Ionosphere Of Saturn Research Articles

Modification of Titan's ion tail and the Kronian magnetosphere: Coupled magnetospheric simulations

Three‐dimensional multifluid/multiscale modeling is used to study the coupled interaction between the Kronian magnetosphere and Titan's induced magnetosphere. The multifluid aspect of the model allows ions from the solar wind, Saturn's ionosphere, Enceladus torus, and Titan's ionosphere to be tracked separately. The multiscale aspect of the model allows simultaneous resolution of global and local processes down to 100 km near Titan. In the simulation, Titan is placed in the premidnight sector at 2100 Saturn local time. Two orientations of interplanetary magnetic field (IMF) are considered: parallel and antiparallel to Saturn's planetary field. During parallel IMF, plasma conditions near Titan are relatively stable and Titan's ion tail extends to 10 Saturn radii. The ion tail produces mass loading of the Kronian plasma disk and leads to a modification of the position of the plasma sheet. During antiparallel IMF, fingers of cold dense plasma from the inner magnetosphere propagate outward, driven by the centrifugal interchange instability. Titan's interaction with the fingers causes enhanced outflow as well as large changes in the orientation of Titan's ion tail. The plasma fingers are also disrupted by the interaction with Titan. In both IMF configurations, ions lost from Titan do not produce a complete ion torus. However, Titan's ion tail has global importance because it influences the inner boundary of the plasma sheet and has the potential to modify Kronian auroral processes when it is in this position in the Kronian magnetosphere.

Midlatitude and high‐latitude electron density profiles in the ionosphere of Saturn obtained by Cassini radio occultation observations

Nineteen new radio occultations of the ionosphere of Saturn have been obtained since 2006. Sixteen of these occultations were from midlatitude and high latitudes and thus provided important, new information of the ionosphere for these regions. A high degree of variability in the electron densities were observed, but grouping and averaging the observations as low‐, middle‐, and high‐latitude ones clearly showed that the electron densities increase with latitude. The topside scale heights also indicate small increases with latitude, but these changes are small enough so these increases may not be statistically significant.

Transient auroral features at Saturn: Signatures of energetic particle injections in the magnetosphere

We report for the first time transient isolated auroral spots at Saturn's southern polar region, based on Hubble Space Telescope (HST) FUV images. The spots last several minutes and appear distinct from the rest of the auroral emissions. We study two sets of HST and Cassini observations during which Cassini instrumentation detected signatures of energetic particle injections close to the region where, on the same day, HST observed transient auroral spots. On the basis of the simultaneous remote and in situ observations, we discuss the possibility that the transient features are associated with the dynamical processes taking place in the Kronian magnetosphere. Given the limitations in the available observations, we suggest the following possible explanations for the transient aurora. The injection region could directly be coupled to Saturn's ionosphere by pitch angle diffusion and electron scattering by whistler waves, or by the electric current flowing along the boundary of the injected cloud. The energy contained in the injection region indicates that electron scattering could account for the transient aurora process.

Plasma temperatures in Saturn's ionosphere

We have calculated self‐consistent electron and ion temperatures in Saturn's ionosphere using a series of coupled fluid and kinetic models developed to help interpret Cassini observations and to examine the energy budget of Saturn's upper atmosphere. Electron temperatures in the midlatitude topside ionosphere during solar maximum are calculated to range between 500 and 560 K during the Saturn day, approximately 80–140 K above the neutral temperature. Ion temperatures, calculated for only the major ions H+ and H3+, are nearly equal to the neutral temperature at altitudes near and below the height of peak electron density, while they can reach 500 K during the day at the topside. Plasma scale heights of the dusk electron density profile from radio occultation measurements of the Voyager 2 flyby of Saturn have been used to estimate plasma temperature as a comparison. Such an estimate agrees well with the temperatures calculated here, although there is a topside enhancement in electron density that remains unexplained by ionospheric calculations that include photochemistry and plasma diffusion. Finally, parameterizations of the heating rate from photoelectrons and secondary electrons to thermal, ambient electrons have been developed. They may apply for other conditions at Saturn and possibly at other giant planets and exoplanets as well.

Interaction evidence between Enceladus' atmosphere and Saturn's magnetosphere

Saturn's ultraviolet (UV) auroras are highly variable, with much of the activity believed to be controlled by conditions in the solar wind. Like Jupiter, Saturn is also expected to have an electrodynamic interaction between the planet's ionosphere and its satellites' atmospheres, a process that is responsible for the moon's auroral footprint components at Jupiter. Recent observations from several instruments on the Cassini spacecraft, e.g., the UltraViolet Imaging Spectrograph (UVIS) and the Cassini Plasma Spectrometer (CAPS), show strong evidence that Enceladus makes significant plasma contributions to Saturn's magnetosphere. Modeling by Pontius and Hill (2006) suggests that Enceladus' mass loading region may be comparable in extent to Io's, whose auroral footprint emissions are clearly detected. However, the upper limits of Enceladus' magnetic footprint emissions are predicted to be only a few tenths of kilo‐Rayleighs (kR). We present results of a search for Enceladus' auroral footprint using series of UV images of Saturn's aurora taken by the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) in January 2004 and the Advanced Camera for Surveys (ACS) between February 2005 and January 2007. We detected no auroral evidence of the electromagnetic connection between Enceladus' atmosphere and Saturn's ionosphere, in agreement with modeling predictions. As our study could detect no signature of currents linking Saturn's ionosphere and Enceladus' interaction region, the implication is that the field‐aligned currents associated with mass loading at Enceladus are too weak to cause auroral emission.

Statistical morphology of ENA emissions at Saturn

The Magnetospheric Imaging Instrument (MIMI) on the Cassini spacecraft is providing the first energetic neutral particle (ENA) measurements in the magnetosphere of Saturn. Advantageous spacecraft orbits during the first 120 days of 2007 allowed ENA observations to be mapped to the equatorial plane of the planet and surveyed as a statistical ensemble in a Sun‐synchronous coordinate system. When projected onto the equatorial plane, emissions from both energetic hydrogen atoms (20–50 keV) and energetic oxygen atoms (64–144 keV) form toroidal distributions nearly concentric with the planet. The hydrogen torus is continuous in local time, but the oxygen torus has a gap from dawn to noon. When fitted to circles, the H torus has a mean radius of 11.0 ± 0.5 RS, while the O torus has a mean radius of 7.9 ± 0.8 RS (1 RS = 60268 km). Both tori display peaks at just before midnight at local times of 23.6 h (H) and 21.8 h (O). These maxima seem to be regular features over the 120‐day interval surveyed, suggesting the hot spot may be caused by injection of particles from Saturn's magnetotail, as would be the case during substorms. However, the persistence of the ENA emissions suggests they are continuously driven by processes internal to Saturn's magnetosphere. When mapped along magnetic field lines to Saturn's ionosphere, both H and O emissions appear equatorward of the aurora and are distinct from it.

Dependence of the open-closed field line boundary in Saturn's ionosphere on both the IMF and solar wind dynamic pressure: comparison with the UV auroral oval observed by the HST

Abstract. We model the open magnetic field region in Saturn's southern polar ionosphere during two compression regions observed by the Cassini spacecraft upstream of Saturn in January 2004, and compare these with the auroral ovals observed simultaneously in ultraviolet images obtained by the Hubble Space Telescope. The modelling employs the paraboloid model of Saturn's magnetospheric magnetic field, whose parameters are varied according to the observed values of both the solar wind dynamic pressure and the interplanetary magnetic field (IMF) vector. It is shown that the open field area responds strongly to the IMF vector for both expanded and compressed magnetic models, corresponding to low and high dynamic pressure, respectively. It is also shown that the computed open field region agrees with the poleward boundary of the auroras as well as or better than those derived previously from a model in which only the variation of the IMF vector was taken into account. The results again support the hypothesis that the auroral oval at Saturn is associated with the open-closed field line boundary and hence with the solar wind interaction.

Magnetic field structure of Saturn's dayside magnetosphere and its mapping to the ionosphere: Results from ring current modeling

Ring current modeling in Saturn's magnetosphere using Pioneer‐11, Voyager, and Cassini data has shown that the size and strength of the current system grows with the extension of the magnetosphere, governed by the solar wind dynamic pressure. Here we show that the consequent middle magnetosphere field is quasi‐dipolar in form when the magnetosphere is strongly compressed, but extends into a magnetodisc when it is strongly expanded. We also show that the region occupied by the modeled ring current corresponds to an essentially fixed shell of field lines that expands and contracts with the size of the system, thus mapping to an almost fixed co‐latitude range in Saturn's ionosphere, between ∼14° and ∼20° in the northern hemisphere, and ∼16° and ∼22° in the southern hemisphere. The median dayside UV auroral oval is found to map from near the poleward edge of the modeled ring current toward the boundary of open field lines at smaller co‐latitudes. In the equatorial plane this corresponds to a layer ∼2–5 Saturn radius wide (depending on magnetosphere size), extending from near the outer edge of the ring current to the vicinity of the magnetopause.

Polarization measurements of Saturn Electrostatic Discharges with Cassini/RPWS below a frequency of 2 MHz

[1] Early in 2006 the RPWS (Radio and Plasma Wave Science) instrument and the ISS (Imaging Science Subsystem) onboard the Cassini spacecraft detected a lightning storm on Saturn that lasted for about one month. The RPWS measured the so-called SEDs (Saturn Electrostatic Discharges), which are high frequency radio signals produced by lightning discharges. The ISS imaged a remarkable cloud system associated with these SEDs at a latitude of 35° South. Below the frequency of 1825 kHz the RPWS was in a mode capable of measuring the polarization of the SEDs. A surprising result was gained; SEDs appeared to be highly polarized (80%) and were exclusively right-handed polarized with a high degree of circular polarization. We will present an explanation for this diagnosis involving magneto-ionic modes and their differential absorption in the magnetoplasma of Saturn's ionosphere.

IMF dependence of the open-closed field line boundary in Saturn's ionosphere, and its relation to the UV auroral oval observed by the Hubble Space Telescope

Abstract. We study the dependence of Saturn's magnetospheric magnetic field structure on the interplanetary magnetic field (IMF), together with the corresponding variations of the open-closed field line boundary in the ionosphere. Specifically we investigate the interval from 8 to 30 January 2004, when UV images of Saturn's southern aurora were obtained by the Hubble Space Telescope (HST), and simultaneous interplanetary measurements were provided by the Cassini spacecraft located near the ecliptic ~0.2 AU upstream of Saturn and ~0.5 AU off the planet-Sun line towards dawn. Using the paraboloid model of Saturn's magnetosphere, we calculate the magnetospheric magnetic field structure for several values of the IMF vector representative of interplanetary compression regions. Variations in the magnetic structure lead to different shapes and areas of the open field line region in the ionosphere. Comparison with the HST auroral images shows that the area of the computed open flux region is generally comparable to that enclosed by the auroral oval, and sometimes agrees in detail with its poleward boundary, though more typically being displaced by a few degrees in the tailward direction.

Are plasma depletions in Saturn's ionosphere a signature of time‐dependent water input?

Recent radio occultation measurements by the Cassini spacecraft reveal the presence of numerous “ionospheric holes”, or plasma depletions, in Saturn's upper atmosphere that cannot be explained with standard photochemical theory. The holes are remarkably similar in size and shape to artificially‐created depletions first observed in the terrestrial ionosphere during the 1970s. At Earth, such vertical structures are typically caused by the enhanced loss of electrons and ions resulting from the introduction of spacecraft exhaust products (e.g., H2O) into the atmosphere. Using a new model of Saturn's upper atmosphere, we show that a time‐variable influx of water into Saturn's ionosphere could explain the observed plasma depletions. The required influxes present a target to assess for the possible sources and consequences of water processes throughout the Saturnian system.

Energization of charged particles in planetary magnetospheres

A model is presented to describe the energiza- tion of charged particles in planetary magnetospheres. The model is based on the stochastic acceleration produced by a random electric field that is induced by the magnetic field fluctuations measured within the magnetospheres. The sto- chastic behavior of the electric field is simulated through a Monte Carlo method. We solve the equation of motion for a single charged particle—which comprises the stochastic acceleration due to the stochastic electric field, the Lorentz acceleration (containing the local magnetic field and the corotational electric field) and the gravitational planetary ac- celeration of the particle—under several initial conditions. The initial conditions include the ion species and the ve- locity distribution of the particles which depends on the sources they come from (solar wind, ionospheres, rings and satellites). We applied this model to Saturn's inner magne- tosphere using a sample of particles (H + ,H 2O + ,N + ,O + and OH + ) initially located on Saturn's north pole, above the C-Ring, on the south pole of Enceladus, in the north pole of Dione and above the E-Ring. The results show that the particles tend to increase the value of their energy with time reaching several eV in a few seconds and the large en- ergization is observed far from the planet. We can distin- guish three main energization regions within Saturn's inner magnetosphere: minimum (Saturn's ionosphere), intermedi- ate (Dione) and high-energy (Enceladus and the E-ring). The resulting energy spectrum follows a power-law distribution

The Source of Saturn's Kilometric Radiation and the Magnetic Field Line's Spirality

AbstractWe put forward a model and deduce the distribution of Saturn's Kilometric Radiation (SKR) Source. The model is as follows, the Kelvin‐Helmholtz Instability (KHI) will generate kinds of waves, in which there is the Alfven wave. It will propagate to the Saturn's polar area by following the bending magnetic field line. It will also cause the Cyclotron Maser Instability (CMI) leading to the generation of extraordinary wave (R‐X mode) in the process. In our views, the region of the CMI, near the ionosphere of Saturn (1Rs above the polar area, Rs is Saturn radius), is regarded as the same area as the Saturn's Kilometric Radiation Source.In this paper, we deduce that the KHI will appear before 11 LT (Saturn's Local Time), the Alfven wave will take 38.3 min to reach Saturn, the sunward magnetic field line will bend almost 80°. Finally, we get the distribution of SKR source. It locates from 7~9 LT at a relatively lower latitude to 18 LT at the eveningside.

How much oxygen is too much? Constraining Saturn's ring atmosphere

The discovery of a molecular oxygen atmosphere around Saturn's rings has important implications for the electrodynamics of the ring system. Its existence was inferred from the Cassini in situ detection of molecular oxygen ions above the rings during Saturn Orbit Insertion in 2004. Molecular oxygen is difficult to observe remotely, and theoretical estimates have yielded only a lower limit ( N n ≳ 10 13 cm −2 ) to the O 2 column density. Comparison with observations has previously concerned matching ion densities at spacecraft altitudes far larger than the scale height of the neutral atmosphere. This is further complicated by charged particle propagation effects in Saturn's offset magnetic field. In this study we adopt a complementary approach, by focusing on bulk atmospheric properties and using additional aspects of the Cassini observations to place an upper limit on the column density. We develop a simple analytic model of the molecular atmosphere and its photo-ionization and dissociation products, with N n a free parameter. Heating of the neutrals by viscous stirring, cooling by collisions with the rings, and torquing by collisions with pickup ions are all included in the model. We limit the neutral scale height to h ≲ 3000 km using the INMS neutral density nondetection over the A ring. A first upper limit to the neutral column is derived by using our model to reassess O 2 production and loss rates. Two further limits are then obtained from Cassini observations: corotation of the observed ions with the planet implies that the height-integrated conductivity of the ring atmosphere is less than that of Saturn's ionosphere; and the nondetection of fluorescent atomic oxygen over the rings constrains the molecular column from which it is produced via photo-dissociation. These latter limits are independent of production and loss rates and are only weakly dependent on temperature. From the three independent methods described, we obtain similar limits: N n ≲ 2 × 10 15 cm −2 . The mean free path for collisions between neutrals thus cannot be very much smaller than the scale height.

Cassini radio occultations of Saturn's ionosphere: Model comparisons using a constant water flux

Recent radio occultations of Saturn's equatorial ionosphere by the Cassini spacecraft provide important insight into this poorly constrained region. Twelve new electron density profiles identify a clear dawn/dusk asymmetry as well as two apparently separate electron density peaks. This study uses a 3D general circulation model along with 1D water diffusion calculations to examine the possibility that a topside flux of neutral water into Saturn's atmosphere may provide a loss mechanism—via charge exchange with protons—that is sufficient to reproduce the ionosphere observed by Cassini. Results indicate that a constant influx of water of (0.5–1.0) × 107 H2O cm−2 sec−1 is adequate for reproducing Cassini measurements, providing a good match to the main electron density peak at dawn and dusk. In addition, these calculations use a reduced rate for the reaction H+ + H2(ν ≥ 4) → H2+ + H, significantly diminishing its importance in Saturn's ionospheric photochemistry.

Enceladus: A significant plasma source for Saturn's magnetosphere

The Cassini Plasma Spectrometer has reported dramatic perturbations of the magnetospheric plasma flow in a region extending at least 30 satellite radii away from Saturn's small but active icy satellite Enceladus. We interpret these observations here by means of a steady state model of the electrodynamic coupling between Enceladus and Saturn. Neutral water molecules from Enceladus are ionized, predominantly by charge exchange with ambient ions, to produce a pickup current that accelerates them to the local plasma velocity. The consequent addition of angular momentum requires Birkeland (magnetic field‐aligned) currents that couple the newly injected plasma to distant parts of the flux tube and ultimately to Saturn's ionosphere. The rate of local ionization in our model varies with the inverse square of distance from the satellite and is scaled by a free parameter proportional to the ratio of the total mass‐loading rate to Saturn's ionospheric Pedersen conductance. To explain the observed velocity perturbations, we require a total mass‐loading rate ≳100 kg/s if the conductance is ≳0.1 S as expected. If the mass‐loading region is not strongly coupled to Saturn's ionosphere, then the appropriate conductance is the Alfvén “wing” conductance ∼2 S, requiring more than an order of magnitude more mass loading. In either case, Enceladus is clearly implicated as a significant, if not dominant, source of Saturn's magnetospheric plasma.

Proposed model for Saturn's auroral response to the solar wind: Centrifugal instability model

We present a model of Saturn's global auroral response to the solar wind as observed by simultaneous Hubble Space Telescope (HST) auroral images and Cassini upstream measurements of the solar wind taken during the month of January 2004. These observations show a direct correlation between solar wind dynamic pressure and (1) auroral brightening toward dawn local time, (2) an increase of rotational movement of auroral features to as much as 75% of the corotation speed, (3) the movement of the auroral oval to higher latitudes, and (4) an increase in the intensity of Saturn Kilometric Radiation (SKR). Our model, referred to as the centrifugal instability model, provides an alternative to the reconnection model of Cowley et al. (2004a, 2004b, 2005); we suggest the above observations result from Saturn's magnetosphere being a fast rotator. Since the torques on Saturn's outer magnetosphere are relatively low, its outer magnetosphere will tend to conserve angular momentum. When compressed on the dayside, the outer magnetosphere spins up to higher angular velocities, and when it expands, the outer magnetosphere spins down to lower angular velocities. This response occurs since Saturn's ionosphere is unable to enforce corotation. The outer boundary of the plasma sheet at L ∼ 15 is identified as the primary source location for the auroral precipitating particles. Enhanced wave activity, which can precipitate the auroral producing particles, may be present at this boundary. If radial transport is dominated by centrifugally driven flux tube interchange motions, when the magnetosphere spins up, outward transport will increase, and the precipitating particles will move radially outward (since the radial gradient in electron energy flux is negative). This mechanism will cause the auroral oval to move to higher latitudes as observed. The Kelvin‐Helmholtz instability may contribute to the enhanced emission along the dawn meridian, as observed by HST, via enhanced wave activity and corresponding charged particle precipitation.

First results from the ionospheric radio occultations of Saturn by the Cassini spacecraft

The first set of near‐equatorial occultations of the Saturn ionosphere was obtained by the Cassini spacecraft between May and September of 2005. The occultations occurred at near‐equatorial latitudes, between 10°N and 10°S, at solar zenith angles from about 84° to 96°. The entry observations correspond to dusk conditions and the exit ones to dawn. An initial look at the data indicates that the average peak densities are lower and the peak altitude higher at dawn than at dusk, possibly the result of ionospheric decay during the night hours. There are also significant differences between individual dawn and dusk occultations; the initial thought is that this variation must be connected to changes in the water inflow into the upper atmosphere and/or variations in the particle impact ionization rates.

A model of the ionosphere of Saturn's rings and its implications

The detection of cold O + and O + ions in the vicinity of Saturn’s rings during the Cassini Orbiter orbit insertion confirmed expectations that the rings would have a water product atmosphere and ionosphere. These observations prompted a new look at their origin and nature by Johnson et al. [Johnson, R.E., Luhmann, J.G., Tokar, R.L., Bouhram, M., Berthelier, J.J., Sittler, E.C., Cooper, J.F., Hill, T.W., Crary, F.J., Young, D.T., 2006. Icarus 180, 393–402], but also raised questions about the ionosphere’s spatial distribution and fate that inspired the ionospheric model described in this report. Here a test particle model with some Monte Carlo aspects is used to consider the behavior of the O + and O + ions produced in the atmosphere of Saturn’s rings. Key features of these calculations include the Johnson et al. description of the production of the ring atmosphere, and the effects of the offset dipole magnetic field of Saturn. The results suggest that the latter should produce some possibly observable asymmetries in both the inner ring ionosphere and the precipitation of ring ions into the atmosphere of Saturn. Further in situ observations of the rings are not currently planned, but remote sensing instruments on Cassini may provide future observational tests of the model.  2005 Elsevier Inc. All rights reserved.

Effects of ring shadowing on the detection of electrostatic discharges at Saturn

A long‐standing discrepancy exists in determinations from observations and modeling of the diurnal variation of the peak electron density of Saturn's ionosphere. Using a new Saturn‐Thermosphere‐Ionosphere‐Model (STIM), we examine the suggestion by Burns et al. (1983) that Saturn's rings shadow its ionosphere causing radio penetration “holes”, thereby allowing lightning‐induced radio signals (Saturn electrostatic discharges, SEDs) to escape. This lessens the need to invoke globally enhanced loss processes to account for Voyager era observations of nighttime peak density (Nmax) as low as 103 e/cm3 from the SEDs. We find radio frequency “windows” produced by ring shadowing that were narrow and confined to low latitudes during the Voyager era, but are now broadly distributed over northern mid‐latitudes during the Cassini era. If lightning sources occur only at near‐equatorial latitudes, then the far less frequent detection of SEDs by Cassini early in its mission would be consistent with the current ionospheric morphology.