References of "Gustin, Jacques"
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See detailCharacteristics of Saturn's polar atmosphere and auroral electrons derived from HST/STIS, FUSE and Cassini/UVIS spectra
Gustin, Jacques ULg; Gérard, Jean-Claude ULg; Pryor, Wayne et al

in Icarus: International Journal of Solar System Studies (2009), 200

Ultraviolet (UV) spectra of Saturn's aurora obtained with the Hubble Space Telescope Imaging Spectrograph (STIS), the Cassini Ultraviolet Imaging Spectrograph (UVIS) and the Far Ultraviolet Spectroscopic ... [more ▼]

Ultraviolet (UV) spectra of Saturn's aurora obtained with the Hubble Space Telescope Imaging Spectrograph (STIS), the Cassini Ultraviolet Imaging Spectrograph (UVIS) and the Far Ultraviolet Spectroscopic Explorer (FUSE) have been analyzed. Comparisons between the observed spectra and synthetic models of electron-excited H[SUB]2[/SUB] have been used to determine various auroral characteristics. Far ultraviolet (FUV: 1200 1700 Å) STIS and UVIS spectra exhibit, below 1400 Å, weak absorption due to methane, with a vertical column ranging between 1.4×10[SUP][/SUP] and 1.2×10[SUP][/SUP]cm[SUP][/SUP]. Using the low-latitude Moses et al. [Moses, J.I., Bézard, B., Lellouch, E., Feuchtgruber, H., Gladstone, G.R., Allen, M., 2000. Icarus, 143, 244 298] atmospheric model of Saturn and an electron energy H[SUB]2[/SUB] column relationship, these methane columns are converted into the mean energy of the primary precipitating electrons, estimated to lie in the range 10 18 keV. This result is confirmed by the study of self-absorption with UVIS and FUSE extreme ultraviolet (EUV: 900 1200 Å) spectra. Below 1200 Å, it is seen that transitions connecting to the v[SUP][/SUP]<2 vibrational levels of the H[SUB]2[/SUB] electronic ground state are partially self-absorbed by H[SUB]2[/SUB] molecules overlying the auroral emission. Because of its low spectral resolution (Ë 5.5 Å), the UVIS EUV spectrum we analyzed does not allow us to unequivocally determine reasonable ranges of temperatures and H[SUB]2[/SUB] columns. On the other hand, the high spectral resolution (Ë 0.2 Å) of the FUSE LiF1a and LiF2a EUV spectra we examined resolve the H[SUB]2[/SUB] rotational lines and makes it possible to determine the H[SUB]2[/SUB] temperature. The modeled spectrum best fitting the FUSE LiF1a observation reveals a temperature of 500 K and self-absorption by a H[SUB]2[/SUB] vertical column of 3×10[SUP][/SUP]cm[SUP][/SUP]. When converted to energy of precipitating electrons, this H[SUB]2[/SUB] column corresponds to primary electrons of Ë 10 keV. The model that best fits the LiF2a spectrum is characterized by a temperature of 400 K and is not self-absorbed, making this segment ideal to determine the H[SUB]2[/SUB] temperature at the altitude of the auroral emission. The latter value is in agreement with temperatures obtained from H3+ infrared polar spectra. Self-absorption is detectable in the LiF2a segment for H[SUB]2[/SUB] columns exceeding 6×10[SUP][/SUP]cm[SUP][/SUP], which sets the maximum mean energy determined from the FUSE observations to Ë 15 keV. The total electron energy range of 10 18 keV deduced from FUV and EUV observations places the auroral emission peak between the 0.1 and 0.3 mubar pressure levels. These values should be seen as an upper limit, since most of the Voyager UVS spectra of Saturn's aurora examined by Sandel et al. [Sandel, B.R., Shemansky, D.E., Broadfoot, A.L., Holberg, J.B., Smith, G.R., 1982. Science 215, 548] do not exhibit methane absorption. The auroral H[SUB]2[/SUB] emission is thus likely located above but close to the methane homopause. The H[SUB]2[/SUB] auroral brightness in the 800 1700 Å bandwidth varies from 2.9 kR to 139 kR, comparable to values derived from FUV Faint Object Camera (FOC) and STIS images. [less ▲]

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See detailAltitude of Saturn's aurora and its implications for the characteristic energy of precipitated electrons
Gérard, Jean-Claude ULg; Bonfond, Bertrand ULg; Gustin, Jacques ULg et al

in Geophysical Research Letters (2009), 36

Images of Saturn's aurora at the limb have been collected with the Advanced Camera for Surveys on board the Hubble Space Telescope. They show that the peak of Saturn's nightside emission is generally ... [more ▼]

Images of Saturn's aurora at the limb have been collected with the Advanced Camera for Surveys on board the Hubble Space Telescope. They show that the peak of Saturn's nightside emission is generally located 900-1300 km above the 1-bar level. On the other hand, methane and H[SUB]2[/SUB] columns overlying the aurora have been determined from the analysis of FUV and EUV spectra, respectively. Using a low-latitude model, these columns place the emission layer at or above 610 km. One possibility to solve this apparent discrepancy between imaging and spectral observations is to assume that the thermospheric temperature in the auroral region sharply increases at a higher pressure level than in the low-latitude regions. Using an electron transport code, we estimate the characteristic energy of the precipitated electrons derived from these observations to be in the range 1-5 keV using a low latitude model and 5-30 keV in case of the modified model. [less ▲]

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See detailAnalysis of Titan airglow UV spectra from Cassini Ultraviolet Imaging Spectrograph (UVIS)
Gustin, Jacques ULg; Ajello, J.M.; Stevens, M.H. et al

Poster (2009)

We present the analysis of FUV (1150-1750 Å) limb dayglow spectra of Titan’s atmosphere obtained on 13 December 2004 at 5 Å resolution by the Ultraviolet Imaging Spectrograph (UVIS) onboard Cassini. The ... [more ▼]

We present the analysis of FUV (1150-1750 Å) limb dayglow spectra of Titan’s atmosphere obtained on 13 December 2004 at 5 Å resolution by the Ultraviolet Imaging Spectrograph (UVIS) onboard Cassini. The fit to the data show that Titan’s airglow consists of four principal emissions: 1) the N2 Lyman-Birge-Hopfield (LBH) band system, which peaks in intensity at 1150 +/- 50 km, 2) N I multiplets peaking at 1050km +/- 50 km, 3) sunlight reflected by N2 between 0 and 300 km and 4) H Ly alpha which grows in intensity with increasing altitude. Comparisons with limb spectra obtained by the Voyager 1 Ultraviolet spectrometer (V1/UVS) show that the vertically integrated brightness are larger for V1/UVS than for UVIS by a factor of 3, consistent with the XUV solar flux ratio at Titan at the time of these observations. The N2 LBH and N I profiles obtained from the regression to the Titan data are compared to models obtained by the Atmospheric Ultraviolet Radiance Integrated Code (AURIC), adapted from Earth’s atmosphere to Titan’s. [less ▲]

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See detailSaturn Auroral Images and Movies from Cassini UVIS
Pryor, Wayne; Esposito, L.W.; Stewart, A.I.F. et al

Conference (2009)

Cassini’s Ultraviolet Imaging Spectrograph (UVIS) has completed five years of study of Saturn’s atmosphere and auroras. Two long slit spectral channels are used to obtain EUV data from 56.3-118.2 nm and ... [more ▼]

Cassini’s Ultraviolet Imaging Spectrograph (UVIS) has completed five years of study of Saturn’s atmosphere and auroras. Two long slit spectral channels are used to obtain EUV data from 56.3-118.2 nm and FUV data from 111.5-191.3 nm. 64 spatial pixels along each slit are combined with slit motion to construct spectral images of Saturn. Auroral emissions are seen from electron-excited molecular and atomic hydrogen. In 2008-2009 UVIS obtained data with the spacecraft well out of Saturn’s ring plane, permitting UVIS to obtain a number of short movies of the rotating auroral structures. In some movies a cusp-like feature is present near noon inside the oval. One movie from 2008 day 201 shows parallel linear features on the day side almost at right angles to the main auroral oval that appear, then lengthen, separate in the middle, and then fade away. The same movie also shows one bright "polar flare" inside the oval. A few of the most recent images were obtained at sufficiently close range that 2 spacecraft slews were needed to completely cover the oval. These images provide almost 100 pixels of information across the oval and clearly show multiple arcs of emission on the main oval and scattered emissions inside the oval. We will discuss these features, their locations, and possible interpretations. We also report on a search for an Enceladus auroral footprint on Saturn. [less ▲]

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See detailCharacteristics of Saturn's polar atmosphere and auroral electrons derived from HST/STIS, FUSE and Cassini/UVIS spectra
Gustin, Jacques ULg; Pryor, W.; Feldman, P. et al

Poster (2008, December 01)

Ultraviolet spectra of Saturn's aurora obtained with the Hubble Space Telescope Imaging Spectrograph (STIS), the Cassini Ultraviolet Imaging Spectrograph (UVIS) and the Far Ultraviolet Spectroscopic ... [more ▼]

Ultraviolet spectra of Saturn's aurora obtained with the Hubble Space Telescope Imaging Spectrograph (STIS), the Cassini Ultraviolet Imaging Spectrograph (UVIS) and the Far Ultraviolet Spectroscopic Explorer (FUSE) have been compared to synthetic spectra of electron-excited H2 in order to derive various auroral characteristics, such as the energy of the primary precipitating electrons and the H2 temperature at the altitude of the aurora. Two physical processes have been exploited: the absorption by hydrocarbons in the FUV and H2 self-absorption in the EUV. We find energies in the range 10-18 keV, which locates Saturns's aurora between 0.1 and 0.3 μ bar. We also determined that the auroral H2 emission is characterized by a temperature of ~400K, consistent with temperatures measured in the infrared, but much higher than what is expected from equatorial atmospheric models. These new results bring valuable constraints on both polar atmospheric models and theoretical studies of the ionosphere-magnetosphere coupling. [less ▲]

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See detailCharacteristics of Saturn's polar atmosphere and auroral electrons derived from HST/STIS, FUSE and Cassini/UVIS spectra
Gustin, Jacques ULg; Pryor, W.; Feldman, P. et al

Poster (2008, December 01)

Ultraviolet spectra of Saturn's aurora obtained with the Hubble Space Telescope Imaging Spectrograph (STIS), the Cassini Ultraviolet Imaging Spectrograph (UVIS) and the Far Ultraviolet Spectroscopic ... [more ▼]

Ultraviolet spectra of Saturn's aurora obtained with the Hubble Space Telescope Imaging Spectrograph (STIS), the Cassini Ultraviolet Imaging Spectrograph (UVIS) and the Far Ultraviolet Spectroscopic Explorer (FUSE) have been compared to synthetic spectra of electron-excited H2 in order to derive various auroral characteristics, such as the energy of the primary precipitating electrons and the H2 temperature at the altitude of the aurora. Two physical processes have been exploited: the absorption by hydrocarbons in the FUV and H2 self-absorption in the EUV. We find energies in the range 10-18 keV, which locates Saturns's aurora between 0.1 and 0.3 μ bar. We also determined that the auroral H2 emission is characterized by a temperature of ~400K, consistent with temperatures measured in the infrared, but much higher than what is expected from equatorial atmospheric models. These new results bring valuable constraints on both polar atmospheric models and theoretical studies of the ionosphere-magnetosphere coupling. [less ▲]

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See detailRecent results from HST and ground-based observations of Saturn's aurora
Grodent, Denis ULg; Stallard, T.; Gérard, Jean-Claude ULg et al

Conference (2008, December 01)

Current observations of Saturn's aurora performed from Earth-orbit with HST and ground based instruments more than complement the in situ measurements obtained by the Cassini spacecraft. These remote ... [more ▼]

Current observations of Saturn's aurora performed from Earth-orbit with HST and ground based instruments more than complement the in situ measurements obtained by the Cassini spacecraft. These remote observations focus on two spectral windows revealing different facets of the same auroral phenomenon. The auroral photons captured in the ultraviolet bandwidth result from direct impact excitation of H and H2 by charged particles accelerated along magnetic field lines, while the thermal infrared emission involves additional steps in order to produce hot H3+ from the auroral energy. Each spectral window presents its own advantages. The high spatial and temporal resolution of the recent UV images obtained with HST make it possible to discriminate auroral sub-structures, such as short lived arcs and spots, and to map them into the magnetosphere where they can be associated with in situ observations. Infrared high resolution spectroscopy and emission-line imaging from ground observatories (IRTF, UKIRT) have more modest spatial resolution; however they recently pinned down emissions barely observed in the UV. Furthermore, they offer a direct measurement of the ion wind velocities in the auroral ionosphere. These ion flow patterns might then be used to untangle the origin of the auroral particles. The complementarity of observations obtained in the UV and IR bandwidths provides a powerful tool to study the auroral mechanisms in the Kronian magnetosphere and the atmospheric response to the auroral input. [less ▲]

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See detailSaturn Auroral Movies from Cassini UVIS
Pryor, W. R.; Stewart, I.; Esposito, L. et al

Conference (2008, December 01)

Cassini's Ultraviolet Imaging Spectrograph (UVIS) has completed four years of study of Saturn's atmosphere and auroras. Two long slit spectral channels are used to obtain EUV data from 56.3-118.2 nm and ... [more ▼]

Cassini's Ultraviolet Imaging Spectrograph (UVIS) has completed four years of study of Saturn's atmosphere and auroras. Two long slit spectral channels are used to obtain EUV data from 56.3-118.2 nm and FUV data from 111.5-191.3 nm. 64 spatial pixels along each slit are combined with slit motion to construct spectral images of Saturn. Auroral emissions are seen from electron-excited molecular and atomic hydrogen. In 2008 UVIS obtained data with the spacecraft well out of Saturn's ring plane, permitting UVIS to obtain a number of short movies of the rotating auroral structures. In some movies a cusp-like feature is present near noon inside the oval. One movie from 2008 day 201 shows parallel linear features on the day side almost at right angles to the main auroral oval that appear, then lengthen, separate in the middle, and then fade away. The same movie also shows one bright "polar flare" inside the oval. A few of the most recent images were obtained at sufficiently close range that 2 spacecraft slews were needed to completely cover the oval. These images provide almost 100 pixels of information across the oval and clearly show multiple arcs of emission on the main oval and scattered emissions inside the oval. We will discuss these features, their locations, and possible interpretations. [less ▲]

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See detailThree-dimensional extension of the Io UV footprint emissions
Bonfond, Bertrand ULg; Grodent, Denis ULg; Gérard, Jean-Claude ULg et al

Conference (2008, September 23)

The Io UV footprint is an auroral feature observed close to the feet of the field lines passing through Io on both Jovian hemispheres. These light emissions are caused by the electromagnetic interaction ... [more ▼]

The Io UV footprint is an auroral feature observed close to the feet of the field lines passing through Io on both Jovian hemispheres. These light emissions are caused by the electromagnetic interaction between the satellite Io and the Jovian magnetosphere. For both the north and south poles, the Io footprint appears as a bright spot followed by a faint trailing tail and occasionally followed or preceded by secondary spots. The footprint morphology and the spots multiplicity have been found to vary with the location of Io in the plasma torus. [less ▲]

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See detailThe altitude of Saturn’s aurora: implications for the auroral electron energy and location of the polar homopause
Gérard, Jean-Claude ULg; Bonfond, Bertrand ULg; Gustin, Jacques ULg et al

Conference (2008, September 23)

The Space Telescope Imaging Spectrograph (STIS) and the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST) have been used to study the characteristics of Saturn’s FUV aurora ... [more ▼]

The Space Telescope Imaging Spectrograph (STIS) and the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST) have been used to study the characteristics of Saturn’s FUV aurora, which is dominated by the Lyman and Werner bands of H2 and the H Lyman-α line. Images have been analyzed to determine the altitude of the emission above the limb and infer the characteristic energy of the auroral electrons. Our study shows that the peak of the emission is located 1000-1300 km above the limb, indicating that the precipitated electrons are mostly in the 0.1-1 keV energy range, that is significantly less than on Jupiter. Another source of information on the electron energy are the FUV spectra obtained with Voyager UVS (Sandel et al., 1982) and the HST-STIS (Gérard et al., 2004) and Cassini-UVIS spectrographs which only occasionally show a signature of absorption by hydrocarbons. Consequently, the emission is located above but close to the methane homopause. Finally, EUV spectra collected with the FUSE satellite (Gustin et al., 2008) provide information on the H2 column overlying the aurora through analysis of self-absorption and rotational temperature of the emitting layer from the intensity distribution among the H2 lines. If Moses et al. (2000)’s low latitude model is used to convert altitudes into pressure levels, H2 and hydrocarbon columns and temperature, discrepancies appear between the observed temperature, ultraviolet colour ratio and geometric altitude of the emission. One possibility is that the temperature near the homopause sharply increases at a pressure level higher than in the equatorial regions. [less ▲]

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See detailNew results on the UV Io footprint morphology and brightness
Bonfond, Bertrand ULg; Grodent, Denis ULg; Gérard, Jean-Claude ULg et al

Poster (2008, April 18)

The Io UV footprint is an auroral feature on Jupiter caused by the electromagnetic interaction between the satellite Io and the Jovian magnetosphere. The footprint morphology and the spots multiplicity ... [more ▼]

The Io UV footprint is an auroral feature on Jupiter caused by the electromagnetic interaction between the satellite Io and the Jovian magnetosphere. The footprint morphology and the spots multiplicity have been found to vary with the location of Io in the plasma torus. We show recent Hubble Space Telescope (HST) images that reveal a new feature in the footprint: a faint leading spot that appears upstream of the main spot in one hemisphere when Io is close to the opposite border of the torus. A possible interpretation relates the leading spots and one downward secondary spot to electron beams generated by downstream currents in the opposite hemisphere. We also present a 3D model of the Io footprint emissions in the 100 to 170 nm wavelength range. Comparisons between this model and the HST images enable us to study the actual size and shape of the different Io footprint features. It also allows to measure the footprint brightness on the new images with a better estimation of the geometric effects (e.g. limb brightening). The observations presented here provide critical constraints to the Io-plasma torus interaction modeling. [less ▲]

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See detailJupiter’s diffuse auroral emissions - Comparison of HST and Galileo data
Radioti, Aikaterini ULg; Tomás, A. T. M.; Grodent, Denis ULg et al

Conference (2008, April 18)

Based on an extensive HST FUV image database obtained between 1997 and 2007, we have studied the morphology and brightness of the equatorward diffuse auroral emissions in both Jovian hemispheres. The ... [more ▼]

Based on an extensive HST FUV image database obtained between 1997 and 2007, we have studied the morphology and brightness of the equatorward diffuse auroral emissions in both Jovian hemispheres. The emissions are wider and brighter on the dusk side than on the dawn and they often form multiple discrete arcs parallel to the main oval. What could be the origin of these equatorward diffuse emissions and their local time variations is still unclear. Galileo observations have shown changes in the electron pitch angle distributions between the inner and middle magnetosphere of Jupiter (10 to 17 RJ ) which could be associated with auroral emissions, without the need of field aligned currents. We derive the electron precipitation flux for the first time in a global scale, based on Galileo electron measurements between 10 and 17 RJ . We magnetically map this region in the ionosphere and compare the derived energy flux with the brightness of the diffuse emissions. We discuss the possibility that the energetic particle distribution in the middle magnetosphere could account for the multiple structured equatorward diffuse emissions and their local time variations. [less ▲]

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See detailEvolution of the Io footprint morphology
Bonfond, Bertrand ULg; Grodent, Denis ULg; Gérard, Jean-Claude ULg et al

Conference (2008, April)

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See detailJupiter’s diffuse auroral emissions - Comparison of HST and Galileo data,
Radioti, Aikaterini ULg; Tomàs, A. T. M.; Grodent, Denis ULg et al

Conference (2008, April)

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See detailTitan airglow spectra from the Cassini Ultraviolet Imaging Spectrograph: FUV disk analysis
Ajello, Joseph M.; Gustin, Jacques ULg; Stewart, Ian et al

in Geophysical Research Letters (2008)

We present a spectral analysis of the far ultraviolet (FUV: 1150–1900 A ° ) disk airglow observations of Titan’s atmosphere by the Cassini Ultraviolet Imaging Spectrograph (UVIS). The FUV spectrum ... [more ▼]

We present a spectral analysis of the far ultraviolet (FUV: 1150–1900 A ° ) disk airglow observations of Titan’s atmosphere by the Cassini Ultraviolet Imaging Spectrograph (UVIS). The FUV spectrum consists of emissions from the Lyman-Birge-Hopfield (LBH) band system of N2 excited by photoelectrons (a 1Pg ! X 1Sg+), N I multiplets from solar photodissociative excitation of N2, resonantly scattered solar H Ly-a and sunlight reflected by N2 in the mesosphere-stratosphere and modified by aerosols (e.g.,tholins) and hydrocarbon absorption. Below 1450 A, the strongest emissions arise from H Ly-a with an intensity of 208 Rayleighs (R), LBH bands with an intensity of 43 R, and the N I multiplets with a combined intensity of 16 R. Above 1450 A , most of the UVIS signal is due to reflected sunlight. Mixing ratios of tholins, C2H2, C2H4 and C4H2 have been derived from the reflected sunlight using a Rayleigh scattering model. The derived mixing ratios are in good agreement with Voyager infrared observations and with FUV photochemical models, assuming solar energy deposition above 1450 A occurs near 250 km (Wilson and Atreya, 2004). We also present the first geometric albedo measurement of Titan from 1500–1900 A [less ▲]

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See detailAuroral evidence of a localized magnetic anomaly in Jupiter's northern hemisphere
Grodent, Denis ULg; Bonfond, Bertrand ULg; Gérard, Jean-Claude ULg et al

in Journal of Geophysical Research (2008), 113(A9),

We analyze more than 1000 HST/Advanced Camera for Survey images of the ultraviolet auroral emissions appearing in the northern hemisphere of Jupiter. The auroral footprints of Io, Europa, and Ganymede ... [more ▼]

We analyze more than 1000 HST/Advanced Camera for Survey images of the ultraviolet auroral emissions appearing in the northern hemisphere of Jupiter. The auroral footprints of Io, Europa, and Ganymede form individual footpaths, which are fitted with three reference contours. The satellite footprints provide a convenient mapping between the northern Jovian ionosphere and the equatorial plane in the middle magnetosphere, independent of any magnetic field model. The VIP4 magnetic field model is in relatively good agreement with the observed footprint of Io. However, in the auroral kink sector, between the 80 degrees and 150 degrees System III meridians, the model significantly departs from the observation. One possible way to improve the agreement between the VIP4 model and the observed footprints is to include a magnetic anomaly. We suggest that this anomaly is characterized by a weakening of the surface magnetic field in the kink sector and by an added localized tilted dipole field. This dipole rotates with the planet at a depth of 0.245 R-J below the surface, and its magnitude is set to similar to 1% of Jupiter's dipole moment. The anomaly has a very limited influence on the magnetic field intensity in the equatorial plane between the orbits of Io and Ganymede. However, it is sufficient to bend the field lines near the high-latitude atmosphere and to reproduce the observed satellite ultraviolet footpaths. JUNO's in situ measurements will determine the structure of Jupiter's magnetic field in detail to expand on these results. [less ▲]

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See detailTitan Airglow Spectra from Cassini UVIS
Ajello, J. M.; Gustin, Jacques ULg; Stevens, M. et al

Conference (2007, December 01)

We present the first UV airglow observations of Titan's atmosphere by the Ultraviolet Imaging Spectrograph (UVIS) on Cassini. The known UV emissions of Titan have been examined with higher spectral ... [more ▼]

We present the first UV airglow observations of Titan's atmosphere by the Ultraviolet Imaging Spectrograph (UVIS) on Cassini. The known UV emissions of Titan have been examined with higher spectral resolution (0.4 nm FWHM) by the Cassini UVIS than in the past by the Voyager Ultraviolet Spectrometer (UVS) (3.0 nm FWHM). The UVIS observations, confirming Voyager UVS results, have shown that molecular nitrogen is the major constituent of the upper atmosphere of Titan. Using one spectral channel in the EUV from 56--118 nm and one in the FUV from 112--191 nm, the UVIS observed the disk on 13 December, 2004. The combined EUV and FUV spectral region is a probe of the stratosphere-mesosphere-thermosphere-exosphere region from about 300--2000 km. The EUV spectrum consists of three band systems of N2 (b {1}Πu, b' {1}Σu+, c{4'} {1}Σ{u+}--X {1}Σ{g+}), while the FUV spectrum consists of one N2 (a {1}Πg--X {1}Σ{g+}). The UVIS observations reveal that the c{4'}(0)--X(0) vibrational band near 95.8 nm is suppressed, and that N \sc{i} multiplets near 95.32 and 96.45 nm are present instead. Magnetospheric particle excitation is weak on this orbit, since the nightside EUV spectrum shows no observable N2 emission features and only H Lyman-β. The absence of significant darkside emission demonstrates that nitrogen emissions are predominantly excited on this orbit by photoelectrons near 900 km in the thermosphere. Above 145 nm most of the observed signal is due to sunlight reflected by N2 in the Titan mesosphere-stratosphere and modified by aerosol and hydrocarbon absorption. Mixing ratios of C2H2, C4H2, C2H4 and tholins have been derived from the reflected sunlight emission between 145 and 190 nm, using a Rayleigh scattering model. Assuming that the energy deposition at these wavelengths occurs near 300 km, these mixing ratios are in good agreement with recent photochemical models and previous Voyager observations in the IR. We also present the first geometric albedo measurement of Titan from 150--190 nm. [less ▲]

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See detailTitan airglow spectra from Cassini Ultraviolet Imaging Spectrograph (UVIS): EUV analysis
Ajello, Joseph M.; Stevens, Michael H.; Stewart, Ian et al

in Geophysical Research Letters (2007), 34

We present the first UV airglow observations of Titan's atmosphere by the Ultraviolet Imaging Spectrograph (UVIS) on Cassini. Using one spectral channel in the EUV from 561-1182 Å and one in the FUV from ... [more ▼]

We present the first UV airglow observations of Titan's atmosphere by the Ultraviolet Imaging Spectrograph (UVIS) on Cassini. Using one spectral channel in the EUV from 561-1182 Å and one in the FUV from 1115-1913 Å, UVIS observed the disk on 13 December, 2004 at low solar activity. The EUV spectrum consists of three band systems of N[SUB]2[/SUB] (b [SUP]1[/SUP]∏[SUB]u[/SUB], b' [SUP]1[/SUP]∑[SUB]u[/SUB] [SUP]+[/SUP], c[SUB]4[/SUB]' [SUP]1[/SUP]∑[SUB]u[/SUB] [SUP]+[/SUP] -> X [SUP]1[/SUP]∑[SUB]g[/SUB] [SUP]+[/SUP]), while the FUV spectrum consists of one (a [SUP]1[/SUP]∏[SUB]g[/SUB] -> X [SUP]1[/SUP]∑[SUB]g[/SUB] [SUP]+[/SUP]). Both the EUV and FUV spectra contain many N I and N II multiplets that are produced primarily by photodissociative ionization. Spectral intensities of the N[SUB]2[/SUB] c[SUB]4[/SUB]' [SUP]1[/SUP]∑[SUB]u[/SUB] [SUP]+[/SUP](v' = 0) -> X [SUP]1[/SUP]∑[SUB]g[/SUB] [SUP]+[/SUP](v'' = 0-2) progression from 950-1010 Å are resolved for the first time. The UVIS observations reveal that the c[SUB]4[/SUB]' [SUP]1[/SUP]∑[SUB]u[/SUB] [SUP]+[/SUP](0) -> X [SUP]1[/SUP]∑[SUB]g[/SUB] [SUP]+[/SUP] (0) vibrational band near 958 Å is weak and undetectable, and that N I multiplets near 953.2 and 964.5 Å are present instead. Magnetospheric particle excitation may be weak or sporadic, since the nightside EUV spectrum on this orbit shows no observable nitrogen emission features and only H Ly-β. [less ▲]

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See detailAuroral Movies and Spectroscopy from Cassini UVIS
Pryor, W. R.; West, R.; Stewart, I. et al

Conference (2007, December 01)

Cassini's Ultraviolet Imaging Spectrograph (UVIS) has completed three years of study of Saturn's atmosphere and auroras. Two long slit spectral channels are used to obtain EUV data from 56.3-118.2 nm and ... [more ▼]

Cassini's Ultraviolet Imaging Spectrograph (UVIS) has completed three years of study of Saturn's atmosphere and auroras. Two long slit spectral channels are used to obtain EUV data from 56.3-118.2 nm and FUV data from 111.5-191.3 nm. 64 spatial pixels along each slit are combined with slit motion to construct spectral images of Saturn. Auroral emissions are seen from electron-excited molecular and atomic hydrogen. In 2007 UVIS obtained data with the spacecraft well out of Saturn's ring plane, permitting us to create images, spectra, and at times movies. We will present an auroral movie from 2007-145 that has been processed to remove flat-fielding artifacts and deconvolved to remove scattering along the slit. The movie shows near co- rotation of N polar auroral features with the planet's rotation. An auroral oval is present. The oval appears doubled on the midnight side. Other images from this year show emissions inside the auroral oval. We will discuss these images and their spectra. Additional images and movies are planned in coming months. [less ▲]

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