Observations Of Saturn Research Articles

New astrometric reduction of old photographic observations of Saturn’s moons based on digitizing of astronegatives

The paper presents new results of the reduction of photographic observations made at the Pulkovo Observatory in 1972–1974 using digitized photographic plates of Saturn’s moons obtained by the 26-inch refractor and normal astrograph. Lists of exact positions of 2–6th and 8th moons and Saturn itself are obtained. The results of the new reduction are stored in the Pulkovo database on the bodies of the solar system and are available online at www.puldb.ru. The comparison of the results of the old and new reductions with the latest ephemeris of Saturn’s moons has shown that the positioning accuracy of the moons of Saturn has increased approximately twofold. A conclusion on the feasibility of reprocessing old observations using the new method is made.

Polarimetry of Saturnian satellite Enceladus

We present results of polarimetric observations of Saturn's moon Enceladus carried out from April 14, 2010 to April 13, 2013 in WR spectral band (550-750 nm). We used 2.6-m telescope equipped with a one-channel photoelectric photometer-polarimeter (Crimean Astrophysical Observatory). The measurements were performed at phase angles ranging from 1.65° to 5.71°. The phase-angle dependence of linear polarization of Enceladus was obtained using the results of our observations. Results obtained are discussed in terms of existing models of light scattering by regolith surfaces.

A multi-scale magnetotail reconnection event at Saturn and associated flows: Cassini/UVIS observations

We present high-resolution Cassini/UVIS (Ultraviolet Imaging Spectrograph) observations of Saturn’s aurora during May 2013 (DOY 140–141). The observations reveal an enhanced auroral activity in the midnight-dawn quadrant in an extended local time sector (∼02 to 05LT), which rotates with an average velocity of ∼45% of rigid corotation. The auroral dawn enhancement reported here, given its observed location and brightness, is most probably due to hot tenuous plasma carried inward in fast moving flux tubes returning from a tail reconnection site to the dayside. These flux tubes could generate intense field-aligned currents that would cause aurora to brighten. However, the origin of tail reconnection (solar wind or internally driven) is uncertain. Based mainly on the flux variations, which do not demonstrate flux closure, we suggest that the most plausible scenario is that of internally driven tail reconnection which operates on closed field lines. The observations also reveal multiple intensifications within the enhanced region suggesting an x-line in the tail, which extends from 02 to 05LT. The localised enhancements evolve in arc and spot-like small scale features, which resemble vortices mainly in the beginning of the sequence. These auroral features could be related to plasma flows enhanced from reconnection which diverge into multiple narrow channels then spread azimuthally and radially. We suggest that the evolution of tail reconnection at Saturn may be pictured by an ensemble of numerous narrow current wedges or that inward transport initiated in the reconnection region could be explained by multiple localised flow burst events. The formation of vortical-like structures could then be related to field-aligned currents, building up in vortical flows in the tail. An alternative, but less plausible, scenario could be that the small scale auroral structures are related to viscous interactions involving small-scale reconnection.

Saturn ring seismology: Evidence for stable stratification in the deep interior of Saturn

Seismology allows for direct observational constraints on the interior structures of stars and planets. Recent observations of Saturn’s ring system have revealed the presence of density waves within the rings excited by oscillation modes within Saturn, allowing for precise measurements of a limited set of the planet’s mode frequencies. We construct interior structure models of Saturn, compute the corresponding mode frequencies, and compare them with the observed mode frequencies. The fundamental mode frequencies of our models match the observed frequencies (of the largest amplitude waves) to an accuracy of ∼1%, confirming that these waves are indeed excited by Saturn’s f-modes. The presence of the lower amplitude waves (finely split in frequency from the f-modes) can only be reproduced in models containing gravity modes that propagate in a stably stratified region of the planet. The stable stratification must exist deep within the planet near the large density gradients between the core and envelope. Our models cannot easily reproduce the observed fine splitting of the m=-3 modes, suggesting that additional effects (e.g., significant latitudinal differential rotation) may be important.

Cassini–VIMS observations of Saturn’s main rings: I. Spectral properties and temperature radial profiles variability with phase angle and elevation

The spectral properties and thermal behavior of Saturn’s rings are determined from a dataset of ten radial mosaics acquired by Cassini–VIMS (Visual and Infrared Mapping Spectrometer) between October 29th 2004 and January 27th 2010 with phase angle ranging between 5.7° and 132.4° and elevation angles between −23.5° and 2.6°. These observations, after reduction to spectrograms, e.g. 2D arrays containing the VIS–IR (0.35–5.1μm) spectral information versus radial distance from Saturn (from 73.500 to 141.375km, 400km/bin), allow us to compare the derived spectral and thermal properties of the ring particles on a common reference. Spectral properties: rings spectra are characterized by an intense reddening at visible wavelengths while they maintain a strong similarity with water ice in the infrared domain. Significant changes in VIS reddening, water ice abundance and grain sizes are observed across different radial regions resulting in correlation with optical depth and local structures. The availability of observations taken at very different phase angles allows us to examine spectrophotometric properties of the ring’s particles. When observed at high phase angles, a remarkable increase of visible reddening and water ice band depths is found, probably as a consequence of the presence of a red-colored contaminant intimately mixed within water ice grains and of multiple scattering. At low phases the analysis of the 3.2–3.6μm range shows faint spectral signatures at 3.42–3.52μm which are compatible with the CH2 aliphatic stretch. The 3.29μm PAH aromatic stretch absorption is not clearly detectable on this dataset. VIMS results indicate that ring particles contain about 90–95% water ice while the remaining 5–10% is consistent with different contaminants like amorphous carbon or tholins. However, we cannot exclude the presence of nanophase iron or hematite produced by iron oxidation in the rings tenuous oxygen atmosphere, intimately mixed with the ice grains. Greater pollution caused by meteoritic material is seen in the C ring and Cassini division while the low levels of aliphatic material observed by VIMS in the A and B rings particles are an evidence that they are pristine. Thermal properties: the ring-particles’ temperature is retrieved by fitting the spectral position of the 3.6μm continuum peak observed on reflectance spectra: in case of pure water ice the position of the peak, as measured in laboratory, shifts towards shorter wavelengths when temperature decreases, moving from about 3.65μm at 123K to about 3.55μm at 88K. When applied to VIMS rings observations, this method allows us to infer the average temperature across ring regions sampled through 400km-wide radial bins. Comparing VIMS temperature radial profiles with similar CIRS measurements acquired at the same time we have found a substantial agreement between the two instruments’ results across the A and B rings. In general VIMS measures higher temperatures than CIRS across C ring and Cassini division as a consequence of the lower optical depth and the resulting pollution that creates a deviation from pure water ice composition of these regions. VIMS results point out that across C ring and CD the 3.6μm peak wavelength is always higher than across B and A rings and therefore C ring and CD are warmer than A and B rings. VIMS observations allow us to investigate also diurnal and seasonal effects: comparing antisolar and subsolar ansae observations we have measured higher temperature on the latter. As the solar elevation angle decreases to 0° (equinox), the peak’s position shifts at shorter wavelengths because ring’s particles becomes colder. Merging multi-wavelength data sets allow us to test different thermal models, combining the effects of particle albedo, regolith composition, grain size and thermal properties with the ring structures.

Plasma loss mechanisms from Saturn's magnetosphere

Since the first up‐close observations of Saturn, made by the Pioneer 11 probe in 1979, a great deal has been learned about the dynamics of the gas giant's magnetosphere. In‐depth observations made by the Cassini orbiter, which has been circling Saturn since 2004, have revealed fundamental differences between the behavior of Saturn's magnetosphere and that of the Earth's magnetosphere. Earth's magnetospheric plasma is largely populated by ions captured from the solar wind, whereas Saturn's plasma comes predominantly from water vapor that spews from massive geysers on the southern end of its icy moon Enceladus. Ionized water vapor from Enceladus streams out at 12 to 250 kilograms per second, yet observations show that the concentration of plasma in Saturn's magnetosphere is at a relatively steady level. This discrepancy has left researchers searching for potential plasma loss mechanisms. In a review, Thomsen highlights the progress made in recent years in understanding this question.

Conjugate observations of Saturn’s northern and southern [formula omitted] aurorae

We present an analysis of recent high spatial and spectral resolution ground-based infrared observations of H3+ obtained with the 10-m Keck II telescope in April 2011. We observed H3+ emission from Saturn’s northern and southern auroral regions, simultaneously, over the course of more than 2h, obtaining spectral images along the central meridian as Saturn rotated. Previous ground-based work has derived only an average temperature of an individual polar region, summing an entire night of observations. Here we analyse 20 H3+ spectra, 10 for each hemisphere, providing H3+ temperature, column density and total emission in both the northern and southern polar regions simultaneously, improving on past results in temporal cadence and simultaneity. We find that: (1) the average thermospheric temperatures are 527±18K in northern Spring and 583±13K in southern Autumn, respectively; (2) this asymmetry in temperature is likely to be the result of an inversely proportional relationship between the total thermospheric heating rate (Joule heating and ion drag) and magnetic field strength – i.e. the larger northern field strength leads to reduced total heating rate and a reduced temperature, irrespective of season, and (3) this implies that thermospheric heating and temperatures are relatively insensitive to seasonal effects.

Cassini ISS observation of Saturn’s String of Pearls

We present the dynamics of the String of Pearls (SoPs) feature observed by the Cassini spacecraft’s Imaging Science Subsystem (ISS) camera between 2007 and 2010. The SoPs was originally discovered in the 5μm images captured by Cassini VIMS instrument, where it appeared as a chain of infrared-bright spots (Momary, T.W., et al. [2006]. The Zoology of Saturn: The Bizarre Features Unveiled by the 5 Micron Eyes of Cassini/VIMS. AAS/Division for Planetary Sciences Meeting Abstracts 38, 499). Using ISS images of Saturn, we found a chain of 23–26 dark spots at 33.2°N planetocentric latitude with characteristics that are consistent with those of SoPs. Our measurements imply that the feature propagated at −2.26±0.02°day−1 in longitude (−22.27±0.2ms−1, negative values denote westward) during the observed period that spans three Earth years. Our measurements imply that the SoPs is a chain of cyclones, which we infer from the motion of clouds on the periphery of the individual pearls. We tracked the motion of 26 pearls for 6months in 2008 and noted a few pearls appearing and disappearing, all near the east–west termini of the SoPs feature. During this period, a few of the pearls, varying between 6 and 10, harbored a small circular cloud at the center, which we call the central peaks. In general, a group of vortices with the same sign of vorticity tend to merge; however, our measurements did not detect merger of pearls. The interest in the feature was heightened when the latest planet-encircling storm erupted from the SoPs on December 5, 2010 (Sayanagi, K.M., Dyudina, U.A., Ewald, S.P., Fischer, G., Ingersoll, A.P., Kurth, W.S., Muro, G.D., Porco, C.C., West, R.A. [2013]. Icarus 223, 460–478). The storm severely disrupted the region; the SoPs was last seen on December 24, 2010 in the turbulent wake of the storm, and has not reappeared as of August 2013.

DISTRIBUTION OF CO2IN SATURN'S ATMOSPHERE FROMCASSINI/CIRS INFRARED OBSERVATIONS

This paper focuses on the CO2 distribution in Saturn's atmosphere based on analysis of infrared spectral observations of Saturn made by the Composite Infrared Spectrometer aboard the Cassini spacecraft. The Cassini spacecraft was launched in 1997 October, inserted in Saturn's orbit in 2004 July, and has been successfully making infrared observations of Saturn, its rings, Titan, and other icy satellites during well-planned orbital tours. The infrared observations, made with a dual Fourier transform spectrometer in both nadir- and limb-viewing modes, cover spectral regions of 10-1400?cm?1, with the option of variable apodized spectral resolutions from 0.53 to 15?cm?1. An analysis of the observed spectra with well-developed radiative transfer models and spectral inversion techniques has the potential to provide knowledge of Saturn's thermal structure and composition with global distributions of a series of gases. In this paper, we present an analysis of a large observational data set for retrieval of Saturn's CO2 distribution utilizing spectral features of CO2 in the Q-branch of the ?2 band, and discuss its possible relationship to the influx of interstellar dust grains. With limited spectral regions available for analysis, due to low densities of CO2 and interference from other gases, the retrieved CO2 profile is obtained as a function of a model photochemical profile, with the retrieved values at atmospheric pressures in the region of ~1-10 mbar levels. The retrieved CO2 profile is found to be in good agreement with the model profile based on Infrared Space Observatory measurements with mixing ratios of ~4.9 ? 10?10 at atmospheric pressures of ~1 mbar.

Strong Temporal Variation Over One Saturnian Year: From Voyager to Cassini

Here we report the combined spacecraft observations of Saturn acquired over one Saturnian year (~29.5 Earth years), from the Voyager encounters (1980–81) to the new Cassini reconnaissance (2009–10). The combined observations reveal a strong temporal increase of tropic temperature (~10 Kelvins) around the tropopause of Saturn (i.e., 50 mbar), which is stronger than the seasonal variability (~a few Kelvins). We also provide the first estimate of the zonal winds at 750 mbar, which is close to the zonal winds at 2000 mbar. The quasi-consistency of zonal winds between these two levels provides observational support to a numerical suggestion inferring that the zonal winds at pressures greater than 500 mbar do not vary significantly with depth. Furthermore, the temporal variation of zonal winds decreases its magnitude with depth, implying that the relatively deep zonal winds are stable with time.

Observing extended sources with theHerschelSPIRE Fourier Transform Spectrometer

The Spectral and Photometric Imaging Receiver (SPIRE) on the European Space Agency's Herschel Space Observatory utilizes a pioneering design for its imaging spectrometer in the form of a Fourier Transform Spectrometer (FTS). The standard FTS data reduction and calibration schemes are aimed at objects with either a spatial extent much larger than the beam size or a source that can be approximated as a point source within the beam. However, when sources are of intermediate spatial extent, neither of these calibrations schemes is appropriate and both the spatial response of the instrument and the source's light profile must be taken into account and the coupling between them explicitly derived. To that end, we derive the necessary corrections using an observed spectrum of a fully extended source with the beam profile and the source's light profile taken into account. We apply the derived correction to several observations of planets and compare the corrected spectra with their spectral models to study the beam coupling efficiency of the instrument in the case of partially extended sources. We find that we can apply these correction factors for sources with angular sizes up to \theta_{D} ~ 17". We demonstrate how the angular size of an extended source can be estimated using the difference between the sub-spectra observed at the overlap bandwidth of the two frequency channels in the spectrometer, at 959<\nu<989 GHz. Using this technique on an observation of Saturn, we estimate a size of 17.2", which is 3% larger than its true size on the day of observation. Finally, we show the results of the correction applied on observations of a nearby galaxy, M82, and the compact core of a Galactic molecular cloud, Sgr B2.

Multispectral simultaneous diagnosis of Saturn's aurorae throughout a planetary rotation

From 27 to 28 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 coincidentally 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 for the 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 subcorototational dynamics of emissions associated with the main auroral oval. Precisely, we show that the atmospheric main oval hosts short‐lived subcorotating isolated features together with a bright, longitudinally extended, corotating region locked at the southern SKR phase. We assign the substorm‐like event to a regular, internally driven, nightside ion injection possibly triggered 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 search‐light 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 and we estimate the fraction of visible southern power 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 subcorotating magnetospheric cold plasma, the occurrence of plasmoid activity, and the comparison of auroral fluxes radiated at different wavelengths.

Mathematical analysis of a model for moon-triggered clumping in Saturn’s rings

Spacecraft observations of Saturn’s rings show evidence of an active aggregation–disaggregation process triggered by periodic influences from the nearby moons. This leads to clumping and break-up of the ring particles at timescales of the order of a few hours. A mathematical model has been developed to explain these dynamics in Saturn’s F-ring and B-ring (Esposito et al., 2012) [4], the implications of which are in close agreement with the empirical results. In this paper, we conduct a rigorous analysis of the proposed forced dynamical system for a class of continuous, periodic and zero-mean forcing functions that model the ring perturbations caused by the moon flybys. In specific, we derive the existence of at least one periodic solution to the dynamic system with the period equal to the forcing period of the moon. Further, conditions for the uniqueness and stability of the solution and bounds for the amplitudes of the periodic solution are derived.

Babylonian Observations of Venus: Arcus Visionis, Atmospheric Extinction and Observational Practice

1. INTRODUCTIONIn this paper analyse Babylonian observations of Venus from the second half of the first millennium b.c. to derive physical parameters determine its first and appearance as morning and/or evening star. The new feature in my approach is the use of a physical model for the visibility of Venus in a twilight atmosphere. Moreover, in this study make use of the most recent editions of the Babylonian observational material. The results provide a major advance in understanding the visibility phases of Venus compared to earlier attempts based on the use of the concept of the arcus visionis. ' The method used here has been applied before to analyse Babylonian observations of Saturn in the sixth and the seventh centuries b.c., of stars in the thirteenth century b.c., and of Venus in the seventeenth century b.c.22. THE OBSERVATIONSThe observational material analysed in this paper consists of all Babylonian records of first appearance and appearance of Venus in the Astronomical diaries and related texts from Babylonia, vols i-iii, ? and vi.3 This body of observational texts (henceforth referred to as Diaries and related texts) contains a large - although quite fragmentary - body of naked-eye astronomical observations of the Moon and the planets covering a period of many centuries (650-50 b.c.), and thereby form a unique database for the study of the astronomical practice in Babylon during the second half of the first millennium b.c.Among the observational material in the Diaries and related texts, the first appearances and appearances of the planets take a prominent place. In total the Diaries and related texts contain about 180 records of the first and appearance of Venus. These observations are scattered over the six centuries b.c., and due to the arbitrary nature of the preservation process they form a random selection of all first and appearances of Venus over period. Since one of the purposes of this paper is to study the effects of the variable atmosphere made an effort to select only 'bona fide' observations. This is not a trivial matter because the texts are often damaged and some dates are apparently estimated or computed rather than observed. The observations have discarded fall in several categories:* those with uncertain dates due to omissions or damaged passages in the text;* those with qualifications around, possible, expected;* those where the observer indicated no observations were carried out by recording clouds (DIR) or stating I did not watch (NU PAP); and* those given in the monthly summaries at the end of the month, because in cases where it can be verified these often turn out to be the expected 'ideal' dates rather than actually observed dates.The total number of observational records retained amounts to 92 observations (about 50% of all preserved records) over the period 525-80 b.c., and may be considered as a randomly selected sample comprising 8% of all first and appearances of Venus over period.Throughout the Diaries and related texts the Babylonian term SU (setting) has been translated as last Based on an analysis of Venus observations available at the time, Huber et al.4 pointed out the data make more sense if SU is translated as disappearance, consistent with its actual meaning. This is in agreement with the fact in the earliest set of Venus observations from the time of Ammisaduqa (seventeenth century b.c.) the verb it-bal is used which means disappeared.5 In a footnote on page X of the Diaries and related texts, vol. vi, the editors confess to having become convinced that by SU the texts refer to the first day when a planet could not be seen anymore but in the translation they would stick to last appearance for reasons of consistency. In the analysis below take SU to refer to the disappearance of Venus, which falls one day later than the date of appearance. …

Earliest recorded ground-based decameter wavelength observations of Saturn’s lightning during the giant E-storm detected by Cassini spacecraft in early 2006

We report the history of the first recorded ground-based radio detection of Saturn’s lightning using the Ukrainian UTR-2 radiotelescope at frequencies from 20 to 25MHz. The observations were performed between 29 January and 3 February 2006, during which lighting activity (E-storm) on Saturn was detected by the radio experiment onboard Cassini spacecraft. The minimum detectable flux density (1σ-level) at UTR-2 reached 40Jy (1Jy=10-26Wm-2Hz-1) for narrowband observations (Δf=10kHz) and 4Jy for broadband observations (Δf=1MHz), for an effective telescope area of ≈100,000m2 and integration time of 20ms. Selection criteria including comparison of simultaneous ON/OFF-source observations were applied to distinguish detection of lightning-associated radio pulses from interference. This allowed us to identify about 70 events with signal-to-noise ratio more than 5. Measured flux densities (between 50 and 700Jy) and burst durations (between 60 and 220ms) are in good agreement with extrapolation of previous Cassini measurements to a ground-based observer. This first detection demonstrates the possibility of Solar System planetary lightning studies using large, present and future ground-based radio instruments. The developed methods of observations and identification criteria are also implemented on the UTR-2 radio telescope for the investigation of the next Saturn’s storms. Together with recently published UTR-2 measurements of activity measured after the 2006 storm reported here, the results have significant implications for detectable planetary radio emission in our Solar System and beyond.

Rotational modulation and local time dependence of Saturn's infrared H3+ auroral intensity

Planetary auroral emissions reveal the configuration of magnetospheric field‐aligned current systems. In this study, Cassini Visual and Infrared Mapping Spectrometer (VIMS) observations of Saturn's pre‐equinox infrared H3+ aurorae were analysed to show (a) rotational modulation of the auroral intensity in both hemispheres and (b) a significant local time dependence of the emitted intensity. The emission intensity is modulated by the ‘planetary period’ rotation of auroral current systems in each hemisphere. The northern auroral intensity also displays a lesser anti‐phase dependence on the southern rotating current system, indicating that part of the southern current system closes in the northern hemisphere. The southern hemisphere aurorae were most intense in the post‐dawn sector, in agreement with some past measurements of auroral field‐aligned currents, UV aurora and SKR emitted power. A corresponding investigation of the northern hemisphere auroral intensity reveals a broader dawn‐noon enhancement, possibly due to the interaction of the southern rotating current system with that of the north. The auroral intensity was reduced around dusk and post‐midnight in both hemispheres. These observations can be explained by the interaction of a rotating field‐aligned current system in each hemisphere with one fixed in local time, which is related to the solar wind interaction with magnetospheric field lines.

On the feasibility of characterizing jovian auroral electrons via [formula omitted] infrared line-emission analysis

Here we investigate the feasibility of characterizing the jovian auroral electron energy and flux via H3+ infrared (IR) emission line analysis. Ground based telescopes can monitor jovian infrared auroral activities continuously for an extended time interval compared to the more restricted temporal coverage of ultraviolet (UV) observations. Since the departure from local thermodynamic equilibrium (LTE) varies with vibrational levels and altitude, measurements of the relative emission line intensities reveal the altitude of emission and hence the electron energy. The combination of three H3+ line-intensity ratios is required to determine the electron energy and the background temperature. The feasibility issue is evaluated by studying how the observational error propagates into the error of the estimated electron energy. We have found several best sets of H3+ lines from which the intensity-ratios can be utilized for the present purpose. Using these lines in the observed 2- and 4- micron wavelength ranges, we can estimate the electron energy and the background temperature within errors of a factor of ∼3.5% and 3%, respectively, if the observation error is 1%. Since saturnian H3+ emissions vary far more substantially according to temperature variations, the method described here is not applicable to observations of Saturn.

Characterizing the limitations to the coupling between Saturn's ionosphere and middle magnetosphere

Observations of Saturn's ultraviolet and infrared aurora show structures that, when traced along the planetary magnetic field, map to the inner, middle, and outer magnetosphere. From low to high latitudes the structures seen in the UV are the Enceladus footprint, which maps to an equatorial radius of 4 RS (Saturn radii); a diffuse emission that maps to a broad equatorial region from 4–11 RS on the nightside; and a bright ring of emission that maps to ∼15 RS. With the exception of the Enceladus spot, the magnetospheric drivers for these auroral emissions are not yet fully understood. We apply a 1D spatial, 2D velocity space Vlasov solver to flux tubes mapping from equatorial radii of 4, 6, 9, and 13 RS to Saturn's southern atmosphere. The aim is to globally characterize the field‐aligned potential structure and plasma density profiles. The ionospheric properties ‐ the field‐aligned current densities at the ionospheric boundary, energy intensity profiles and fluxes of the electrons precipitating into the ionosphere ‐ are also determined. We then couple our results to an ionospheric model to calculate the Pedersen conductances at the foot of the relevant flux tubes. We find that for a zero net potential drop between the ionosphere and magnetosphere, there exists a sharp potential drop at ∼1.5 RS along the magnetic field line as measured from the planetary center. The strength of this potential drop is approximately equal to that of the ambipolar potential resulting from the centrifugal confinement of the heavy, cold magnetospheric ion population. We also find that the ionospheric properties respond to changes in the magnetospheric plasma population, which are reflected in the nature of the precipitating electron population. For the flux tube mapping to 9 RS (−70°), the incident electron energy flux into the ionosphere resulting from a magnetospheric plasma population with a small fraction of hot electrons is an order of magnitude less than that inferred from observations, implying that significant high‐latitude field‐aligned potentials (up to 1.5 keV) may exist in the saturnian magnetosphere. Calculated ionospheric Pedersen conductances range from 3.0–18.9 mho, and are thus not expected to limit the currents flowing between the ionosphere and magnetosphere.

The morphology of Saturn's ultraviolet aurora

Spectrographic images in the far ultraviolet, obtained from observations made from 2007 to 2009, are used to quantify and compare the morphology of Saturn's northern and southern aurora in local time. Made by the UVIS imager on the Cassini spacecraft, these observations differ from previous UV observations of Saturn's aurora by offering complete local time coverage of the aurora from small zenith angles. A statistical picture emerges after combining over 105 spectrographic images of the north and south aurora. From its composite, the northern aurora exhibits an intensity peak of ∼40 kR near 05 h local time, while the southern aurora peaks at ∼60 kR near the same local time. Aurora intensities in the post‐noon sectors are essentially constant at ∼10 kR. The half‐power points of latitudinal profiles reveal widths of ∼4° near dawn and ∼8° near dusk in both north and south. The aurora has an approximately circular shape (not oval!), although harmonic fits suggest the northern aurora is more nearly circular than the southern. The center of the northern aurora has an offset of ∼1–2° from the pole, while the southern center is offset by ∼2–3°, with both offsets in the midnight‐to‐dawn quadrant. When mapped to the polar regions, precipitation of field‐aligned energetic electrons (&gt;20 keV) generally lies equatorward of the main auroral intensity at all local times.

Astrometric observations of major satellites of Saturn with a 26-inch refractor

The results of observations of Saturn and its satellites with the 26-inch refractor at Pulkovo are presented. Over the observing period from January 2008 until May 2009, results were found from more than 5000 CCD frames suitable for measurement. On the basis of these frames, 183 positions of major satellites of Saturn (with the exception of Mimas) were obtained. The astrometric reduction was based on the Turner method, with the use of the UCAC2 catalog as a reference. The obtained equatorial coordinates of satellites were compared with the TASS 1.7 theory, and results of comparison are presented. The accuracy of observed positions is 0.05″ on average. Positions of Saturn, calculated on the basis of positions of satellites and their theoretical saturnocentric coordinates according to the TASS 1.7, and the differential coordinates of satellites relative to each other, are also given.