References of "Gustin, Jacques"
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See detailVariability of the Jovian aurorae: focus on a selection of recent results
Bonfond, Bertrand ULg; Grodent, Denis ULg; Gérard, Jean-Claude ULg et al

Conference (2013, July 11)

The aurorae at Jupiter can be separated into four main components: the satellite footprints, the outer emissions, the main emissions and the polar emissions. Each of these components displays some form of ... [more ▼]

The aurorae at Jupiter can be separated into four main components: the satellite footprints, the outer emissions, the main emissions and the polar emissions. Each of these components displays some form of variability in location, brightness and/or shape. The nature and the timescale of these changes is particularly revealing of the processes at play. The footprints of Io and Ganymede are often made of several spots. The distance between these spots and their brightness essentially varies as the planetary magnetic dipole rotates relative to the moons and as the plasma torus or plasma sheet wobble across the satellite orbital plane. However, the spots brightness can also considerably vary on a timescale of minutes as well as from one year to another. The outer emissions are made of diffuse, patchy or arc-shaped emissions. Two different sources have been proposed to explain these features: injections of hot plasma from the outer magnetosphere and the pitch angle scattering boundary. These features usually last for a few Jovian rotations, but their occurrence rate appears to be related to the global dynamics of the inner magnetosphere on timescales of months. The main emissions sometimes appear as a complete oval, but they usually have a more chaotic appearance, with broken arcs, gaps and forks. Their brightness and morphology respond to changes in the solar wind characteristics. Nevertheless, the dawn portion of the main emissions sometimes displays spectacular brightening apparently unrelated to the solar wind: the dawn storms. Moreover, on timescales of several months, the statistical location of the main emissions also evolves as the material input from Io increases or decreases. Globally speaking, the polar emissions also respond to the solar wind input. However, the term “polar emissions” encompasses many different auroral features obviously driven from different mechanisms. Spots and arcs, located just inside the main emissions on the dawn and night side and lasting for a few tens of minutes, have been seen to re-occur every 2 to 3 days. They have thus been associated with night-side reconnection related to the Vasyliũnas cycle. On the other hand, the dusk-side of the polar region is the locus of quasi-periodic UV flares on timescales of 2 to 3 minutes, while periodicities of 20 to 45 minutes have been identified for their X-ray counterpart. The central part of the polar region is very dynamic, with patches of emissions constantly appearing, moving and disappearing within minutes. However, along with these patches, elongated auroral arcs dubbed “polar auroral filaments” may remain present for several consecutive days. As we will see in this review talk, the current data set of UV images from the Hubble Space Telescope, including the brand new time-tag sequences from the latest 2012-2013 campaign, gives access to a wide range of auroral phenomena that only begin to reveal their secrets. [less ▲]

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See detailJupiter’s elusive bald patch
Grodent, Denis ULg; Bonfond, Bertrand ULg; Gustin, Jacques ULg et al

Conference (2013, July)

The detailed morphology of Jupiter’s UV auroral emissions is definitely very complex. To some extent, this complexity depicts the zoo of processes taking place inside, and sometimes, outside Jupiter’s ... [more ▼]

The detailed morphology of Jupiter’s UV auroral emissions is definitely very complex. To some extent, this complexity depicts the zoo of processes taking place inside, and sometimes, outside Jupiter’s enormous magnetosphere. One is naturally more inclined to focus on the bright emissions, but recent progresses in cosmology teach us that there is also important information in the darkness. In this present, preliminary study, we are exploring a dark region of Jupiter’s polar aurora -“Jupiter’s bald patch”- located poleward of the main emission (oval). It appears to be bordered by patchy features belonging to auroral regions often referred to as the swirl and flare regions. These regions contain the poleward most auroral features. Therefore, it is legitimate to ask whether this dark region, even closer to the pole, is actually the polar cap, implying some level of reconnection of Jupiter’s strong magnetic field with the interplanetary magnetic field. An ongoing HST campaign is providing stunning high temporal and spatial (and spectral) resolution time tagged images of Jupiter’s northern and southern aurora. They show that the bald patch is conspicuous on some images but much less obvious in others. They also suggest that it is not always completely devoid of emission, possibly alluding to a weak, intermittent, Dungey-like cycle. [less ▲]

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See detailStatistical analysis of Saturn's UV auroral outer emission
Grodent, Denis ULg; Radioti, Aikaterini ULg; Schippers, Patricia et al

Conference (2013, March 15)

Recent observations of Saturn’s aurora with the UVIS spectrograph on-board Cassini not only confirm the presence of a quasi-permanent partial ring of emission equatorward of the main auroral oval, but ... [more ▼]

Recent observations of Saturn’s aurora with the UVIS spectrograph on-board Cassini not only confirm the presence of a quasi-permanent partial ring of emission equatorward of the main auroral oval, but they also increase the number of positive cases and allow for a statistical analysis of the characteristics of this outer emission. This faint but distinct auroral feature appears at both hemispheres in the nightside sector. It magnetically maps to relatively large distances in the nightside magnetosphere, on the order of 9 RS. It was initially thought that pitch angle scattering of electrons into the loss cone by whistler waves would be responsible for the outer auroral emission. Rough estimates suggested that a suprathermal electron population observed with Cassini in the nightside sector between 7 and 10 RS might power this process. However, a new analysis of 7 years of Cassini electron plasma data indicates the presence of layers of upward and downward field aligned currents. They appear to be part of a large-scale current system involving dayside-nightside asymmetries as well as trans- hemispheric variations. This system comprises a net upward current layer, carried by warm electrons, limited to the nightside sector which may as well generate the outer UV auroral emission. The growing dataset of UVIS spectro- images is used to find any such asymmetries in the outer auroral emission. [less ▲]

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See detailRemote sensing of the energy of auroral electrons in Saturn’s atmosphere: Hubble and Cassini spectral observations
Gérard, Jean-Claude ULg; Gustin, Jacques ULg; Pryor, Wayne et al

in Icarus (2013), 223

Saturn’s north ultraviolet aurora has been successfully observed twice between March and May 2011 with the STIS long-slit spectrograph on board the Hubble Space Telescope. Spatially resolved spectra at ... [more ▼]

Saturn’s north ultraviolet aurora has been successfully observed twice between March and May 2011 with the STIS long-slit spectrograph on board the Hubble Space Telescope. Spatially resolved spectra at ∼12 Å spectral resolution have been collected at different local times from dawn to dusk to determine the amount of hydrocarbon absorption. For this purpose, the HST telescope slewed across the auroral oval from mid-latitudes up to beyond the limb while collecting spectral data in the timetag mode. Spectral images of the north ultraviolet aurora were obtained within minutes and hours with the UVIS spectrograph on board Cassini. Several daytime sectors and one nightside location were observed and showed signatures of weak absorption by methane present in (or above) the layer of the auroral emission. No absorption from other hydrocarbons (e.g. C2H2) has been detected. For the absorbed spectra, the overlying slant CH4 column varies from 3x1015 to 2x1016 cm-2, but no clear dependence on local time is identified. A Monte Carlo electron transport model is used to calculate the vertical distribution of the H2 emission and to relate the observed spectra to the energy of the primary auroral electrons. Assuming electron precipitation with a Maxwellian energy distribution into a standard model atmosphere, we find that the mean energy ranges from less than 3 to ∼10 keV. These results are compared with previous determinations of the energy of Saturn’s aurora based on ultraviolet spectra and limb images. We conclude that the energies derived from spectral methods indicate a wide range of electron energies while the nightside limb images suggest that the auroral precipitation is consistently soft. We emphasize the need for more realistic model atmospheres with temperature and hydrocarbon distributions appropriate to high-latitude conditions. [less ▲]

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See detailSignatures of magnetospheric injections in Saturn's aurora
Radioti, Aikaterini ULg; Roussos, E.; Grodent, Denis ULg et al

in Journal of Geophysical Research. Space Physics (2013)

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See detailAuroral signatures of multiple magnetopause reconnection at Saturn
Radioti, Aikaterini ULg; Grodent, Denis ULg; Gérard, Jean-Claude ULg et al

in Geophysical Research Letters (2013)

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See detailUnraveling electron acceleration mechanisms in Ganymede's space environment through N-S conjugate imagery of Jupiter's aurora
Grodent, Denis ULg; Bonfond, Bertrand ULg; Gérard, Jean-Claude ULg et al

E-print/Working paper (2013)

There is strong scientific interest in Ganymede (Jupiter's third Galilean moon) and its surrounding environment, which stems from the likely presence of a liquid water ocean underneath its icy crust and ... [more ▼]

There is strong scientific interest in Ganymede (Jupiter's third Galilean moon) and its surrounding environment, which stems from the likely presence of a liquid water ocean underneath its icy crust and from its internally driven magnetic field. The interaction of the latter with Jupiter's magnetospheric plasma and its magnetic field gives rise to a unique situation in our solar system implying a mini-magnetosphere embedded within a giant-magnetosphere. This interaction generates Ganymede's ultraviolet auroral footprint in Jupiter's atmosphere. We propose to investigate the strong auroral connection between Jupiter and Ganymede and the variable characteristics of Ganymede's magnetosphere with an innovative approach, taking advantage of the large scale north-south asymmetries of Jupiter's magnetic field. The results obtained for Ganymede will be compared with the case of small injected hot plasma bubbles observed by the Galileo spacecraft and whose size and location are similar to those of Ganymede's magnetosphere. HST is currently the sole instrument capable of obtaining this information which pins down the proposed mechanisms linking the source and sink regions of auroral particles in the giant planets' magnetospheres. [less ▲]

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See detailEffects of methane on giant planet’s UV emissions and implications for the auroral characteristics
Gustin, Jacques ULg; Gérard, Jean-Claude; Grodent, Denis ULg et al

in Journal of Molecular Spectroscopy (2013)

This study reviews methods used to determine important characteristics of giant planet’s UV aurora (brightness,energy of the precipitating particles, altitude of the emission peak,. . .), based on the ... [more ▼]

This study reviews methods used to determine important characteristics of giant planet’s UV aurora (brightness,energy of the precipitating particles, altitude of the emission peak,. . .), based on the absorbing properties of methane and other hydrocarbons. Ultraviolet aurorae on giant planets are mostly caused by inelastic collisions between energetic magnetospheric electrons and the ambient atmospheric H2 molecules. The auroral emission is situated close to a hydrocarbon layer and may be attenuated by methane (CH4), ethane (C2H6) and acetylene (C2H2) at selected wavelengths. As methane is the most abundant hydrocarbon, it is the main UV absorber and attenuates the auroral emission shorward of 1350 Å. The level of absorption is used to situate the altitude/pressure level of the aurora, hence the energy of the precipitated electrons, whose penetration depth is directly related to their mean energy. Several techniques are used to determine these characteristics, from the color ratio method which measures the level of absorption from the ratio between an absorbed and an unabsorbed portion of the observed auroral spectrum, to more realistic methods which combine theoretical distributions of the precipitating electrons with altitude dependent atmospheric models. The latter models are coupled with synthetic or laboratory H2 spectra and the simulated emergent spectra are compared to observations to determine the best auroral characteristics. Although auroral characteristics may be very variable with time and locations, several typical properties may be highlighted from these methods: the Jovian aurora is the most powerful, with brightness around 120 kR produced by electrons of mean energy 100 keV and an emission situated near the 1 lbar level ( 250 km above the 1 bar level) while Saturn’s aurora is fainter ( 10 kR), produced by electrons less than 20 keV and situated near the 0.2 lbar level ( 1100 km). [less ▲]

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See detailCassini UVIS observations of Titan nightglow spectra
Ajello, Joseph M.; West, Robert A.; Gustin, Jacques ULg et al

in Journal of Geophysical Research. Space Physics (2012), 117

In this paper we present the first nightside EUV and FUV airglow limb spectra of Titan showing molecular emissions. The Cassini Ultraviolet Imaging Spectrograph (UVIS) observed photon emissions of Titan's ... [more ▼]

In this paper we present the first nightside EUV and FUV airglow limb spectra of Titan showing molecular emissions. The Cassini Ultraviolet Imaging Spectrograph (UVIS) observed photon emissions of Titan's day and night limb-airglow and disk-airglow on multiple occasions, including during an eclipse observation. The 71 airglow observations analyzed in this paper show EUV (600-1150 Å) and FUV (1150-1900 Å) atomic multiplet lines and band emissions arising from either photoelectron induced fluorescence and solar photo-fragmentation of molecular nitrogen (N[SUB]2[/SUB]) or excitation by magnetosphere plasma. The altitude of the peak UV emissions on the limb during daylight occurred inside the thermosphere at the altitude of the topside ionosphere (near 1000 km altitude). However, at night on the limb, a subset of emission features, much weaker in intensity, arise in the atmosphere with two different geometries. First, there is a twilight photoelectron-excited glow that persists with solar depression angle up to 25-30 degrees past the terminator, until the solar XUV shadow height passes the altitude of the topside ionosphere (1000-1200 km). The UV twilight glow spectrum is similar to the dayglow but weaker in intensity. Second, beyond 120° solar zenith angle, when the upper atmosphere of Titan is in total XUV darkness, there is indication of weak and sporadic nightside UV airglow emissions excited by magnetosphere plasma collisions with ambient thermosphere gas, with similar N[SUB]2[/SUB] excited features as above in the daylight or twilight glow over an extended altitude range. [less ▲]

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See detailUltraviolet Auroral Pulsations on Saturn from Cassini UVIS
Pryor, Wayne R.; Esposito, L. W.; Jouchoux, A. et al

in AAS/Division for Planetary Sciences Meeting Abstracts (2012, October 01)

Cassini Ultraviolet Imaging Spectrograph (UVIS) observations of Saturn were obtained on 2009 days 278-280 with the UVIS long slit aligned east-west along the northern auroral oval. Bright quasi-periodic ... [more ▼]

Cassini Ultraviolet Imaging Spectrograph (UVIS) observations of Saturn were obtained on 2009 days 278-280 with the UVIS long slit aligned east-west along the northern auroral oval. Bright quasi-periodic localized bursts of UV emission were often observed with 1 hour spacing that slowly moved sub-corotationally along the main auroral arc. We will report on an apparent correlation of the bursts with the locations of Saturn's moons, and a search for other such examples. We will also compare the UVIS results with simultaneous Cassini Imaging Science Subsystem (ISS) auroral images. [less ▲]

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See detailOn the origin of Saturn's polar auroral arcs
Radioti, Aikaterini ULg; Grodent, Denis ULg; Gérard, Jean-Claude ULg et al

Conference (2012, September 27)

Saturn’s main auroral emission similarly to Earth’s is suggested to be associated with the open-closed field line boundary. The polar auroral emissions at Saturn, emissions located poleward of the main ... [more ▼]

Saturn’s main auroral emission similarly to Earth’s is suggested to be associated with the open-closed field line boundary. The polar auroral emissions at Saturn, emissions located poleward of the main emission consist of several arc-like and spotty features. In this study we focus on the arc-like structures observed with the UVIS instrument onboard Cassini and we characterize them into three groups: ‘bending arcs’ arcs whose one end is connected to the main emision, ‘oval aligned arcs’ arcs oriented parallel to the main emission and ‘moving arcs’ arcs which move with time inside the main emission. We study their occurrence rate, location, size as well as their associated expansion or contraction of the main emission. Finally, we compare the auroral arcs at Saturn with those in the terrestrial aurora and we examine their relation to a combination of solar wind parameters such as northward IMF, strong IMF magnitude and high solar wind speed. [less ▲]

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See detailConversion from HST ACS and STIS auroral counts into brightness, precipitated power, and radiated power for H2 giant planets
Gustin, Jacques ULg; Bonfond, Bertrand ULg; Grodent, Denis ULg et al

in Journal of Geophysical Research. Space Physics (2012), 117

The STIS and ACS instruments onboard HST are widely used to study the giant planet's aurora. Several assumptions have to be made to convert the instrumental counts into meaningful physical values (type ... [more ▼]

The STIS and ACS instruments onboard HST are widely used to study the giant planet's aurora. Several assumptions have to be made to convert the instrumental counts into meaningful physical values (type and bandwidth of the filters, definition of the physical units, etc…), but these may significantly differ from one author to another, which makes it difficult to compare the auroral characteristics published in different studies. We present a method to convert the counts obtained in representative ACS and STIS imaging modes/filters used by the auroral scientific community to brightness, precipitated power and radiated power in the ultraviolet (700-1800 Å). Since hydrocarbon absorption may considerably affect the observed auroral emission, the conversion factors are determined for several attenuation levels. Several properties of the auroral emission have been determined: the fraction of the H[SUB]2[/SUB] emission shortward and longward of the HLy-α line is 50.3% and 49.7% respectively, the contribution of HLy-α to the total unabsorbed auroral signal has been set to 9.1% and an input of 1 mW m[SUP]-2[/SUP] produces 10 kR of H[SUB]2[/SUB] in the Lyman and Werner bands. A first application sets the order of magnitude of Saturn's auroral characteristics in the total UV bandwidth to a brightness of 10 kR and an emitted power of ˜2.8 GW. A second application uses published brightnesses of Europa's footprint to determine the current density associated with the Europa auroral spot: 0.21 and 0.045 μA m[SUP]-2[/SUP] assuming no hydrocarbon absorption and a color ratio of 2, respectively. Factors to extend the brightnesses observed with Cassini-UVIS to total H[SUB]2[/SUB] UV brightnesses are also provided. [less ▲]

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