P51D-01
A Diffusive Equilibrium Density Model for a Two-Species Plasma in Saturn's Magnetosphere
Measurements of the upper hybrid resonance frequency by the Radio and Plasma Wave Science (RPWS) instrument have been used to derive electron densities for more than 50 Cassini passes through Saturn's inner magnetosphere between July 1, 2004 and September 1, 2007. As the latitudinal and radial distributions of these density measurements have expanded, increasingly more complex density models have been developed to describe the distribution of plasma inside L=9. The early radial outflow model [Persoon et al., Geophys. Res. Lett., 32, L23105, 2005] found that the electron density has a radial dependence of R- 3.63, but the model was restricted to density measurements in the equatorial plane. As density measurements at higher latitudes became available, a simple centrifugal potential model [Persoon et al., Geophys. Res. Lett., 33, L18106, 2006] was developed for the plasma distribution along the planetary magnetic field lines. This early centrifugal potential model, however, had a limited latitudinal distribution and was able to resolve only one ion species in the plasma distribution. Recent plasma density measurements acquired at higher latitudes have made it possible to resolve two distinct plasma components, assumed to be the water group ions (W+) and the hydrogen ions (H+). When compared to the centrifugal potential model, the density measurements for 5<L<9 yield a good fit to an equatorial density profile that varies as L-4 RS for the water group ions and as L-5 RS for the hydrogen ions and plasma scale heights that vary as (0.1)L1.3 RS for the water group ions and (0.3)L1.3 RS for the hydrogen ions, where RS is the radius of Saturn. Although centrifugal force is the dominant force acting on the co-rotating plasma at these radial distances, the ambipolar electric field force becomes increasingly significant at higher latitudes and must be considered in a credible plasma density model for Saturn's magnetosphere. Plasma densities and anisotropies are determined from the comparison of the RPWS densities to a diffusive equilibrium model, derived from a solution to the full force balance equation for a plasma distribution along Saturn's magnetic field lines, originally presented by Richardson and Sittler [ J. Geophys. Res., 95, 12019, 1990] using the Voyager data. A contour plot of the plasma density in Saturn's inner magnetosphere is constructed from the fit of the electron density measurements to the diffusive equilibrium model.
P51D-02 INVITED
Saturn's ring ionosphere
The arrival of Cassini at Saturn on 1 July 2004 included a spectacular trajectory close to Saturn's rings. This allowed the first direct measurements of the ring ionosphere of Saturn, long postulated but somewhat controversial, and never measured in-situ before. This confirmed the existence of an atmosphere and ionosphere of the rings, and there were several surprising results. First, the atmosphere and ionosphere appears to be dominated by molecular oxygen – so Saturn's rings join Europa and Earth as having a molecular oxygen component to the atmosphere. Second, photoelectrons from the far side of the rings were attenuated by regions of higher optical density in the rings leading to a remarkable anticorrelation between electron fluxes and ring images. In addition, electron spectra contained photoelectrons from the ring ionosphere. Third, it has recently been postulated that currents driven by Saturn lightning may play a role in producing ring spokes. In this paper we review the relevant results from Cassini measurements, and current theory and ideas on production and loss processes. We present our current understanding of this remarkable phenomenon.
P51D-03 INVITED
Titan's magnetospheric interaction
Titan's magnetospheric interaction has been extensively studied with observations from both the Voyager and Cassini spacecraft and investigated with various numerical models. Titan has a well extended atmosphere/ionosphere system and no appreciable intrinsic magnetic field. Its interaction with the rotating plasma flow in the Saturnian magnetosphere is controlled by both the upstream plasma and Titan's Saturn local time. The interaction results in a complex plasma environment near Titan: the plasma flow slows down due to ion pick-up and the resulting mass loading, the magnetic field lines pile-up in front of the obstacle and form an induced bipolar magnetotail in the wake region. Kinetic processes are believed to be important in the interaction process due to the large gyroradii of the heavy ion plasma component. In this presentation, we will review Titan's interaction process and discuss the applicability of various numerical models, including a comparison of the pros and cons of contemporary hybrid models and fluid models in particular MHD-models.
P51D-04
The electron density structure of the Titan ionosphere
Eight vertical profiles of the electron density in Titan's ionosphere were derived from the Cassini radio occultations of March 26, and May28, 2007 (T27 and T31), as well as those of March 19, 2006 (T12), and May 20,2006 (T14) . The 2006 occultations occurred at low Southern latitudes of 14.7S, 36.2S, 19.8S, and 21.9S. The 2007 occultations were nearly polar, at latitudes of 75S and 61N for T27. and 75S and 74N for T31.. The solar zenith angles for all occultations were near the terminator, ranging from 85 to 95 deg. The ionosphere peak was observed to lie close to an altitude of 1200 km, and the observed peak densities ranged from about 1.2 to 2.2 x 103 cm-3, which is in good agreement with other Cassini observations and the previous Voyager radio occultation results. In all measurements, the peak densities are about 30% higher near the dusk terminator, showing the influence of solar EUV. Radio occultation observations of the Titan ionosphere are difficult because of its low density and small size, and it was facilitated by the unprecedented Cassini radio science system, which has three frequencies that can operate simultaneously: S-band (2.3 GHz), X-band (8.4 GHz), and Ka-band (32 GHz). In particular, Ka-band had never been used before to probe Titan's ionosphere, and the signal-to-noise ratios at all frequencies of 42, 54, and 48 dB-Hz., respectively, have never before been achieved. The results indicate a clear dusk-dawn asymmetry in peak electron density, with the average dusk density being about 450% greater than the dawn side. . The dusk profiles also have more structure both above and below the main peak, including a peak at about 500 km. altitude.
P51D-05
Models of Titan's Ionosphere
During the TA and T18 encounters with Titan, in situ measurements were made of Titan's atmosphere and ionosphere by several instruments on board the Cassini Orbiter, including the Ion and Neutral Mass Spectrometer (INMS), the Langmuir probe on the Cassini Radio and Plasma Wave Experiment (RPWS), and the Cassini Plasma Spectrometer Subsystem (CAPS). Both of these encounters were on the day as well as the night side of Titan. The model uses neutral densities measured by the INMS instrument and the electron temperature was measured by the RPWS instrument. The model also includes energetic electron fluxes measured by the CAPS instrument, which act as an important source of ionization on the night side. The modeled ion densities are compared with densities measured by INMS in its Open Source mode.
P51D-06
Holes in the Nightside Ionosphere of Titan
Flybys of Titan by Cassini reveal a highly variable and complex structure of its ionised environment. On July 2, 2006, Cassini flew across the wake and nightside of Titan. Measurements of the electron density and temperature by the Radio and Plasma Waves Science instrument (RPWS) have been employed to investigate the presence of density holes in the antisolar ionosphere of Titan. During this flyby (T15), three depletions of the electron number density by a factor 10 to 50 are observed on the nightside. These holes are large scale structures which extend ~10° in longitude (500-700 km). Furthermore, the magnetic pressure in these holes is larger than the thermal pressure, suggesting that the magnetic pressure is adequate to prevent ionospheric plasma from quickly refilling holes. A larger plasma speed in these structures is also implied by the RPWS observations and supports a change in the ion plasma composition. Presence of magnetospheric plasma (ions and electrons) in these holes is investigated by the Cassini Plasma Spectrometer (CAPS) observations. The magnetic field topology provided by the Cassini magnetometer instrument (MAG) and different scenarios of hole formations are examined. Analogous structures have been observed in the Venusian ionosphere by Pioneer Venus Orbiter and a comparison between Titan and Venus is presented.
P51D-07 INVITED
Rotational Dynamics of Saturn's Magnetosphere
Saturn's magnetosphere exhibits periodicities at something close to Saturn's rotation period in many different plasma parameters. Best known is the periodic intensification in the Saturn kilometric radiation (SKR), with observations going back as far as Voyager 25 years ago, and also including Ulysses and Cassini. Although the Voyager SKR results were long assumed to represent Saturn's internal rotation rate, variations in the period of order 0.01 observed first by Ulysses and more recently by Cassini (Kurth et al. 2007) have cast considerable doubt on that scenario. With the reanalysis of the Saturn Pioneer and Voyager magnetometer data by Espinosa et al., 2003, similar periodicities in Saturn's magnetic field were revealed. Cassini measurements have confirmed and refined our characterization of the magnetic rotational signatures, which seem to follow the SKR period quite closely (e.g., Giampieri et al., 2006). Less precisely determined, but with a similar repetition rate, energetic particle periodicities have been identified in the outer magnetosphere (Carbary et al., 2007). Energetic neutral atom emissions also vary nearly sinusoidally at times (Paranicas et al., 2005) at a similar period. Plasma density in the inner magnetosphere also follows the SKR period very closely (Gurnett et al., 2007). At the same time, solar wind effects at Saturn are clearly important, and modify the dynamics of Saturn's&p magnetosphere in very direct ways (Crary et al., 2004; Bunce et al., 2007) as well as more subtly (Cecconi and Zarka 2005). While much of this behavior has been documented, understanding it has been elusive. Models of Saturn, some of which include coupling between the ionosphere and the magnetosphere, have been put forward in the hopes of explaining various features of different data sets. None seem to be sufficient to satisfy all the demands of the data, or at least our understanding of them, at this time. The rotation period exhibited in the data is remarkably stable, yet it varies too much to represent direct driving from the interior. The SKR periodicity is considered to be modulation of a high latitude, field aligned current driven instability. The plasma density at 3 to 5 Rs is clearly on relatively low latitude field lines. The ENA periodicity is on mid-latitude field lines. Coupling between low altitudes and high altitudes should be a strong function of latitude, yet similar periods and stability are observed at both low and high latitudes. The very existence of a rotational periodicity in what appears to be a very symmetric dipole is not explained. What generates the asymmetry? What sustains it over hundreds, perhaps thousands of rotations? How is the period maintained in a system that shows evidence (in both auroral forms [Gerard et al.2006] and in situ plasma [Sittler et al. 2005]) of plasma corotation well below the rate implied by the SKR period? This paper will discuss these mysteries, will try to organize them toward a common explanation, and will concentrate on posing questions that must be answered by any comprehensive model. References not listed, as they exceed the character count.