided important information about the thermal structure of Jupiter's auroral atmosphere and the altitude distribution of auroral particle energy deposition, while Lyman alpha line profiles yield clues to the nature of thermospheric dynamical effects. Galileo observations at visible wavelengths on the nightside offer a new view of the jovian aurora with unprecedented spatial information. Infrared observations have added much to the understanding of thermal structure at all latitudes, the dynamics of the thermospheric wind system, and auroral morphology, and may hold the key to understanding the role of Joule heating in Jupiter's thermosphere. ROSAT observations have revealed soft x-ray emissions from Jupiter's lower latitudes as well as from the auroral zones, implying that energetic particle precipitation also occurs at low latitudes in regions magnetically linked to the inner radiation belts. In this review, multispectral observations of jovian auroral emissions are presented within a theoretical/modeling framework that is intended to provide some insight into magnetosphere-ionosphere coupling and its effects on the upper atmosphere. (C) 2000 COSPAR. Published by Elsevier Science Ltd. All rights reserved. [less ▲]

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See detailDiagnostics of the jovian aurora deduced from ultraviolet spectroscopy: Model and HST/GHRS observations
Dols, V.; Gérard, Jean-Claude ULg; Clarke, J. T. et al

in Icarus: International Journal of Solar System Studies (2000), 147(1), 251-266

A model coupling an electron energy degradation code with a detailed synthetic spectrum of the H-2 Lyman and Werner band system is used to calculate the emerging auroral ultraviolet spectra from Jupiter's ... [more ▼]

A model coupling an electron energy degradation code with a detailed synthetic spectrum of the H-2 Lyman and Werner band system is used to calculate the emerging auroral ultraviolet spectra from Jupiter's atmosphere excited by electrons with different initial energy distributions. The atmospheric model is adapted from the vertical P-T profile measured by the Galileo probe and midlatitude model hydrocarbon photochemistry. Each altitude layer, with its own gas temperature, contributes to the emergent ultraviolet spectrum and the absorbers are vertically distributed within the source region of the auroral emissions. Examples of the calculated spectra are shown to validate the synthetic spectrum and to illustrate the importance of the electron energy distribution and the vertical structure. The model is then applied to the analysis of seven HST/GHRS spectra of the 1200-1700 Angstrom region obtained with 5-Angstrom resolution at various locations in the north and south Jovian aurora. These spectra have different color ratios which characterize the energy of the precipitated electrons, although they do not have a high enough spectral resolution to permit a determination of the H-2 temperature. We find that the characteristic energy of the assumed initial Maxwellian distribution ranges between 17 and 40 keV. A clear signature of acetylene absorption is observed near 1520, 1480, and 1440 Angstrom where the C2H2 cross section shows strong absorption peaks. The acetylene column abundance overlying the emission peak varies from 0.02 to 0.2 of the methane column. A better fit is obtained for some spectra when ethane absorption is added. The C2H6 column abundance varies from 0 to 0.5 of the methane column. These changes relative to methane are presumably the result of perturbations by heat released by the fast electron thermalization and/or perturbations to the hydrocarbon chemistry resulting from the production of H atoms by the aurora, A spectrum of the Io flux tube footprint and its trailing tail shows an ultraviolet color and hydrocarbon absorption quite similar to some of the main oval spectra, This observation suggests that the electrons of the Io flux tube are energized to a few tens of keV, similar to the electron precipitated in the main ovals and polar caps. Echelle spectra between 1216 and 1220 Angstrom at 0.07 Angstrom resolution are also compared with the model fitting best the mid-resolution spectra. It is found that the effective H-2 rovibrational temperature associated with the echelle spectra are significantly higher than predicted by the mid-latitude model. A large vertical temperature gradient just above the methane homopause due to large heating by auroral precipitation is a plausible explanation for this difference. (C) 2000 Academic Press. [less ▲]

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See detailImpact of crops on biomass and soil carbon: steady state simulations
Nemry, B.; François, Louis ULg; Gérard, Jean-Claude ULg et al

in Cramer, W.; Doherty, R.; Hulme, M. (Eds.) et al Climate Scenarios for Agricultural, Forest and Ecosystem Impacts (2000)

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See detailEvolution de l'environnement planétaire : réchauffement climatique et conséquences
Gérard, Jean-Claude ULg

Scientific conference (1999, November 17)

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See detailJovian thermal structure inferred from the energy degradation of auroral electrons
Grodent, Denis ULg; Gérard, Jean-Claude ULg; Waite, J. H.

in Bulletin of the American Astronomical Society (1999, October 10)

A one--dimensional model has been developed to investigate the links between auroral heat input and the atmospheric temperature and composition structure of Jupiter. Different energy distributions are ... [more ▼]

A one--dimensional model has been developed to investigate the links between auroral heat input and the atmospheric temperature and composition structure of Jupiter. Different energy distributions are used to evaluate the importance of the energy spectrum of the incident electrons for the thermal balance of Jupiter's auroral thermosphere. Radiative cooling by H_3(+) and hydrocarbon (CH_4, C_2H_2) and downward conduction are calculated to solve the heat conduction equation. The values of observable quantities such as the altitude of the H_2 emission peak, infrared (IR) and ultraviolet (UV) emissions and temperatures associated with H_2 and H_3(+) optical signatures are used to constrain the parameters of the auroral electron energy distributions. From these simulations, it appears that the precipitated auroral energy is not able to directly provide the necessary heat to balance the hydrocarbon cooling below the homopause. It is suggested however that the auroral upper stratosphere is warmer than the equatorial upper stratosphere measured by Galileo. A Maxwellian energy distribution with a total flux of 20 ergs cm(-2) s(-1) and a characteristic energy of 22 keV added to a soft Maxwellian component of 1 erg cm(-2) s(-1) and 350 eV produces results in good agreement with thermospheric observations. [less ▲]

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See detailFar ultraviolet Observations of Jovian low latitude regions with HST/STIS
Gustin, Jacques ULg; Grodent, Denis ULg; Gérard, Jean-Claude ULg et al

in Bulletin of the American Astronomical Society (1999, September 01), 30(11),

Far ultraviolet observations of the Jovian disk were made at low and mid-latitudes with FUV MAMA/STIS on board HST in January 1999 both in the imaging and spectroscopic modes. An image was obtained with ... [more ▼]

Far ultraviolet observations of the Jovian disk were made at low and mid-latitudes with FUV MAMA/STIS on board HST in January 1999 both in the imaging and spectroscopic modes. An image was obtained with the Lyalpha filter in the hydrogen bulge region for comparison with the expected Lyman-alpha brightness distribution for Ly-alpha resonance scattering. Other images in the 1200-1700 { Angstroms} region show band structures parallel to the equator with fading contrast toward the center and the limb. Spectroscopic observations were made in the 1200-1700 { Angstroms} (G140L) and 1245-1298 { Angstroms} (G140M) regions at ~ 5 { Angstroms} resolution to map the H_2 airglow and the UV absorbents along the STIS slit. Preliminary results indicate that a C_2H_2 absorption signature is clearly observed in the solar ultraviolet reflected spectrum. The ethylene absorption may be mapped to derive variations of the acetylene abundance. The H_2 FUV airglow shows both the fluorescence and the electron impact components. Its spatial variation is described and compared with the expected airglow distribution. We acknowledge funding by NASA and by the PRODEX program of the European space agency. [less ▲]

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