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The Atmospheric Remote-Sensing Infrared Exoplanet Large-survey, ARIEL, has been selected to be the next (M4) medium class space mission in the ESA Cosmic Vision programme. From launch in 2028, and during the following 4 years of operation, ARIEL will perform precise spectroscopy of the atmospheres of ~1000 known transiting exoplanets using its metre-class telescope. A three-band photometer and three spectrometers cover the 0.5 µm to 7.8 µm region of the electromagnetic spectrum. This paper gives an overview of the mission payload, including the telescope assembly, the FGS (Fine Guidance System) - which provides both pointing information to the spacecraft and scientific photometry and low-resolution spectrometer data, the ARIEL InfraRed Spectrometer (AIRS), and other payload infrastructure such as the warm electronics, structures and cryogenic cooling systems.

Previous observations of Saturn's infrared aurorae have shown that a mid-latitude aurora exists significantly equatorward of the main auroral oval. Here, we present new results using data from four separate observing runs in 1998, 2003, 2008, and 2010. When combined, these provide a view of the mid-latitude aurora under a considerable range of viewing conditions, allowing the first calculation of the latitudinal position of this aurora to be made. This has shown that the mid-latitude aurora is located at the magnetic footprint of the region within the magnetosphere where the initial breakdown in corotation occurs, between 3 R S and the orbit of Enceladus (~3.95 R S). We also confirm that this aurora is a continuous stable feature over a period of more than a decade and that an oval morphology is likely. When combined, these results indicate that the mid-latitude auroral oval is formed by currents driven by the breakdown process within the magnetosphere, in turn caused by mass loading from the torus of Enceladus, analogous with the volcanic moon Io's dominant role in the formation of Jupiter's main auroral oval.

Planetary aurorae are generally produced by currents flowing between the planet's ionosphere and magnetosphere, which accelerate energetic charged particles that then hit the upper atmosphere. Recent models of Saturn's aurora predict only weak emission away from the main auroral oval. Stallard et al. now present Cassini infrared images taken from a novel angle, providing the first nightside auroral view. They reveal emissions both poleward and equatorward of the main oval. The polar emissions vary with time, and seem not to be linked with strong magnetospheric compressions. This aurora appears to be unique to Saturn and cannot be explained by current models of Saturn's magnetosphere. The majority of planetary aurorae are produced by electrical currents flowing between the ionosphere and the magnetosphere which accelerate energetic charged particles that hit the upper atmosphere. At Saturn, these processes collisionally excite hydrogen, causing ultraviolet emission1,2,3,4,5,6,7,8, and ionize the hydrogen, leading to H3+ infrared emission9,10,11,12,13,14,15. Although the morphology of these aurorae is affected by changes in the solar wind6,11, the source of the currents which produce them is a matter of debate16,17. Recent models predict only weak emission away from the main auroral oval18. Here we report images that show emission both poleward and equatorward of the main oval (separated by a region of low emission). The extensive polar emission is highly variable with time, and disappears when the main oval has a spiral morphology; this suggests that although the polar emission may be associated with minor increases in the dynamic pressure from the solar wind, it is not directly linked to strong magnetospheric compressions. This aurora appears to be unique to Saturn and cannot be explained using our current understanding of Saturn’s magnetosphere. The equatorward arc of emission exists only on the nightside of the planet, and arises from internal magnetospheric processes that are currently unknown.

We present the first vertical ion density profiles of Jupiter's upper atmosphere derived directly from ground-based observations. Observations of infrared H+3 emissions in Jupiter's auroral/polar regions were collected by the high-resolution spectrometer NIRSPEC on the Keck II telescope. We have calculated vertical density profiles for a latitude in the southern auroral region using the measured column densities and a shell model of the Jovian ionospheric H+3 emission. We compare our resultant profiles to those generated by a recent one-dimensional model in both local thermodynamic equilibrium (LTE) and non-LTE conditions. We find good agreement with the model profiles up to 1800 km. Above that, however, our measurements show that more H+3 is produced than is predicted by the model. Our observational method is a new tool for probing Jupiter's upper atmosphere from Earth and can possibly be extended to the study of other gas giant planets.

We present the first-ever infrared image of Saturn's auroral emission taken from Earth, using the CSHELL instrument on NASA's InfraRed Telescope Facility. We adapt the emission-line imaging technique to use the spectrometer as an ultra-narrowband imager, measuring the H3+ line at 3.953 μm with a spectral resolution of ~5000. Calibrating this technique with Jupiter shows a significant increase in sensitivity over the narrowest H3+ filters available. The resulting image of Saturn's auroral region confirms that the infrared main auroral oval is broadly similar to that seen in ultraviolet observations with a partially filled-in polar cap. It also shows that significant variations in brightness occur within the oval itself.

We report on a unique dusk-brightening event within Saturn's aurorae. Measurements of the H infrared aurora using the CSHELL instrument on NASA's Infrared Telescope Facility (IRTF), made in 2005 December, show an auroral intensity structure unlike anything previously detected. The aurora has a significantly brighter dusk sector over three Earth nights, a period in excess of 5 Saturnian days, suggesting a consistent source for this emission, stable in position within the magnetosphere. However, unlike previously detected dawn-brightening events, the overall auroral brightness remains low and the ion wind structure appears unaffected. Using the location of magnetopause crossings as a proxy for the solar wind pressure, the solar wind appears to be exceptionally rarefied. This leads us to conclude that the dusk-brightening event is strongly linked with the unusual solar wind conditions at the time of the observations. © 2008. The American Astronomical Society. All rights reserved.

We present a detailed analysis of the H + 3 intensity and velocity profiles crossing Saturn's auroral/polar region, as described by Stallard et al. [Stallard, T., Miller, S., Melin, H., Lystrup, M., Dougherty, M., Achilleos, N., 2007. Icarus 189, 1–13], with a view to understanding the magnetospheric processes with which they are connected. The data are not consistent with the theory that Saturn's main auroral oval is associated with corotation enforcement currents in the middle magnetosphere. This implies that the main auroral oval can be associated with the open–closed field line boundary [Cowley, S.W.H., Bunce, E.J., O'Rourke, J.M., 2004. J. Geophys. Res. 109. A05212]; a third model, by Sittler et al. [Sittler, E.C., Blanc, M.F., Richardson, J.D., 2006. J. Geophys. Res. 111. A06208] associates the main oval with centrifugal instabilities in the outer magnetosphere, but does not make predictions about ionospheric plasma flows with which we can compare our data. We do, however, tentatively identify emission at latitudes lower than the main auroral oval which may be associated with the corotation enforcement currents in the middle magnetosphere. We also find that at latitudes higher than the main auroral oval there is often a region of the ionosphere that is in rigid corotation with the planet. We suggest that this region corresponds to field lines embedded in the centre of the magnetotail which are shielded from the solar wind such that their rotation is controlled only by the neutral atmosphere.

We present an analysis of an extensive dataset of observations of the auroral/polar regions of Saturn, carried out in 2004 and 2005 using the high-resolution facility spectrometer CSHELL on the NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. The paper outlines the morphology of emission in the H + 3 ν 2 Q ( 1 , 0 − ) line at 3.953 μm across the planet's southern auroral/polar region, and gives a description of the main classes of morphology observed. Similarly, it presents the ion velocity as a function of location across the auroral/polar region, with the main classes of structure observed. In presenting these, it sets out the baseline phenomenological parameters, both averaged across our entire dataset and for specific nights, for use in future studies. Averaged across our dataset, the (planetary) west–east infrared H + 3 emission profile of Saturn's auroral/polar regions indicates the presence of a clear auroral oval, roughly coincident with that delineated by EUV images, although there is clearly relatively more H + 3 emission across the polar cap than would be expected from a straight correlation with the EUV images. The integrated H + 3 emission can vary by a factor of ∼50 between observing runs, and a factor of two on a night-to-night basis. Again averaged across our dataset, the lag to co-rotation of ions in the auroral/polar regions is considerable. We also find that the lag to co-rotation shows large variations, and that it is possible to distinguish sub-regions within the auroral/polar region and characterise them by the rate at which the velocity changes as a function of distance from the centre of the planet.

We present a review of recent developments in the use of molecular ion as a probe of physics and chemistry of the upper atmospheres of giant planets. This ion is shown to be a good tracer of energy inputs into Jupiter (J), Saturn (S) and Uranus (U). It also acts as a ‘thermostat’, offsetting increases in the energy inputs owing to particle precipitation via cooling to space (J and U). Computer models have established that is also the main contributor to ionospheric conductivity. The coupling of electric and magnetic fields in the auroral polar regions leads to ion winds, which, in turn, drive neutral circulation systems (J and S). These latter two effects, dependent on , also result in very large heating terms, approximately 5×10 12 W for Saturn and greater than 10 14 W for Jupiter, planet-wide; these terms compare with approximately 2.5×10 11 W of solar extreme UV absorbed at Saturn and 10 12 W at Jupiter. Thus, is shown to play a major role in explaining why the temperatures of the giant planets are much greater (by hundreds of kelvin) at the top of the atmosphere than solar inputs alone can account for.

A new study of Saturn's auroral emissions has uncovered a secondary auroral oval, a quarter as bright as the main one. The main auroral oval was first seen in Hubble Space Telescope images more than a decade ago and its morphology has since been determined in detail, yet there is continued debate about its origins. One theory was that Saturn's auroral emissions were a hybrid between those seen on Earth, primarily formed by interaction with the solar wind, and those on Jupiter, formed by interaction with plasma flows. But the properties of the secondary oval at Saturn show it to be a weak equivalent of Jupiter's main oval, its relative dimness due to the lack of as large a source of ions as Jupiter's volcanic moon Io. Now it seems that the underlying aurora formation processes on Saturn and Jupiter are very similar, with scaling differences accounting for the differences in appearance. Planetary aurorae are formed by energetic charged particles streaming along the planet's magnetic field lines into the upper atmosphere from the surrounding space environment. At Saturn only the main auroral oval has been observed, but this paper reports the discovery of a secondary oval, ∼25 per cent as bright as the main oval. Planetary aurorae are formed by energetic charged particles streaming along the planet’s magnetic field lines into the upper atmosphere from the surrounding space environment. Earth’s main auroral oval is formed through interactions with the solar wind1, whereas that at Jupiter is formed through interactions with plasma from the moon Io inside its magnetic field (although other processes form aurorae at both planets2,3). At Saturn, only the main auroral oval has previously been observed and there remains much debate over its origin. Here we report the discovery of a secondary oval at Saturn that is ∼25 per cent as bright as the main oval, and we show this to be caused by interaction with the middle magnetosphere around the planet. This is a weak equivalent of Jupiter’s main oval, its relative dimness being due to the lack of as large a source of ions as Jupiter’s volcanic moon Io. This result suggests that differences seen in the auroral emissions from Saturn and Jupiter are due to scaling differences in the conditions at each of these two planets, whereas the underlying formation processes are the same.

We present a review of recent developments in the use of H3+ molecular ion as a probe of physics and chemistry of the upper atmospheres of giant planets. This ion is shown to be a good tracer of energy inputs into Jupiter (J), Saturn (S) and Uranus (U). It also acts as a 'thermostat', offsetting increases in the energy inputs owing to particle precipitation via cooling to space (J and U). Computer models have established that H3+ is also the main contributor to ionospheric conductivity. The coupling of electric and magnetic fields in the auroral polar regions leads to ion winds, which, in turn, drive neutral circulation systems (J and S). These latter two effects, dependent on H3+, also result in very large heating terms, approximately 5 x 10(12) W for Saturn and greater than 10(14) W for Jupiter, planet-wide; these terms compare with approximately 2.5 x 10(11) W of solar extreme UV absorbed at Saturn and 10(12) W at Jupiter. Thus, H3+ is shown to play a major role in explaining why the temperatures of the giant planets are much greater (by hundreds of kelvin) at the top of the atmosphere than solar inputs alone can account for.

This paper presents the setup for empirical validations of the Pair-based Analytical model for Segmented Telescope Imaging from Space (PASTIS) tolerancing model for segmented coronagraphy. We show the hardware configuration of the High-contrast imager for Complex Aperture Telescopes (HiCAT) testbed on which these experiments will be conducted at an intermediate contrast regime between $10^{-6}$ and $10^{-8}$. We describe the optical performance of the testbed with a classical Lyot coronagraph and describe the recent hardware upgrade to a segmented mode, using an IrisAO segmented deformable mirror. Implementing experiments on HiCAT is made easy through its top-level control infrastructure that uses the same code base to run on the real testbed, or to invoke the optical simulator. The experiments presented in this paper are run on the HiCAT testbed emulator, which makes them ready to be performed on actual hardware. We show results of three experiments with results from the emulator, with the goal to demonstrate PASTIS on hardware next. We measure the testbed PASTIS matrix, and validate the PASTIS analytical propagation model by comparing its contrast predictions to simulator results. We perform the tolerancing analysis on the optical eigenmodes (PASTIS modes) and on independent segments, then validate these results in respective experiments. This work prepares and enables the experimental validation of the analytical segment-based tolerancing model for segmented aperture coronagraphy with the specific application to the HiCAT testbed., Comment: 18 pages, 9 figures, published

ISBN: 9781510601871; International audience; PICARD is a mission devoted to solar variability observation, through imagery and radiometric measurements, with the goal to provide data for scientific investigation first in the area of solar physics, and second in the assessment of the influence of the solar variability on the Earth climate variability. PICARD contains a double program with in-space and on-ground measurements. The PICARD spacecraft was launched on June 15, 2010, commissioned in-flight in October of the same year, and was retired in April 2014. PICARD ground-based is functional since May 2011, and is still operational today. We shall give an overview of the PICARD instrumentation. New estimates of the absolute values of the total solar irradiance, of the solar spectral irradiance at typical wavelengths, and of the solar oblateness will be given. We will also report about helioseismic studies. Finally, we will present our current results about solar radius variations after six years of solar observations.

International audience; The scientific objectives of a space mission translate into instrumental developments and satellites operations to observe the astronomical objects of interest. The payload in its space environment is however subject to important thermal variations that affect the observations. This is well observed when images of the Sun are recorded with the constraint of having the solar rotational axis in a constant direction relatively to the camera reference system. Consequences are then clearly observed on the image positions that follow the thermal variations on the satellite orbit. This is particularly the case for the PICARD space mission. This phenomenon is similar to the defocus and motions of the images recorded with ground-based telescopes. We will present in this work some simulations showing this effect in space. We will then compare our results with real data obtained from the PICARD solar mission.