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International audience; Stratospheric temperatures on Saturn imply a strong decay of the equatorial winds with altitude. If the decrease in winds reported from recent Hubble Space Telescope images is not a temporal change, then the features tracked must have been at least 130 kilometers higher than in earlier studies. Saturn's south polar stratosphere is warmer than predicted from simple radiative models. The C/H ratio on Saturn is seven times solar, twice Jupiter's. Saturn's ring temperatures have radial variations down to the smallest scale resolved (100 kilometers). Diurnal surface temperature variations on Phoebe suggest a more porous regolith than on the jovian satellites.

International audience; The Auroral Planetary Imaging and Spectroscopy (APIS) service, accessible online, provides an open and interactive access to processed auroral observations of the outer planets and their satellites. Such observations are of interest for a wide community at the interface between planetology and magnetospheric and heliospheric physics. APIS consists of (i) a high level database, built from planetary auroral observations acquired by the Hubble Space Telescope (HST) since 1997 with its mostly used Far-UltraViolet spectro-imagers, (ii) a dedicated search interface aimed at browsing efficiently this database through relevant conditional search criteria and (iii) the ability to interactively work with the data online through plotting tools developed by the Virtual Observatory (VO) community, such as Aladin and Specview. This service is VO compliant and can therefore also been queried by external search tools of the VO community. The diversity of available data and the capability to sort them out by relevant physical criteria shall in particular facilitate statistical studies, on long-term scales and/or multi-instrumental multi-spectral combined analysis.

Terrestrial contamination by 137 Cs and 134 Cs since 1986 in France is held to originate in atmospheric nuclear weapon tests and/or the releases from the Chernobyl accident. Monthly measurements of radioactivity in terrestrial mosses are part of the monitoring protocol carried out by the Conseil General of Tarn-et-Garonne to assess the environmental impact of the Golfech nuclear power plant radioactive releases. Since the commissioning of the NPP in 1990, these monthly measurements revealed a contamination by 137 Cs and 134 Cs (up to 700 and 60 BqKg-1 DW) in both sampling sites. This contamination decreased by a factor about 10 over 1990-2004 through three stages (134 Cs undetectable since 1995). It is significantly correlated over large periods of time with the monthly levels of atmospheric halogens-aerosols releases by the Golfech NPP (95%, r2 = 0.35 to >0.84). These results are discussed in the light of the time since radioactive releases, of the operating and release conditions of/from the Golfech NPP and of the mosses sampled. These preliminary results bear evidence of an impact of the Golfech NPP on the terrestrial environment. They confirm that species of terrestrial mosses are particularly sensitive to low levels of atmospheric radioactive caesium contamination and are a good indicator of a radioactive fallout. They suggest that terrestrial mosses may serve to elaborate a predictive model of a caesium release, together with some relevant parameters and in the framework of a carefully defined monitoring protocol.

We consider the flows and currents in Saturn's polar ionosphere which are implied by a three-component picture of large-scale magnetospheric flow driven both by planetary rotation and the solar wind interaction. With increasing radial distance in the equatorial plane, these components consist of a region dominated by planetary rotation where planetary plasma sub-corotates on closed field lines, a surrounding region where planetary plasma is lost down the dusk tail by the stretching out of closed field lines followed by plasmoid formation and pinch-off, as first described for Jupiter by Vasyliunas, and an outer region driven by the interaction with the solar wind, specifically by reconnection at the dayside magnetopause and in the dawn tail, first discussed for Earth by Dungey. The sub-corotating flow on closed field lines in the dayside magnetosphere is constrained by Voyager plasma observations, showing that the plasma angular velocity falls to around half of rigid corotation in the outer magnetosphere, possibly increasing somewhat near the dayside magnetopause, while here we provide theoretical arguments which indicate that the flow should drop to considerably smaller values on open field lines in the polar cap. The implied ionospheric current system requires a four-ring pattern of field-aligned currents, with distributed downward currents on open field lines in the polar cap, a narrow ring of upward current near the boundary of open and closed field lines, and regions of distributed downward and upward current on closed field lines at lower latitudes associated with the transfer of angular momentum from the planetary atmosphere to the sub-corotating planetary magnetospheric plasma. Recent work has shown that the upward current associated with sub-corotation is not sufficiently intense to produce significant auroral acceleration and emission. Here we suggest that the observed auroral oval at Saturn instead corresponds to the ring of upward current bounding the region of open and closed field lines. Estimates indicate that auroras of brightness from a few kR to a few tens of kR can be produced by precipitating accelerated magnetospheric electrons of a few keV to a few tens of keV energy, if the current flows in a region which is sufficiently narrow, of the order of or less than ~1000 km (~1° latitude) wide. Arguments are also given which indicate that the auroras should typically be significantly brighter on the dawn side of the oval than at dusk, by roughly an order of magnitude, and should be displaced somewhat towards dawn by the down-tail outflow at dusk associated with the Vasyliunas cycle. Model estimates are found to be in good agreement with data derived from high quality images newly obtained using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope, both in regard to physical parameters, as well as local time effects. The implication of this picture is that the form, position, and brightness of Saturn's main auroral oval provide remote diagnostics of the magnetospheric interaction with the solar wind, including dynamics associated with magnetopause and tail plasma interaction processes. Key words. Magnetospheric physics (auroral phenomena, magnetosphere-ionosphere interactions, solar windmagnetosphere interactions)

We consider the flows and currents in Saturn's polar ionosphere which are implied by a three-component picture of large-scale magnetospheric flow driven both by planetary rotation and the solar wind interaction. With increasing radial distance in the equatorial plane, these components consist of a region dominated by planetary rotation where planetary plasma sub-corotates on closed field lines, a surrounding region where planetary plasma is lost down the dusk tail by the stretching out of closed field lines followed by plasmoid formation and pinch-off, as first described for Jupiter by Vasyliunas, and an outer region driven by the interaction with the solar wind, specifically by reconnection at the dayside magnetopause and in the dawn tail, first discussed for Earth by Dungey. The sub-corotating flow on closed field lines in the dayside magnetosphere is constrained by Voyager plasma observations, showing that the plasma angular velocity falls to around half of rigid corotation in the outer magnetosphere, possibly increasing somewhat near the dayside magnetopause, while here we provide theoretical arguments which indicate that the flow should drop to considerably smaller values on open field lines in the polar cap. The implied ionospheric current system requires a four-ring pattern of field-aligned currents, with distributed downward currents on open field lines in the polar cap, a narrow ring of upward current near the boundary of open and closed field lines, and regions of distributed downward and upward current on closed field lines at lower latitudes associated with the transfer of angular momentum from the planetary atmosphere to the sub-corotating planetary magnetospheric plasma. Recent work has shown that the upward current associated with sub-corotation is not sufficiently intense to produce significant auroral acceleration and emission. Here we suggest that the observed auroral oval at Saturn instead corresponds to the ring of upward current bounding the region of open and closed field lines. Estimates indicate that auroras of brightness from a few kR to a few tens of kR can be produced by precipitating accelerated magnetospheric electrons of a few keV to a few tens of keV energy, if the current flows in a region which is sufficiently narrow, of the order of or less than ~1000 km (~1° latitude) wide. Arguments are also given which indicate that the auroras should typically be significantly brighter on the dawn side of the oval than at dusk, by roughly an order of magnitude, and should be displaced somewhat towards dawn by the down-tail outflow at dusk associated with the Vasyliunas cycle. Model estimates are found to be in good agreement with data derived from high quality images newly obtained using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope, both in regard to physical parameters, as well as local time effects. The implication of this picture is that the form, position, and brightness of Saturn's main auroral oval provide remote diagnostics of the magnetospheric interaction with the solar wind, including dynamics associated with magnetopause and tail plasma interaction processes. Key words. Magnetospheric physics (auroral phenomena, magnetosphere-ionosphere interactions, solar windmagnetosphere interactions)

Analysis of international ultraviolet explorer (IUE) and Voyager ultraviolet spectrometer (UVS) spectra of the Jovian auroral emission indicates that the Jovian auroral brightness is modulated in longitude (brighter near 180° in the north and 20° in the south) and that there is a color ratio asymmetry associated with this brightening. The purpose of this study is to investigate the origin of this apparent asymmetry. To that end, we use a series of six typical images of the north auroral region taken in the H 2 Lyman bands with the faint object camera (FOC) aboard the Hubble space telescope (HST) and which cover a full Jovian rotation. Although the images do not display any strong brightening near 180°, once we have simulated the signal IUE would see through its aperture, we find the characteristic longitudinal modulation. We attribute most of this modulation to a combination of viewing geometry effects near the east and west ansae of the auroral oval (already taken into account in previous studies) and of the spatial degradation of the source by the IUE instrument function (never considered so far), and we suggest qualitatively that these effects may also affect the color ratio asymmetry. Nevertheless, part of the asymmetry seems to be clue to an intrinsic modulation associated with a bright feature crossing the polar cap along the 160° meridian (transpolar emission) and present in most of the images. We then use a series of FOC images taken during an atypically strong auroral event, and we show that the same effects can again account for the anomalous brightness variations observed simultaneously with IUE.

The limb to limb Lyman-alpha reflectivities observed with the Voyager ultraviolet spectrometer (UVS) instruments during the fly-by of Saturn are reanalyzed using a revised H Lyman-alpha sensitivity for the Voyager 1 instrument. The new sensitivity reconciles the measured intensities to those of Voyager 2 and gives a coherent set of data. To fit the UV airglow observations, four sources are considered: (1) H resonance and H2 Rayleigh scattering of solar Lyman-alpha radiation, (2) the interplanetary Lyman-alpha radiation, (3) a possible internal source of unknown origin, (4) the possibility of atmospheric turbulence recently proposed to explain the Lyman-alpha bulge of Jupiter. The analysis supports neither a dominant collisional excitation source for the UV emissions nor the presence of strong atmospheric turbulence. The best fit, in terms of brightness but also in terms of shape of the limb to limb profile (that is to say independent on the absolute calibrations), is obtained for pure resonance and Rayleigh scattering of solar and interstellar wind line in an atmosphere enriched in atomic hydrogen up to three times the standard model. Influx of water from the rings of Saturn may provide a means for producing such enhanced H densities in the upper atmosphere.

[1] This study is based on multi-planet multi-wavelength observations of planetary aurorae throughout the heliosphere, acquired along the propagation path of a series of consecutive interplanetary shocks. The underlying motivation to track the shocks was to increase the probability of detection of auroral emissions at Uranus. Despite several Earth-based attempts in the past few years, at Far-UV (FUV) and Near-IR (NIR) wavelengths, such emissions have never been unambiguously re-observed since their discovery by Voyager 2 in 1986. Here, we present a campaign of FUV observations of Uranus obtained in November 2011 with the Hubble Space Telescope (HST) during active solar wind conditions. We positively identify auroral signatures in several of these HST measurements, together with some obtained in 1998, representing the first images of Uranus' aurorae. We analyze their characteristics and discuss the implications for the asymmetric Uranian magnetosphere and its highly variable interaction with the solar wind flow from near-solstice (1986) to near-equinox (2011) configurations.

During the flyby of the Earth by Cassini in 1999, the Radio and Plasma Wave Science (RPWS) instrument recorded one month of quasi-continuous observations of Auroral Kilometric Radiation (AKR). Analyzing the Stokes parameters of incoming radio waves, we found AKR to be 100% circular left-handed (LH) or right-handed (RHW). We analyzed separately the northern - RH - emission, from the southern - LH - one with respect to the magnetic equator. AKR power variations reveal (i) a log-normal distribution at time scales of minutes, (ii) bursts of emission at time scales of a few hours, and (iii) a clear modulation at 24 hours, with a weaker modulation at 12 hours (especially visible for LH emissions). The prominent 24 h modulation is found to modulate LH and RH AKR in phase opposition. This is interpreted as being due to visibility effects related to the precession of the terrestrial magnetic dipole, making Cassini oscillate relative to the average AKR beaming pattern from each hemisphere. We accordingly quantified the AKR beaming vs explored latitudes. On time scales shorter than a few hours, LH and RH emissions are found to be correlated. This is attributed to the actual conjugacy of the corresponding sources, simultaneously turned on by substorm occurrence. The geometrical anti-correlation (at 24 h) dominates close to Earth, while the short term correlation (substorms) dominates far from Earth, where the detection threshold makes the visibility less important than the occurrence of substorms. Finally, the 12 h modulation is detected when it is not masked by strong visibility effects, i.e. for the LH emission which is observed mostly near the magnetic equator along the path of Cassini. A 12 h modulation being also observed in some geomagnetic indices, we suggest that a physical process (e.g. semi-diurnal variable efficiency of the reconnection between interplanetary and geomagnetic fields, or magnetotail oscillations) may be responsible of the observed AKR 12 h modulation.

[1] During the flyby of the Earth by Cassini in 1999, the Radio and Plasma Wave Science (RPWS) instrument recorded a month of quasi-continuous observations of auroral kilometric radiation (AKR). Analyzing the full polarization of incoming radio waves, we found AKR to be 100% circular left-handed (LH) or right-handed (RH). As AKR is emitted predominantly on the extraordinary mode, we analyzed separately the northern (RH) emission, from the southern (LH) one with respect to the magnetic equator. AKR power variations reveal (1) a log-normal distribution at time scales of minutes, (2) bursts of emission at time scales of a few hours, and (3) a clear overall modulation at 24 h, together with a weaker modulation at 12 h (especially visible for LH emissions). The prominent 24 h modulation is found to modulate LH and RH AKR in phase opposition. This is interpreted as being due to visibility effects related to the precession of the terrestrial magnetic dipole, making Cassini oscillate relative to the average AKR beaming pattern from each hemisphere. We accordingly quantified the AKR beaming as a function of explored latitudes. On time scales shorter than a few hours, LH and RH emissions are found to be correlated. This is attributed to the actual conjugacy of the corresponding sources, simultaneously turned on by substorm occurrence. The geometrical anticorrelation (at 24 h) dominates close to Earth, while the short-term correlation (substorms) dominates far from Earth, where the detection threshold makes the visibility less important than the occurrence of substorms. Finally, the 12 h modulation is detected when it is not masked by strong visibility effects, namely, for the LH emission, which is observed mostly near the magnetic equator along the path of Cassini. As a 12 h modulation of some geomagnetic indices was also observed, we suggest that the AKR semidiurnal modulation may have a physical cause, for which we favor the variable geometry and efficiency of the reconnection between interplanetary and geomagnetic fields, or magnetotail oscillations.

[1] Similarly to other magnetized planets, Saturn displays auroral emissions generated by accelerated electrons gyrating around high-latitude magnetic field lines. They mainly divide in ultraviolet (UV) and infrared (IR) aurorae, excited by electron collisions with the upper atmosphere, and Saturn's kilometric radiation (SKR), radiated from higher altitudes by electron-wave resonance. Whereas spatially resolved UV and IR images of atmospheric aurorae reveal a continuous auroral oval around each pole, the SKR source locus was only indirectly constrained by the Voyager radio experiment to a limited local time (LT) range on the morningside, leading to interpretation of the SKR modulation as a fixed flashing light. Here, we present resolved SKR maps derived from the Cassini Radio and Plasma Wave Science (RPWS) experiment using goniopolarimetric techniques. We observe radio sources all around the planet, organized along a high-latitude continuous auroral oval. Observations of the Hubble Space Telescope obtained in January 2004 and January 2007 have been compared to simultaneous and averaged Cassini-RPWS measurements, revealing that SKR and UV auroral ovals are very similar, both significantly enhanced on the dawnside. These results imply that the SKR and atmospheric aurorae are triggered by the same populations of energetic electron beams, requiring a unified model of particle acceleration and precipitation on Saturn.

[1] We present goniopolarimetric (also known as direction finding) results of the Saturn kilometric radiation (SKR), using the Cassini Radio and Plasma Wave Science instrument high-frequency radio receiver data. Tools to retrieve the characteristics of the SKR sources have been developed that allow us to measure their 3-D location and beaming angle relative to the magnetic field in the source and, thus, to deduce the location of the footprints of the active magnetic field lines. We present results from these analyses on SKR observed during the revolution 29 perikrone (25–26 September 2006) with a relatively high orbital inclination. These results provide for the first time the observed beaming angle, the invariant latitude, and the local time of the SKR sources. We provide evidence that the SKR is mainly emitted in the right-hand extraordinary (R-X) mode and marginally in the left-hand ordinary (L-O) mode. We observe the footprint of the active magnetic field lines in the ∼70° to ∼80° northern and southern latitudinal range and in the 0400 to 1600 local time range. The northern sources are observed at slightly higher latitude than southern sources. The location matches that of the UV and IR aurorae. Duskside and nightside sources are also detected.

[1] Accelerated electrons gyrating around Saturn's auroral (high latitude) magnetic field lines generate the intense Saturnian kilometric radiation (SKR). This radio emission is thought to be generated via the cyclotron maser instability (CMI) that predicts a strong anisotropy of the beaming pattern of the emission. Resulting visibility effects were suspected to be at the origin of characteristic features observed in Cassini's dynamic spectra as arc-shaped structures and shadow zones. By using the Planetary Radio Emissions Simulator (PRES) code, we model these visibility effects and their consequence on Cassini's observations of SKR: we compute the dynamic spectra resulting from the beaming pattern of CMI-generated SKR and its intersection with Cassini's trajectory. The SKR beaming pattern and its dependence on the frequency is computed for two typical electron distributions observed in auroral regions: a loss cone (favoring oblique emission) or a shell (favoring perpendicular emission). We successfully fit observed arc-shaped structures and shadow zones. Although oblique and perpendicular emissions both allow to produce radio arcs, the fit is better in the oblique case. Similarities and differences between observations and model results are discussed and perspectives are outlined.

[1] Since Cassini entered Saturn's magnetosphere in July 2004, the auroral Saturnian kilometric radiation (SKR), which dominates the kronian radio spectrum, is observed quasi-continuously. Consecutive orbits of the spacecraft covered distances to Saturn down to 1.3 Saturn radii, all local times and, since December 2006, latitudes as high as 60°. On the basis of carefully calibrated and cleaned long-term time series and dynamic spectra, we analyze the average properties, and characteristics of the SKR over 2.75 years starting at Cassini's Saturn orbit insertion. This study confirms and expands previous results from Voyager 1 and 2 studies in the 1980s: the SKR spectrum is found to extend from a few kHz to 1200 kHz; extraordinary mode emission dominates, i.e., left-handed (LH) from the southern kronian hemisphere and right-handed (RH) from the northern one, for which we measure directly a degree of circular polarization up to 100%; the variable visibility of SKR along Cassini's orbit is consistent with sources at or close to the local electron cyclotron frequency f ce , in the Local Time (LT) sector 09 h-12 h, and at latitudes >70°, with emission beamed along hollow cones centered on the local magnetic field vector; this anisotropic beaming results in the existence of an equatorial radio shadow zone, whose extent is quantified as a function of frequency; it also causes the systematic disappearance of emission at high latitudes above 200 kHz and below 30 kHz. In addition, we obtain new results on SKR: LH and RH intensity variations are found to match together at all timescales >30 min; moreover their spectra are found to be conjugated as a function of the latitude of the observer; we use this conjugacy to merge LH and RH spectra and derive pronounced systematic dependences of the SKR spectrum as a function of the spacecraft latitude and LT (that will be the input of a subsequent modeling study); we identify for the first time ordinary mode SKR emission; finally, in addition to the SKR and n-SMR components, we discuss the narrowband kilometric component (named here n-SKR) which extends mainly between 10 and 40 kHz, preferentially observed from high latitudes.

Temperatures obtained from early Cassini infrared observations of Titan show a stratopause at an altitude of 310 kilometers (and 186 kelvin at 15°S). Stratospheric temperatures are coldest in the winter northern hemisphere, with zonal winds reaching 160 meters per second. The concentrations of several stratospheric organic compounds are enhanced at mid- and high northern latitudes, and the strong zonal winds may inhibit mixing between these latitudes and the rest of Titan. Above the south pole, temperatures in the stratosphere are 4 to 5 kelvin cooler than at the equator. The stratospheric mole fractions of methane and carbon monoxide are (1.6 ± 0.5) × 10 -2 and (4.5 ± 1.5) × 10 -5 , respectively.

International audience; The Composite Infrared Spectrometer observed Jupiter in the thermal infrared during the swing-by of the Cassini spacecraft. Results include the detection of two new stratospheric species, the methyl radical and diacetylene, gaseous species present in the north and south auroral infrared hot spots; determination of the variations with latitude of acetylene and ethane, the latter a tracer of atmospheric motion; observations of unexpected spatial distributions of carbon dioxide and hydrogen cyanide, both considered to be products of comet Shoemaker-Levy 9 impacts; characterization of the morphology of the auroral infrared hot spot acetylene emission; and a new evaluation of the energetics of the northern auroral infrared hot spot.

A new efficient radiative transfer algorithm for nonhomogeneous model atmospheres has been applied to the Uranian atmosphere. The contribution of the scatter solar Lyman-alpha to the Uranain emission is of the order of 300 R, and the Rayleigh contribution may reach 450 R for small values of the eddy diffusion coefficient (EDC). The total solar contribution may then reach about 750 R for a solar flux of 2.5 x 10 to the 11th photons/sq cm/s/A. A level of up to 400 R is confirmed in some directions for the interstellar wind contribution. The values of the atmospheric EDC necessary to mimic the observations are 50-100 sq cm/s. A small additional source located on the dayside Uranian atmosphere seems necessary correctly to fit the shape of the limb to limb intensity variation, especially near the limbs. Its contribution to the emergent intensity would range from 100 to 500 R.