References of "Gérard, Jean-Claude"
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See detailThermalization of <formula alphabet="latin">O([SUP]1[/SUP]D) atoms in the thermosphere
Shematovich, Valery; Gérard, Jean-Claude ULg; Bisikalo, Dimitry V et al

in Journal of Geophysical Research (1999), 104

Measurements of the Doppler width of the 6300 Å airglow emission line have been extensively used to determine the thermospheric temperature. This technique is based on the assumption that the bulk of the ... [more ▼]

Measurements of the Doppler width of the 6300 Å airglow emission line have been extensively used to determine the thermospheric temperature. This technique is based on the assumption that the bulk of the emitting O([SUP]1[/SUP]D) atoms are thermalized in the region of the airglow source (200-300 km). A Monte Carlo stochastic model is used to calculate the energy distribution function of O([SUP]1[/SUP]D) atoms in the daytime and nighttime thermosphere. Hot O([SUP]1[/SUP]D) atoms are produced by exothermic processes and their thermalization is controlled by the competition between radiation, collisional quenching, and relaxation. It is found that the O([SUP]1[/SUP]D) temperature departs from the background gas temperature not only in the upper thermosphere but also in the region of the bulk 6300 Å emission. At 300 km for low solar activity conditions, the model predicts an excess O([SUP]1[/SUP]D) temperature of ~180 K during daytime and ~950 K at night. The temperature departure persists at lower altitudes as a result of the major contribution of the O[SUB]2[/SUB][SUP]+[/SUP] dissociative recombination source of hot [SUP]1[/SUP]D atoms. Experimental evidence based on the Fabry-Perot interferometer measurements on board the Dynamics Explorer satellite confirms the existence of an O([SUP]1[/SUP]D) temperature excess over the mass spectrometer/incoherent scatter (MSIS) value. It is concluded that temperatures deduced from the 6300 Å airglow line width may significantly exceed the ambient gas temperature in a way depending on solar activity, local time, and observation geometry. [less ▲]

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See detailThe interannual change of atmospheric CO2: contribution of subtropical ecosystems?
Gérard, Jean-Claude ULg; Nemry, B.; François, Louis ULg et al

in Geophysical Research Letters (1999), 26(2), 243-246

The global terrestrial carbon cycle model CARAIB (CARbon Assimilation In the Biosphere) is used to study the response of the terrestrial ecosystems to the large scale climate variations over the period ... [more ▼]

The global terrestrial carbon cycle model CARAIB (CARbon Assimilation In the Biosphere) is used to study the response of the terrestrial ecosystems to the large scale climate variations over the period 1980-1993. The global net carbon exchange flux with the atmosphere is calculated and compared with the terrestrial contribution derived from the deconvolution of the atmospheric CO2 and delta(13)C measurements. A fairly large CO2 biospheric source is predicted during the strong El Nino events of 1982-83 and 1986-87 as a consequence of the induced global warming. The direct and indirect temperature controls of the primacy production and respiration dominate the CO2 anomaly. An analysis of the relative contribution by latitudinal bands and ecosystems shows that low-latitude vegetation dominates the variability at the El Nino time scale. In savannas, the model indicates that the interannual changes result, to a large extent, from the control of soil water content on gross primary production (GPP). In the tropical cain forests, both respiration and GPP contribute to the response of the net biospheric flux. [less ▲]

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See detailComparing global models of terrestrial net primary productivity (NPP): analysis of the seasonal atmospheric CO2 signal
Nemry, B.; François, Louis ULg; Gérard, Jean-Claude ULg et al

in Global Change Biology (1999), 5(Suppl. 1), 65-76

Eight terrestrial biospheric models (TBMs) calculating the monthly distributions of both net primary productivity (NPP) and soil heterotrophic respiration (R-H) in the Potsdam NPP Model Intercomparison ... [more ▼]

Eight terrestrial biospheric models (TBMs) calculating the monthly distributions of both net primary productivity (NPP) and soil heterotrophic respiration (R-H) in the Potsdam NPP Model Intercomparison workshop are used to simulate seasonal patterns of atmospheric CO2 concentration. For each model, we used net ecosystem productivity (NEP=NPP-R-H) as the source function in the TM2 atmospheric transport model from the Max-Planck Institute for Meteorology. Comparing the simulated concentration fields with detrended measurements from 25 monitoring stations spread over the world, we found that the decreasing seasonal amplitude from north to south is rather well reproduced by all the models, though the amplitudes are slightly too low in the north. The agreement between the simulated and observed seasonality is good in the northern hemisphere, but poor in the southern hemisphere, even when the ocean is accounted for. Based on a Fourier analysis of the calculated zonal atmospheric signals, tropical NEP plays a key role in the seasonal cycle of the atmospheric CO2 in the whole southern hemisphere. The relatively poor match between measured and predicted atmospheric CO2 in this hemisphere suggests problems with all the models. The simulation of water relations, a dominant regulator of NEP in the tropics, is a leading candidate for the source of these problems. [less ▲]

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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, Jr

in Bulletin of the American Astronomical Society (1999)

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See detailHST/STIS images of UV auroral footprints from Io, Europa, and Ganymede.
Clarke, J. T.; Ajello, J.; Ballester, G. E. et al

in Bulletin of the American Astronomical Society (1999)

Ultraviolet images of Jupiter's aurora have been obtained with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) since September 1997 with much higher sensitivity than ... [more ▼]

Ultraviolet images of Jupiter's aurora have been obtained with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) since September 1997 with much higher sensitivity than earlier cameras. Higher sensitivity permits shorter exposures, freezing Jupiter's rotation and providing the highest angular resolution obtained to date. This combination of sensitivity and resolution has revealed new emissions from the magnetic footprints of Io, Europa, and Ganymede, which will be reported and discussed. Io's auroral footprint, while well studied with earlier cameras, appears highly extended at a low brightness in the wake or plasma flow direction. Ganymede's auroral footprint emission is now well established from the repeated appearance of this feature under the magnetic field trace of Ganymede, and there is initial evidence for auroral emission at Europa's magnetic footprint. Ganymede's auroral footprint appears consistently equatorward of the main auroral oval, which clearly constrains the main oval auroral currents to originate from beyond about 20 R_J. The observation that the main oval emissions are observed to corotate with Jupiter's magnetic field further constrains the origin of these currents to be within about 30 R_J, so that the source region for the main oval auroral is now fairly well constrained to a region in Jupiter's middle magnetosphere and within the current sheet. This work was supported by NASA under grant GO-7308.01-96A to the University of Michigan. [less ▲]

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See detailHST/GHRS ultraviolet spectroscopy and model diagnostics of the Jovian aurora.
Gérard, Jean-Claude ULg; Dols, V.; Gustin, Jacques ULg et al

in Bulletin of the American Astronomical Society (1999)

A model coupling a detailed synthetic spectrum of the H_2 Lyman and Werner band systems with an electron energy degradation code is applied to the analysis of 7 GHRS spectra of the 1200-1700 { Angstroms ... [more ▼]

A model coupling a detailed synthetic spectrum of the H_2 Lyman and Werner band systems with an electron energy degradation code is applied to the analysis of 7 GHRS spectra of the 1200-1700 { Angstroms} region obtained with a ~ 5 { Angstroms} resolution at various locations in the north and south Jovian aurora. The observed color ratios indicate that the characteristic energy of the assumed initial Maxwellian distribution ranges between 17 and 40 keV. A clear signature of acetylene is observed near the absorption peaks at 1520, 1480 and 1440 { Angstroms}. The C_2H_2 column 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 changing mixing ratios relative to methane are attributed to 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 magnetic footprint and its trailing tail shows ultraviolet color and hydrocarbon absorption charateristics quite similar to some of the main oval spectra. This observation implies 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 { Angstroms} at 0.07 { Angstroms} resolution are also compared with the model best fitting the closely spaced in time mid-resolution spectrum. It is found that the effective H_2 rovibrational temperature associated with the Echelle spectra is significantly higher than predicted by the model. A steep temperature gradient near the methane homopause due to large heating by auroral precipitation is a plausible explanation for this difference. We acknowledge funding by NASA and the PRODEX program of the European Space Agency. [less ▲]

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See detailHubble Space Telescope imaging of Jupiter's UV aurora during the Galileo orbiter mission
Clarke, John T; Ben Jaffel, Lotfi; Gérard, Jean-Claude ULg

in Journal of Geophysical Research (1998), 103

Hubble Space Telescope (HST) Wide-Field Planetary Camera 2 (WFPC 2) images of Jupiter's aurora have been obtained close in time with Galileo ultraviolet spectrometer (UVS) spectra and in situ particles ... [more ▼]

Hubble Space Telescope (HST) Wide-Field Planetary Camera 2 (WFPC 2) images of Jupiter's aurora have been obtained close in time with Galileo ultraviolet spectrometer (UVS) spectra and in situ particles, fields, and plasma wave measurements between June 1996 and July 1997, overlapping Galileo orbits G1, G2, G7, G8, and C9. This paper presents HST images of Jupiter's aurora as a first step toward a comparative analysis of the auroral images with the in situ Galileo data. The WFPC 2 images appear similar to earlier auroral images, with the main ovals at similar locations to those observed over the preceding 2 years, and rapidly variable emissions poleward of the main ovals. Further examples have been observed of the equatorward surge of the auroral oval over 140-180° longitude as this region moves from local morning to afternoon. Comparison of the WFPC 2 reference auroral ovals north and south with the VIP4 planetary magnetic field model suggests that the main ovals map along magnetic field lines exceeding 15R[SUB]J[/SUB], and that the Io footprint locations have lead angles of 0-10° from the instantaneous magnetic projection. There was an apparent dawn auroral storm on June 23, 1996, and projections of the three dawn storms imaged with HST to date demonstrate that these appear consistently along the WFPC 2 reference oval. Auroral emissions have been consistently observed from Io's magnetic footprints on Jupiter. Possible systematic variations in brightness are explored, within factor of 6 variations in brightness with time. Images are also presented marked with expected locations of any auroral footprints associated with the satellites Europa and Ganymede, with localized emissions observed at some times but not at other times. [less ▲]

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See detailThe Jovian Aurora: Implications of Multiwavelength Auroral Spectra for Auroral Particle Identity and Auroral Microphysics
Waite, J. H.; Gladstone, G. R.; Bolton, S. J. et al

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

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 Jupiter's upper ... [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 Jupiter's upper atmosphere. Both energetic heavy ions and electrons energized in the outer magnetosphere contribute to the auroral excitation, as indicated by the combination of x-ray and ultraviolet observations. Imaging with the HST in the ultraviolet and with the IRTF at infrared wavelengths reveals 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 arc originating in the outer magnetosphere of Jupiter; 3) diffuse emissions associated with the Io plasma torus; and 4) a distinct region associated with the Io Flux Tube footprint. Ultraviolet spectroscopy has provided important information about the thermal structure of the upper atmosphere and altitude distribution of the auroral particle energy deposition, while Lyman alpha line profiles offer clues to the nature of thermospheric dynamical effects. Galileo observations at visible wavelengths on the nightside have provided a new view of the jovian aurora with unprecedented spatial information. Infrared observations have added much to the understanding of thermal structure and morphology and may hold the key to understanding the role of Joule heating. Radio observations imply that energetic particle precipitation extends to low latitudes, a result that has been corroborated at x-ray wavelengths. Multispectral observations of jovian auroral emissions will be discussed within a theoretical/modeling framework that serves to provide some insight into magnetosphere-ionosphere coupling and its effect on the upper atmosphere. Particular emphasis will be placed on the use of auroral spectra to identify incident energetic particles and their energy spectra as a means of elucidating the microphysics of auroral processes. [less ▲]

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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 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)

Detailed reference viewed: 10 (1 ULg)