The aurorae at Jupiter are made up of many different features associated with a variety of genera... more The aurorae at Jupiter are made up of many different features associated with a variety of generation mechanisms. The main auroral emission, also known as the main oval, is the most prominent of them as it accounts for approximately half of the total power emitted by the aurorae in the ultraviolet range. The energy of the precipitating electrons is a crucial parameter to characterize the processes at play which give rise to these auroral emissions, and the altitude of the emissions directly depends on this energy. Here we make use of far-UV (FUV) images acquired with the Advanced Camera for Surveys on board the Hubble Space Telescope and spectra acquired with the Space Telescope Imaging Spectrograph to measure the vertical profile of the main emissions. The altitude of the brightness peak as seen above the limb is ∼ 400 km, which is significantly higher than the 250 km measured in the post-dusk sector by Galileo in the visible domain. However, a detailed analysis of the effect of hydrocarbon absorption, including both simulations and FUV spectral observations, indicates that FUV apparent vertical profiles should be considered with caution, as these observations are not incompatible with an emission peak located at 250 km. The analysis also calls for spectral observations to be carried out with an optimized geometry in order to remove observational ambiguities.
On 21 April 2013, during a coordinated Saturn auroral observing campaign, the northern and southe... more On 21 April 2013, during a coordinated Saturn auroral observing campaign, the northern and southern poles of the planet were observed from the NASA Infrared Telescope Facility (IRTF), Keck, and Hubble Space Telescope (HST) simultaneously with the Cassini infrared, visible, and ultraviolet remote sensing instruments. We present simultaneous multi-scale and mutli-wavelength analysis of the morphology of auroral emissions at Saturn. There is variability on every spatial and temporal scale analysed, and high spatial resolution observations distinguish variable features with sizes tens of km on the planet. The fine scale temporal and spatial features seen in the main oval itself show that complex structures present even during quiet solar wind conditions. We observe diffuse ultraviolet emissions southward of the southern midnight main oval that are not seen in the infrared. This implies a steep temperature gradient, ∼60 K over 2-4 • latitude equator wad of the main oval. Lower spatial resolution observations reveal broad-scale dynamics at both poles, variable over minutes to hours, the morphology of which is only partly consistent with a overlapping local-time fixed and co-rotating current systems. We also present the
UVIS observations of the FUV OI and CO 4P Venus dayglow during the Cassini flyby
Icarus, 2010
... The spectral range of UVIS extends from 111.5 to 191.3 nm in the FUV range, thus including th... more ... The spectral range of UVIS extends from 111.5 to 191.3 nm in the FUV range, thus including the very bright Lyman-α emission of atomic hydrogen as well as the CO A 1 Π → X 1 Σ + 4P band system and the OI emissions at 130.4 and 135.6 nm. ...
Ultraviolet images of Jupiter's northern aurora obtained in 2005 confirm the existence of an elec... more Ultraviolet images of Jupiter's northern aurora obtained in 2005 confirm the existence of an electromagnetic interaction between Europa and the Jovian ionosphere. The auroral signature shows a two-component structure: a quasi-circular Europa spot, followed by a previously undetected faint tail emission trailing in the direction of corotation flow. The characteristic brightness for the auroral spot is $14 ± 1 kR above background, and approximately 7 ± 1 kR for the tail. The spot's size is $1100 km, magnetically mapping to an interaction region 15 Europa diameters. The auroral tail extends over $5000 km, which maps along a region of at least 70 Europa diameters. The ultraviolet power emitted by both components varies from a fraction to several GW. The present study suggests auroral interaction at Europa similar to that at Io, but scaled-down by an order of magnitude, including a subcorotating plasma plume in the geometrical wake of Europa.
Journal of Geophysical Research: Space Physics, 2012
The STIS and ACS instruments onboard HST are widely used to study the giant
planet’s aurora. Sev... 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 H2 emission shortward and longward of the HLy-a line is 50.3% and
49.7% respectively, the contribution of HLy-a to the total unabsorbed auroral signal has
been set to 9.1% and an input of 1 mW m À2 produces 10 kR of H2 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 mA m À2 assuming no hydrocarbon absorption and a color ratio of 2, respectively. Factors to extend the brightnesses observed with Cassini-UVIS to total H2 UV brightnesses are also provided.
Thousands of exoplanets have now been discovered with a huge range of masses, sizes and orbits: f... more Thousands of exoplanets have now been discovered with a huge range of masses, sizes and orbits: from rocky Earth-like planets to large gas giants grazing the surface of their host star. However, the essential nature of these exoplanets remains largely mysterious: there is no known, discernible pattern linking the presence, size, or orbital parameters of a planet to the nature of its parent star. We have little idea whether the chemistry of a planet is linked to its formation environment, or whether the type of host star drives the physics and chemistry of the planet's birth, and evolution. ARIEL was conceived to observe a large number (~1000) of transiting planets for statistical understanding, including gas giants, Neptunes, super-Earths and Earth-size planets around a range of host star types using transit spectroscopy in the 1.25-7.8 μm spectral range and multiple narrow-band photometry in the optical. ARIEL will focus on warm and hot planets to take advantage of their well-mixed atmospheres which should show minimal condensation and sequestration of high-Z materials compared to their colder Solar System siblings. Said warm and hot atmospheres are expected to be more representative of the planetary bulk composition. Observations of these warm/hot exoplanets, and in particular of their elemental composition (especially C, O, N, S, Si), will allow the understanding of the early stages of planetary and atmospheric formation during the nebular phase and the following few million years. ARIEL will thus provide a representative picture of the chemical nature of the exoplanets and relate this directly to the type and chemical environment of the host star. ARIEL is designed as a dedicated survey mission for combined-light spectroscopy, capable of observing a large and welldefined planet sample within its 4-year mission lifetime. Transit, eclipse and phasecurve spectroscopy methods, whereby the signal from the star and planet are differentiated using knowledge of the planetary ephemerides, allow us to measure atmospheric signals from the planet at levels of 10-100 part per million (ppm) relative to the star and, given the bright nature of targets, also allows more sophisticated techniques, such as eclipse mapping, to give a deeper insight into the nature of the atmosphere. These types of observations require a stable payload and satellite platform with broad, instantaneous wavelength coverage to detect many molecular species, probe the thermal structure, identify clouds and monitor the stellar activity. The wavelength range proposed covers all the expected major atmospheric gases from e.g. H 2 O, CO 2 , CH 4 NH 3 , HCN, H 2 S through to the more exotic metallic compounds, such as TiO, VO, and condensed species. Simulations of ARIEL performance in conducting exoplanet surveys have been performedusing conservative estimates of mission performance and a
Cassini UVIS Observations of Saturn during the Grand Finale Orbits
AGUFM, Dec 1, 2017
In 2016 and 2017, the Cassini Saturn orbiter executed a final series of high inclination, low-per... more In 2016 and 2017, the Cassini Saturn orbiter executed a final series of high inclination, low-periapsis orbits ideal for studies of Saturn's polar regions. The Cassini Ultraviolet Imaging Spectrograph (UVIS) obtained an extensive set of auroral images, some at the highest spatial resolution obtained during Cassini's long orbital mission (2004-2017). In some cases, two or three spacecraft slews at right angles to the long slit of the spectrograph were required to cover the entire auroral region to form auroral images. We will present selected images from this set showing narrow arcs of emission, more diffuse auroral emissions, multiple auroral arcs in a single image, discrete spots of emission, small scale vortices, large-scale spiral forms, and parallel linear features that appear to cross in places like twisted wires. Some shorter features are transverse to the main auroral arcs, like barbs on a wire. UVIS observations were in some cases simultaneous with auroral observations from the Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS) that will also be presented. UVIS polar images also contain spectral information suitable for studies of the auroral electron energy distribution. The long wavelength part of the UVIS polar images contains a signal from reflected sunlight containing absorption signatures of acetylene and other Saturn hydrocarbons. The hydrocarbon spatial distribution will also be examined
Abstract. From the 27th to the 28th January 2009, the Cassini spacecraft remotely acquired combin... more Abstract. From the 27th to the 28th January 2009, the Cassini spacecraft remotely acquired combined observations of Saturn’s southern aurorae at radio, ultraviolet and infrared wavelengths, while monitoring ion injections in the middle magnetosphere from energetic neutral atoms. Simultaneous measurements included the sampling of a full plan-etary rotation, a relevant timescale to investigate auroral emissions driven by processes internal to the magnetosphere. In addition, this interval coincidently matched a power-ful substorm-like event in the magnetotail, which induced an overall dawnside intensi-fication of the magnetospheric and auroral activity. We comparatively analyze this unique set of measurements to reach a comprehensive view of kronian auroral processes over the investigated timescale. We identify three source regions in atmospheric aurorae, includ-ing a main oval associated with the bulk of Saturn Kilometric Radiation (SKR), together with polar and equatorward emissions....
The aurorae at Jupiter are made up of many different features associated with a variety of genera... more The aurorae at Jupiter are made up of many different features associated with a variety of generation mechanisms. The main auroral emission, also known as the main oval, is the most prominent of them as it accounts for approximately half of the total power emitted by the aurorae in the ultraviolet range. The energy of the precipitating electrons is a crucial parameter to characterize the processes at play which give rise to these auroral emissions, and the altitude of the emissions directly depends on this energy. Here we make use of far-UV (FUV) images acquired with the Advanced Camera for Surveys on board the Hubble Space Telescope and spectra acquired with the Space Telescope Imaging Spectrograph to measure the vertical profile of the main emissions. The altitude of the brightness peak as seen above the limb is ~ 400 km, which is significantly higher than the 250 km measured in the post-dusk sector by Galileo in the visible domain. However, a detailed analysis of the effect of hy...
High-resolution (∼ 0.22 Å) spectra of the north jovian aurora were obtained in the 905-1180 Å win... more High-resolution (∼ 0.22 Å) spectra of the north jovian aurora were obtained in the 905-1180 Å window with the Far Ultraviolet Spectroscopic Explorer (FUSE) on October 28, 2000. The FUSE instrument resolves the rotational structure of the H 2 spectra and the spectral range allows the study of self-absorption. Below 1100 Å, transitions connecting to the v 2 levels of the H 2 ground state are partially or totally absorbed by the overlying H 2 molecules. The FUSE spectra provide information on the overlying H 2 column and on the vibrational distribution of H 2 . Transitions from high-energy H 2 Rydberg states and treatment of self-absorption are considered in our synthetic spectral generator. We show comparisons between synthetic and observed spectra in the 920-970, 1030-1080, and 1090-1180 Å spectral windows. In a first approach (single-layer model ), the synthetic spectra are generated in a thin emitting layer and the emerging photons are absorbed by a layer located above the source. It is found that the parameters of the single-layer model best fitting the three spectral windows are 850, 800, and 800 K respectively for the H 2 gas temperature and 1.3 × 10 18 , 1.5 × 10 20 , and 1.3 × 10 20 cm -2 for the H 2 self-absorbing vertical column respectively. Comparison between the H 2 column and a 1-D atmospheric model indicates that the short-wavelength FUV auroral emission originates from just above the homopause. This is confirmed by the high H 2 rovibrational temperatures, close to those deduced from spectral analyses of H + 3 auroral emission. In a second approach, the synthetic spectral generator is coupled with a vertically distributed energy degradation model, where the only input is the energy distribution of incoming electrons (multi-layer model ). The model that best fits globally the three FUSE spectra is a sum of Maxwellian functions, with characteristic energies ranging from 1 to 100 keV, giving rise to an emission peak located at 5 µbar, that is ∼ 100 km below the methane homopause. This multi-layer model is also applied to a re-analysis of the Hopkins Ultraviolet Telescope (HUT) auroral spectrum and accounts for the H 2 self-absorption as well as the methane absorption. It is found that no additional discrete soft electron precipitation is necessary to fit either the FUSE or the HUT observations.
From the 27th to the 28th January 2009, the Cassini spacecraft remotely acquired combined observa... more From the 27th to the 28th January 2009, the Cassini spacecraft remotely acquired combined observations of Saturn's southern aurorae at radio, ultraviolet and infrared wavelengths, while monitoring ion injections in the middle magnetosphere from energetic neutral atoms. Simultaneous measurements included the sampling of a full planetary rotation, a relevant timescale to investigate auroral emissions driven by processes internal to the magnetosphere. In addition, this interval coincidently matched a powerful substorm-like event in the magnetotail, which induced an overall dawnside intensification of the magnetospheric and auroral activity. We comparatively analyze this unique set of measurements to reach a comprehensive view of kronian auroral processes over the investigated timescale. We identify three source regions in atmospheric aurorae, including a main oval associated with the bulk of Saturn Kilometric Radiation (SKR), together with polar and equatorward emissions. These observations reveal the coexistence of corotational and sub-corototational dynamics of emissions associated with the main auroral oval. Precisely, we show that the atmospheric main oval hosts short-lived sub-corotating isolated features together with a bright, longitudinally extended, corotating region locked at the southern SKR phase. We assign the susbtorm-like event to a regular, internally driven, nightside ion injection possibly triggerred by a plasmoid ejection. We also investigate the total auroral energy budget, from the power input to the atmosphere, characterized by precipitating electrons up to 20 keV, to its dissipation through the various radiating processes. Finally, through simulations, we confirm the searchlight nature of the SKR rotational modulation and we show that SKR arcs relate to isolated auroral spots. We characterize which radio sources are visible from the spacecraft with the fraction of visible southern power estimated to a few percent. The resulting findings are discussed in the frame of pending questions as the persistence of a corotating field-aligned current system within a sub-corotating magnetospheric cold plasma, the occurrence of plasmoid activity and the comparison of auroral fluxes radiated at different wavelengths.
Ultraviolet aurorae and dayglow in the upper atmospheres of terrestrial planets
Since its discovery in 2005 with the SPICAM spectrograph on board Mars Express, the Mars aurora h... more Since its discovery in 2005 with the SPICAM spectrograph on board Mars Express, the Mars aurora has been further investigated. It is caused by sporadic soft electron precipitation whose signature is clearly observed in the FUV nightglow spectrum. The characteristics of the auroral electrons have been documented with parallel observations. Dayglow UV spectra have been collected with SPICAM over several
We analyzed two observations obtained in Jan. 2013, consisting of spatial scans of the Jovian nor... more We analyzed two observations obtained in Jan. 2013, consisting of spatial scans of the Jovian north ultraviolet aurora with the HST Space Telescope Imaging Spectrograph (STIS) in the spectroscopic mode. The color ratio (CR) method, which relates the wavelength-dependent absorption of the FUV spectra to the mean energy of the precipitating electrons, allowed us to determine important characteristics of the entire auroral region. The results show that the spatial distribution of the precipitating electron energy is far from uniform. The morning main emission arc is associated with mean energies of around 265 keV, the afternoon main emission (kink region) has energies near 105 keV, while the 'flare' emissions poleward of the main oval are characterized by electrons in the 50-85 keV range. A small scale structure observed in the discontinuity region is related to electrons of 232 keV and the Ganymede footprint shows energies of 157 keV. Interestingly, each specific region shows very similar behavior for the two separate observations. The Io footprint shows a weak but undeniable hydrocarbon absorption, which is not consistent with altitudes of the Io emission profiles (~900 km relative to the 1 bar level) determined from HST-ACS observations. An upward shift of the hydrocarbon homopause of at least 100 km is required to reconcile the high altitude of the emission and hydrocarbon absorption. The relationship between the energy fluxes and the electron energies has been compared to curves obtained from Knight's theory of field-aligned currents. Assuming a fixed electron temperature of 2.5 keV, an electron source population density of ~800 m-3 and ~2400 m-3 is obtained for the morning main emission and kink regions, respectively. Magnetospheric electron densities are lowered for the morning main emission (~600 m-3) if the relativistic version of Knight's theory is applied. Lyman and Werner H 2 emission profiles resulting from secondary electrons, produced by precipitation of heavy ions in the 1-2 MeV/u range, have been applied to our model. The low CR
The discovery of almost two thousand exoplanets has revealed an unexpectedly diverse planet popul... more The discovery of almost two thousand exoplanets has revealed an unexpectedly diverse planet population. We see gas giants in few-day orbits, whole multi-planet systems within the orbit of Mercury, and new populations of planets with masses between that of the Earth and Neptune-all unknown in the Solar System. Observations to date have shown that our Solar System is certainly not representative of the general population of planets in our Milky Way. The key science questions that urgently need addressing are therefore: What are exoplanets made of? Why are planets as they are? How do planetary systems work and what causes the exceptional diversity observed as compared to the Solar System? The EChO (Exoplanet Characterisation Observatory) space mission was conceived to take up the challenge to explain this diversity in terms of formation, evolution, internal structure and planet and atmospheric composition. This requires in-depth spectroscopic knowledge of the atmospheres of a large and well-defined planet sample for which precise physical, chemical and dynamical information can be obtained. In order to fulfil this ambitious scientific program, EChO was designed as a dedicated survey mission for transit and eclipse spectroscopy capable of observing a large, diverse and well-defined planet sample within its 4-year mission lifetime. The transit and eclipse spectroscopy method, whereby the signal from the star and planet are differentiated using knowledge of the planetary ephemerides, allows us to measure atmospheric signals from the planet at levels of at least 10 −4 relative to the star. This can only be achieved in conjunction with a carefully designed stable payload and satellite platform. It is also necessary to provide broad instantaneous wavelength coverage to detect as many molecular species as possible, to probe the thermal structure of the planetary atmospheres and to correct for the contaminating effects of the stellar photosphere. This requires wavelength coverage of at least 0.55 to 11 μm with a goal of covering from 0.4 to 16 μm. Only modest spectral resolving power is needed, with R~300 for wavelengths less than 5 μm and R~30 for wavelengths greater than this. The transit
High color ratio and high temperature in Jupiter's auroral atmosphere
A high FUV color ratio usually implies that most of the energy of the impinging auroral particles... more A high FUV color ratio usually implies that most of the energy of the impinging auroral particles is deposited below the methane homopause. In this region, the resulting auroral heating is efficiently balanced by the strong hydrocarbon cooling. Therefore, this auroral process cannot sustain the high temperature observed in the Jovian auroral atmosphere. This work is an attempt to remove
We present the analysis of EUV spatially-resolved dayglow spectra obtained at 0.37 nm resolution ... more We present the analysis of EUV spatially-resolved dayglow spectra obtained at 0.37 nm resolution by the UVIS instrument during the Cassini flyby of Venus, a period of high solar activity level. Emissions from OI, OII, NI, CI and CII and CO have been identified and their disc average intensity has been determined. They are generally somewhat brighter than those determined from the observations made with the HUT spectrograph at a lower activity level, We analyze the brightness distribution along the foot track of the UVIS slit of the OII 83.4 nm, OI 98.9 nm, Lyman-ß + OI 115.2 nm and NI 120.0 nm multiplets, and the CO C-X and B-X Hopfield-Birge bands. We make a detailed comparison of the intensities of the 834 nm, 989 nm, 120.0 nm multiplets and CO B-X band measured along the slit foot track on the disc with those predicted by a detailed airglow model. This model includes the treatment of multiple scattering for the optically thick OI, OII and NI multiplets. It is found that the calculated intensity of the OII emission at 83.4 nm is somewhat larger and the limb brightening more pronounced than predicted by the model. The calculated intensity variation of the CO B-X emission along the track of the UVIS slit is in fair agreement with the observations. The calculated brightness of the NI 120 nm multiplet is larger by a factor of ~2-3 than observed.
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Papers by J Gustin
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 H2 emission shortward and longward of the HLy-a line is 50.3% and
49.7% respectively, the contribution of HLy-a to the total unabsorbed auroral signal has
been set to 9.1% and an input of 1 mW m À2 produces 10 kR of H2 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 mA m À2 assuming no hydrocarbon absorption and a color ratio of 2, respectively. Factors to extend the brightnesses observed with Cassini-UVIS to total H2 UV brightnesses are also provided.