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1.
The properties of Saturn's electrostatic discharges (SED) as observed by the Voyager Planetary Radio Astronomy experiment during the two Voyager encounters with Saturn are summarized. Several models for the formation of SED are discussed in light of these observations. The most likely source regions appear to be either the equatorial zone of the planet or the dense part of the B ring near 1.80 Rs. The strenghts and weaknesses of each of these possibilities are examined. Neither possibility accounts fully for the observed SED properties in a simple way. A search for an anomaly near 1.80 Rs in the data of other experiments aboard Voyager has been carried out, and at least one and possibly more such experiments do indeed obtain anomalous data at this point in the ring system. There thus appears to be unexplained phenomena at this point, independent of the PRA data, and it is a short step to postulate that a single object may be the cause of all such phenomena. 相似文献
2.
During 2004 the Cassini/RPWS (Radio and Plasma Wave Science) instrument recorded about 5400 SEDs (Saturn Electrostatic Discharges), which were organized in 4 storm systems and 95 episodes. A computer algorithm with different intensity thresholds was applied to extract the SEDs from the RPWS data, and a statistical analysis on the main characteristics of these SEDs is performed. Compared to the SEDs recorded by the Voyagers in the early 1980s, some characteristics like SED rate, intensity, signal duration, or power spectrum are similar, but there are also remarkable differences with regard to time occurrence and frequency range: The first appearance of SEDs (storm 0) was recorded by RPWS from a distance of more than 300 Saturn radii at the end of May 2004, followed by storm A in mid-July, storm B at the beginning of August, and the most prominent storm C throughout most of September. There were also significant intervals of time with no detectable SED activity, e.g., SEDs were practically absent from October 2004 until June 2005. No clear indication for SEDs below a frequency of 1.3 MHz could be found. We suggest that the SED storms A, B, C, and possibly also storm 0 originate from the same storm system residing at a latitude of 35° South, which lasted for several months, waxed and waned in strength, and rotated with the Voyager radio period of Saturn. The SED source might be located in the updrafting water clouds beneath the visible cloud features detected in the Cassini images. 相似文献
3.
Radiative-convective equilibrium models for Jupiter and Saturn have been produced in a study centered primarily on the stratospheric energy balance and the possible role of aerosol heating. These models are compared directly to the thermal structure profiles obtained from Voyager radio occultation measurements. The method is based on a straightforward flux divergence formulation derived from earlier work (J. S. Hogan, S. I. Rasool, and T. Encrenaz 1969, J. Atmos. Sci.26, 898–905). The balance between absorbed and emitted energies is computed iteratively at each level in the atmosphere, assuming local thermodynamic equilibrium and employing a standard treatment of opacities. Results for Jupiter indicate that a dust-free model (no aerosol heating) furnishes a good mean thermal profile for the stratosphere when compared with the Voyager 1 radio occultation (RSS) measurements. These observations of the equatorial region (0° and 12°S, respectively) exhibit periodic vertical structure. Of course, among many possible complications, the Voyager profiles may not represent typical excursions from the mean. The aerosol heat depositions required to match these profiles exactly, relative to the nominal dust-free model, are reasonably consistent with independent estimates for “continuum” absorbers. Other interpretations are discussed, along with a survey of problems encountered in intercomparing the lower portions (P ? 300 mb) of the models, the RSS profiles, and a recent IRIS equatorial profile. Although aerosol heating cannot be ruled out at low latitudes on Jupiter, our results indicate that it may not be required to reproduce the Voyager 1 RSS profiles. On the other hand, heating by aerosols or some other absorber seems necessary in order to match the high-latitude Voyager 2 RSS temperature profile. The Saturn models are relatively simple and in good-to-excellent agreement with the Voyager 2 RSS profiles at all levels. Our comparisons indicate that aerosol heating played a minor role in Saturn's midlatitude stratospheric energy balance at the time of the Voyager 2 encounter. These models, however, may need to be reassessed once the hydrocarbon concentrations have been more precisely determined. 相似文献
4.
《Icarus》2002,158(1):272-275
A singular feature in planetary atmospheres, the saturnian midnorthern latitude “ribbon” wave, was discovered in images obtained during the Voyager 1 and 2 encounters with the planet in 1980 and 1981. Here we present evidence of its first detection 14 years later (about half a Saturn year later) using archived Hubble Space Telescope images. The properties of the recovered wave (latitude location, dominant wavenumber, meridional amplitude, and reflectivity contrast) in 1994-1995 are similar to those observed during the Voyager era. 相似文献
5.
《Planetary and Space Science》1999,47(10-11):1175-1182
We present evolutionary sequences for Jupiter and Saturn, based on new non-gray model atmospheres, which take into account the evolution of the solar luminosity and partitioning of dense components to deeper layers. The results are used to set limits on the extent to which possible interior phase separation of hydrogen and helium may have progressed in the two planets. When combined with static models constrained by the gravity field, our evolutionary calculations constrain the helium mass fraction in Jupiter to be between 0.20 and 0.27, relative to total hydrogen and helium. This is consistent with the Galileo determination. The helium mass fraction in Saturn’s atmosphere lies between 0.11 and 0.21, higher than the Voyager determination. Based on the discrepancy between the Galileo and Voyager results for Jupiter, and our models, we predict that revised observational results for Saturn will yield a higher atmospheric helium mass fraction relative to the Voyager value. 相似文献
6.
It is suggested that as a result of the dawn-dusk asymmetry of the magnetospheric structure of Saturn, caused by the interplanetary magnetic field, the Voyager 1 spacecraft remained in the closed region (the extended plasma sheet) during its entire traversal through the tail region, so that it did not have an opportunity to penetrate into the high latitude lobe (the open region). 相似文献
7.
On January 23, 2006, the Cassini/RPWS (Radio and Plasma Wave Science) instrument detected a massive outbreak of SEDs (Saturn Electrostatic Discharges). The following SED storm lasted for about one month and consisted of 71 consecutive episodes. It exceeded all other previous SED observations by Cassini as well as by the Voyagers with regard to number and rate of detected events. At the same time astronomers at the Earth as well as Cassini/ISS (Imaging Science Subsystem) detected a distinctive bright atmospheric cloud feature at a latitude of 35° South, strongly confirming the current interpretation of SEDs being the radio signatures of lightning flashes in Saturn's atmosphere. In this paper we will analyze the main physical properties of this SED storm and of a single small SED storm from 2005. The giant SED storm of 2006 had maximum burst rates of 1 SED every 2 s, its episodes lasted for 5.5 h on average, and the episode's periodicity of about 10.66 h exactly matched the period of the ISS observed cloud feature. Using the low frequency cutoff of SED episodes we determined an ionospheric electron density around 104 cm−3 for the dawn side of Saturn. 相似文献
8.
The region in the Saturn system between the F ring and the outer edge of the A ring is an area that appears, in images from the imaging experiment, to be virtually devoid of material except for three small satellites. Near the orbit of 1980S28, Atlas—the innermost satellite—the Voyager Photopolarimeter Stellar Occultation data show a discontinuity in count rate which marks a boundary between the tenuous materials near the outer edge of the A ring and the orbit of Atlas. The data pertaining to this region have been examined with the aid of statistics and models generated from other similar ring structures. It is concluded that the discontinuity is real, implying the existence of tenuous material of normal optical depth of 0.01 to 0.006 in this region. 相似文献
9.
A time-dependent one-dimensional model of Saturn's ionosphere has been developed as an intermediate step towards a fully coupled Saturn Thermosphere-Ionosphere Model (STIM). A global circulation model (GCM) of the thermosphere provides the latitude and local time dependent neutral atmosphere, from which a globally varying ionosphere is calculated. Four ion species are used (H+, H+2, H+3, and He+) with current cross-sections and reaction rates, and the SOLAR2000 model for the Sun's irradiance. Occultation data from the Voyager photopolarimeter system (PPS) are adapted to model the radial profile of the ultraviolet (UV) optical depth of the rings. Diurnal electron density peak values and heights are generated for all latitudes and two seasons under solar minimum and solar maximum conditions, both with and without shadowing from the rings. Saturn's lower ionosphere is shown to be in photochemical equilibrium, whereas diffusive processes are important in the topside. In agreement with previous 1-D models, the ionosphere is dominated by H+ and H+3, with a peak electron density of ∼104 electrons cm−3. At low- and mid-latitudes, H+ is the dominant ion, and the electron density exhibits a diurnal maximum during the mid-afternoon. At higher latitudes and shadowed latitudes (smaller ionizing fluxes), the diurnal maximum retreats towards noon, and the ratio of [H+]/[H+3] decreases, with H+3 becoming the dominant ion at altitudes near the peak (∼1200-1600 km) for noon-time hours. Shadowing from the rings leads to attenuation of solar flux, the magnitude and latitudinal structure of which is seasonal. During solstice, the season for the Cassini spacecraft's encounter with Saturn, attenuation has a maximum of two orders of magnitude, causing a reduction in modeled peak electron densities and total electron column contents by as much as a factor of three. Calculations are performed that explore the parameter space for charge-exchange reactions of H+ with vibrationally excited H2, and for different influxes of H2O, resulting in a maximum diurnal variation in electron density much weaker than the diurnal variations inferred from Voyager's Saturn Electrostatic Discharge (SED) measurements. Peak values of height-integrated Pedersen conductivities at high latitudes during solar maximum are modeled to be ∼42 mho in the summer hemisphere during solstice and ∼18 mho during equinox, indicating that even without ionization produced by auroral processes, magnetosphere-ionosphere coupling can be highly variable. 相似文献
10.
《Icarus》1986,67(2):189-204
Imaging data are presented which indicate that small satellites exist in the Saturnian system in addition to those whose orbits had been firmly established by the time of the Voyager 2 encounter with Saturn. The noise characteristics of the Voyager camera system, the images themselves, and the best current interpretation of them are discussed. No two observations could be connected with motion of a single object. The orbital distance of the object with the most convincing image was found to lie midway between the orbital radii of Mimas and Enceladus if equatorial motion is assumed. However, if the object has an inclination comparable to Mimas's (∼11°.5°), its orbital distance could lie anywhere in the Mimas-Enceladus region. The object appeared to trail Mimas by ∼ 30° (or ∼ 180° out of phase with the “Mimas ghost”) but also trailed Enceladus by ∼ 65°, opening the possibility that the object could reside at the Enceladus L5 point. At this position it could be a source of E-ring material and the vertical excursions due to its inclined motion (equivalent to ∼ 1 arcsec as viewed from the Earth) could provide an explanation for the vertical extent and structure of the E ring. 相似文献
11.
Full-disk and high-resolution measurements recorded during the Voyager 1 flyby of Saturn by the radiometer of the infrared instrument, IRIS, indicate a geometric albedo of 0.242 ± 0.012, which is lower than previous estimates. The given error is largely due to uncertainties in systematic corrections; random effects are small. Combining this measurement with the Pioneer-derived phase integral yields a Bond albedo of 0.342 ± 0.030. Infrared spectra recorded at the same time by the Michelson interferometer, along with a model extrapolation to wavenumbers not covered by the instrument, yield an effective temperature of 95.0 ± 0.4°K. As in the case of the radiometer, random instrumental errors are small, and the quoted error in the effective temperature reflects primarily uncertainties in systematic corrections. The rings of Saturn significantly affect both the short- and long-wavelength fluxes. From these measurements the internal heat flux of Saturn is 2.01 ± 0.14 10?4W cm?2, and the energy balance, defined as the ratio of total emitted to total absorbed energy, is 1.78 ± 0.09. 相似文献
12.
《Planetary and Space Science》1999,47(10-11):1277-1283
A regular extensive CCD imaging of Saturn allowed us to analyze the discrete cloud activity in the Equatorial Zone from 1995 to 1997. The large-scale storm observed in 1994 at +10° (Sanchez-Lavega et al., 1994, Sanchez-Lavega et al., 1996) was rediscovered in 1995, reaching a lifetime >1 year. Its slow motion characterized by a zonal velocity difference of −150 ms−1 relative to background flow is confirmed. Our red and near infrared observations showed a strong increase of white cloud activity in the southern Equatorial Zone (latitude −13.5°) during 1996, declining later on during 1997. Cloud tracking of two prominent plumes and other features allowed us to measure zonal wind velocities and to compare them to the Voyager zonal flow velocity profile. We note that in general the 1995–1997 features have velocities lower than those measured with the Voyagers. Altitude differences in the clouds and hence different zonal velocities, or real changes in the zonal jet as a consequence of Saturn’s insolation cycle and ring-shadowing, can be the reason for such differences. 相似文献
13.
《Icarus》1987,72(1):69-78
Observations of the Uranian rings were made in several color filters by the Voyager Imaging Science experiment in January 1986 for the purpose of determining the color of the rings. Selected images were taken through the Violet (λ = 0.41 μm), Clear (λ = 0.48 μm), and Green (λ = 0.55 μm) filters of the Voyager 2 narrow angle camera. The results of the analysis are consistent with the α, β, η, γ, δ, and ϵ rings being very dark, with flat spectra throughout the visible, and are comparable to the latest Voyager results showing a lack of color for the Uranian satellites. The general lack of color in the ring/satellite system of Uranus is remarkably different than the case of the distinctly reddish systems of Jupiter and Saturn. The unique combination of low absolute reflectivity and flat spectrum which characterizes the Uranian rings supports the concept that the Uranian ring material is compositionally distinct from either the Si- and S-rich Jovian ring and inner satellites, or the water-ice-rich rings and inner satellites of Saturn. Of all cosmically abundant materials, the candidate which best matches the low brightness and flat spectrum of the Uranian rings is carbon. 相似文献
14.
Io's neutral sodium emission cloud was monitored during the period of Voyager 1 encounter from two independent ground-based sites. Observations from Table Mountain Observatory verified the continued existence of the “near-Io cloud” (d < 1.5 × 105 km, for 4πI > 1 kR; R denotes Rayleigh) while those from Wise Observatory showed a deficiency in the weaker emission at greater distances from Io. The sodium cloud has been monitored from both observatories for several years. These and other observations demonstrate that the behavior of the cloud is complex since it undergoes a variety of changes, both systematic and secular, which can have both time and spatial dependencies. The cloud also displays some characteristics of stability. Table Mountain images and high-dispersion spectra (resolution ) indicate that the basic shape and intensity of the “near cloud” have remained relatively constant at least since imaging observations began in 1976. Wise Observatory low-dispersion spectra (resolution ) which have been obtained since 1974 demonstrate substantial variability of the size and intensity of the “far cloud” (d ? 1.5 × 105 km) on a time scale of months or less. Corresponding changes in the state of the plasma associated with the Io torus are suggested, with the period of Voyager 1 encounter represented as a time of unusually high plasma temperature and/or density. Dynamic models of the sodium cloud employing Voyager 1 plasma data provide a reasonable fit to the Table Mountain encounter images. The modeling assumptions of anisotropic ejection of neutral sodium atoms from the leading, inner hemisphere of Io with a velocity distribution characteristic of sputtering adequately explain the overall intensity distribution of the “near cloud”. During the Voyager 1 encounter period there appeared a region of enhanced intensity projecting outward from Io's orbit and inclined to the orbital plane. This region is clearly distinguished from the sodium emission normally aligned with the plane of Io's orbit. The process responsible for this phenomenon is not yet understood. Similar but less pronounced features are also present in several Table Mountain images obtained over the past few years. 相似文献
15.
Encounter of Voyager with Saturn’s environment revealed the presence of electromagnetic ion-cyclotron waves (EMIC) in Saturnian magnetosphere. Cassini provided the evidence of dynamic particle injections in inner magnetosphere of Saturn. Also inner magnetosphere of Saturn has highest rotational flow shear as compared to any other planet in our solar system. Hence during these injections, electrons and ions are transported to regions of stronger magnetic field, thus gaining energy. The dynamics of the inner magnetosphere of Saturn are governed by wave-particle interaction. In present paper we have investigated those EMIC waves pertaining in background plasma which propagates obliquely with respect to the magnetic field of Saturn. Applying kinetic approach, the expression for dispersion relation and growth rate has been derived. Magnetic field model has been used to incorporate magnetic field strength at different latitudes for radial distance of \(6.18~R_{{s}}\) (\(1~R_{{s}}= 60{,}268~\mbox{km}\)). Various parameters affecting the growth of EMIC waves in cold bi-Maxwellian background and after the hot injections has been studied. Parametric analysis inferred that after hot injections, growth rate of EMIC waves increases till \(10^{\circ}\) and decreases eventually with increase in latitude due to ion density distribution in near-equatorial region. Also, growth rate of EMIC waves increases with increasing value of temperature anisotropy and AC frequency, but the growth rate decreases as the angle of propagation with respect to \(B_{0}\) (Magnetic field at equator) increases. The injection events which assume the Loss-cone distribution of particles, affect the lower wave numbers of the spectra. 相似文献
16.
P. Zarka W.M. Farrell M.L. Kaiser E. Blanc W.S. Kurth 《Planetary and Space Science》2004,52(15):1435-1447
Radio signatures of lightning discharges have been detected by the Voyager spacecraft near Saturn and Uranus up to 40 MHz. Corresponding flux densities at the distance of the Earth are up to 1000 Jansky (Jy) for Saturn (1 event per minute above 50 Jy, with 30–300 ms duration) and up to a few tens of Jansky for Uranus. Low Frequency ARray LOFAR will allow us to detect and monitor the lightning activity at these two planets. Imaging will allow us to locate lightning sources on Saturn's disk (even if with moderate accuracy), which could then be correlated to optical imaging of clouds. Such observations could provide new information on electrification processes, atmospheric dynamics, composition, and geographical and seasonal variations, compared to the Earth. In addition, lightning may play a role in the atmospheric chemistry, through the production of non-equilibrium trace organic constituents potentially important for biological processes. LOFAR observations can also help us to assess the existence of lightning at Neptune (marginally detected by Voyager), at Venus (where their existence is very controversial), and at Mars (possibly resulting from dust cloud charging). At Jupiter, low-altitude ionospheric layers of meteoritic origin and/or intrinsically long discharge duration seem to prevent the emission and escape of high-frequency radio waves associated with lightning. LOFAR thus presents good possibilities for the detection and study of solar system planetary lightning; we also discuss its relevance to bring new information on Terrestrial lightning-related upper atmosphere transient phenomena (sprites, TIPPs…). Instrumental constraints are outlined. 相似文献
17.
Robert S. French Mark R. Showalter Rafael Sfair Carlos A. Argüelles Myriam Pajuelo Patricio Becerra Matthew M. Hedman Philip D. Nicholson 《Icarus》2012,219(1):181-193
Image photometry reveals that the F ring is approximately twice as bright during the Cassini tour as it was during the Voyager flybys of 1980 and 1981. It is also three times as wide and has a higher integrated optical depth. We have performed photometric measurements of more than 4800 images of Saturn’s F ring taken over a 5-year period with Cassini’s Narrow Angle Camera. We show that the ring is not optically thin in many observing geometries and apply a photometric model based on single-scattering in the presence of shadowing and obscuration, deriving a mean effective optical depth τ ≈ 0.033. Stellar occultation data from Voyager PPS and Cassini VIMS validate both the optical depth and the width measurements. In contrast to this decades-scale change, the baseline properties of the F ring have not changed significantly from 2004 to 2009. However, we have investigated one major, bright feature that appeared in the ring in late 2006. This transient feature increased the ring’s overall mean brightness by 84% and decayed with a half-life of 91 days. 相似文献
18.
R. Vasundhara 《Planetary and Space Science》2008,56(14):1791-1796
Salient features of the analysis of the mutual event light curves of planetary satellites are presented. The need to carefully evaluate the flux contribution of the occulting/eclipsing satellite to the total flux is illustrated. Albedo variations on the satellites will produce signatures on the mutual event light curves. The partial events of the upcoming mutual event series of the uranian satellites can be modeled taking into account the albedo variations inferred from the maps of the southern regions imaged by Voyager 2 when only these regions are occulted/eclipsed. This will enable a robust determination of the astrometric parameters. The shape and asymmetry of the mutual event light curves along with the rotational light curves of the satellites obtained simultaneously during the planet's equinox crossing period can be utilized to obtain a coarse albedo map of the northern hemisphere of the satellites. These studies will also help in investigating possible changes in the known southern regions since the 1986 encounter of Voyager 2. 相似文献
19.
《Icarus》1986,68(1):120-166
Diffraction of radio waves is a prominent phenomenon in Voyager 1 radio occultation measurements of Saturn's rings. It limits the effective radial resolution of observed signal intensity and phase to the characteristic Fresnel scale F, which is set by the geometry and wavelength, Λ. For the two Voyager wavelengths at Saturn, F ≅ 9–15 km at 3.6 cm Λ, and F ≅ 17–29 km at 13 cm Λ. This limitation can be largely removed by inverse-Fresnel filtering of the complex (i.e., amplitude and phase) observed signals. An Huygens-Fresnel formulation of the diffracted signal in terms of a circularly symmetric, complex gray-screen model of the rings, valid to second order in phase, leads to an exact Fresnel transform solution for the complex transmittance of the screen, which is useful for analysis. Extension of the formulation to fourth order in the phase of the transform kernel provides a practical implementation where the final resolution is limited by uncertainties in system parameters and noise. Consideration of the effects of uncertainties in the geometry, finite width of the data window employed, analytical approximations used, profile reconstruction fidelity required, system thermal noise, and system phase stability shows the phase stability and thermal noise to be the most critical factors for realistic systems. For Voyager at Saturn, phase instability limits radial resolution to values of the order of F/90, or about 200 m for optically thin rings. For more opaque rings, useful signal-to-noise ratios are the limiting factor: the resolution achieved at 3.6 cm Λ is typically 200–400 m over Ring C and the Cassini Division, 1–4 km over Ring A, and is greater than about 4 km over Ring B. For Voyager 2 at Uranus, the achievable resolution at 3.6 cm Λ is set by system phase stability and should approach 30 m as long as the normal opacity does not exceed ∼2. Reconstructed profiles of limited regions of Saturn's rings illustrate the technique. These reveal a remarkable array of small-scale (∼1 km) ring structures, including very sharp edges, narrow ringlets, gaps with distinctive edge profiles, wakes of embedded satellites, bending waves, density waves, and many unidentified wave-like phenomena. Profiles reconstructed over the full extent of the rings are available currently at 4.2 km and 900 m resolutions, and will be available presently at 400 m resolution. 相似文献
20.
R.H. Hildebrand R.F. Loewenstein D.A. Harper G.S. Orton Jocelyn Keene S.E. Whitcomb 《Icarus》1985,64(1):64-87
We have measured the brightness temperatures of Jupiter, Saturn, Uranus, and Neptune in the range 35 to 1000 μm. The effective temperatures derived from the measurements, supplemented by shorter wavelength Voyager data for Jupiter and Saturn, are 126.8 ± 4.5, 93.4 ± 3.3, 58.3 ± 2.0, and 60.3 ± 2.0°K, respectively. We discuss the implications of the measurements for bolometric output and for atmospheric structure and composition. The temperature spectrum of Jupiter shows a strong peak at ～350 μm followed by a deep valley at ～450 to 500 μm. Spectra derived from model atmospheres qualitatively reproduce these features but do not fit the data closely. 相似文献