References of "Clarke, John"
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See detailOpen flux in Saturn's magnetosphere
Badman, Sarah; Jackman, Caitriuna; Nichols, Jonathan et al

in Icarus (2014), 231

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See detailEvolution of the Io footprint brightness I: Far-UV observations
Bonfond, Bertrand ULg; Hess, Sébastien; Gérard, Jean-Claude ULg et al

in Planetary and Space Science (2013), 88

The Io footprint (IFP) is a set of auroral spots and an extended tail resulting from the strong interaction between Io and the Jovian magnetosphere. For the first time, we present measurements of the ... [more ▼]

The Io footprint (IFP) is a set of auroral spots and an extended tail resulting from the strong interaction between Io and the Jovian magnetosphere. For the first time, we present measurements of the brightness and precipitated power for each individual spot, using the image database gathered from 1997 to 2009 with the Hubble Space Telescope in the Far-UV domain. We show that the relative brightness of the spots varies with the System III longitude of Io. Moreover, our novel measurement method based on 3D simulations of the auroral features allows to derive the precipitated energy fluxes from images on which the emission region is observed at a slant angle. Peak values as high as 2 W/m² are observed for the main spot, probably triggering a localized and sudden heating of the atmosphere. Additionally, strong brightness differences are observed from one hemisphere to another. This result indicates that the location of Io in the plasma torus is not the only parameter to control the brightness, but that the magnetic field asymmetries also play a key role. Finally, we present new data confirming that significant variations of the spots' brightness on timescales of 2-4 minutes are ubiquitous, which suggests a relationship with intermittent double layers close to Jovian surface. [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 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 detailLikely Detection of UV Auroral Emission from the Magnetic Footprint of Callisto
Clarke, John; Bhattacharyya, Dolon; Montgomery, Jordan et al

Poster (2012, December 06)

A large number of UV images of Jupiter's aurora were obtained in 2007/2008 with the Hubble Space Telescope (HST) ACS/SBC camera. The initial results on variations with the solar wind conditions have been ... [more ▼]

A large number of UV images of Jupiter's aurora were obtained in 2007/2008 with the Hubble Space Telescope (HST) ACS/SBC camera. The initial results on variations with the solar wind conditions have been published elsewhere, but the large database permits other studies to be performed. In particular, while auroral emissions have been detected from the magnetic footprints of Io, Europa, and Ganymede in Jupiter's atmosphere, the footprint of Callisto has been located too close to the main auroral oval to be detected. We have thus analyzed images of the ultraviolet auroral emissions of Jupiter taken using the F115LP filter on the HST/ACS instrument. Using a unique co-addition method, we have identified a strong candidate for the footprint of Callisto on May 24, 2007. We tested this finding by applying the same co-addition method to a nearly identical auroral configuration on May 30, 2007 when Callisto was well removed in its orbit. Comparing the two co-added images, we can clearly see the presence of Callisto’s footprint on the 24th and its absence on the 30th. The method relies as well on the motion of Callisto's footprint remaining under the satellite, while most of the auroral emissions rotate with the planet. The images and analysis method will be presented in this presentation. [less ▲]

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