P23C-01 INVITED
Saturn's Interior and Deep Rotation Rate
Temporal changes in the periodicities of Saturn's kilometric radiation and magnetic field show that these signals reflect magnetospheric processes and not the rotation rate of Saturn's deep interior. There is thus no way known at present to directly measure the rotation period of Saturn. We have proposed (Anderson and Schubert, Science 317, 7 September 2007) a plausible, but not definitive, method to infer Saturn's rotation rate. We use Cassini gravitational data (Jacobson et al., 2006, Astron. J. 132, 2520) along with Pioneer and Voyager radio occultation and wind data to simultaneously determine Saturn's rotation rate and internal structure. We hypothesize that Saturn's rotation period can be constrained by minimizing the dynamical height variations with respect to the geoid of the 100~mbar surface of Saturn's atmosphere. The shape of Saturn's 100~mbar isosurface is known, but the shape of the geoid is not because it depends on Saturn's unknown rotation rate. We thus vary the geoid until the 100~mbar isosurface heights measured from the geoid have a minimum variance. There is no dynamical principle that can serve as a basis for this procedure. In a qualitative sense it minimizes the energy and angular momentum of Saturn's winds. It produces a Jovian-like Saturnian wind system with sensible equatorial wind speeds and both eastward and westward jets at higher latitudes. We obtain a rotation period for Saturn of 10~h 32~m 35~s with a standard error of ± 13~s (Anderson and Schubert, Science, 317, 7 September 2007). The corresponding fifth-order reference geoid that best fits the wind and occultation data at the 100~mbar level has a polar radius of 54,438 ± 10~km and an equatorial radius of 60,357 ± 3~km. We represent Saturn's internal density distribution by a single sixth degree polynomial in fractional mean radius r/R, with R equal to 58,256~km, the radius of a sphere with density equal to the mean density of 686.244~kg~m-3 (mass of Saturn divided by the volume internal to the geoid). By constraining the polynomial such that it goes to zero at the surface, and also such that its derivative goes to zero at both the surface and the center, and by using up all the total available mass, four constraints in all, the polynomial has three degrees of freedom. We use the third-order method of level surfaces (Zharkov and Trubitsyn, Physics of Planetary Interiors, W. B. Hubbard, Ed., Pachart Press, Tucson, 1978) to match this polynomial to the three zonal gravitational harmonics J2 = 16290.71 ± 0.27, J4 = - 935.8 ± 2.8 and J6 = 85.3± 8.5, all in units of 10-6 and at a reference radius of 60,330~km. The result is a unique sixth degree polynomial that defines an empirical equation of state for Saturn's interior.
P23C-02
The Structure of Saturn's South Polar Vortex Determined by Cassini VIMS: Constraints on Winds and Horizontal and Vertical Cloud Distributions
We present new imagery and quantitative results for wind and cloud structures in the south polar region of Saturn, obtained by Cassini/VIMS. A hurricane-like vortex feature is well observed in images obtained on October 11, 2006 and May 11, 2007, with a deep "eye" of cloud-free skies extending about 1 bar deeper than the surrounding ring of clouds. Winds measured in both reflected sunlight and in thermal radiation show comparable speeds throughout the region, suggesting little vertical wind shear over the 0.5-3-bar altitude range. Discrete clouds at 88 degrees S. planetographic latitude observed near the 0.5-2-bar level whip around the pole at speeds approaching 200 m/s. At greater latitudes, near the "eye" of the system, winds are much slower: about 45 m/s at 89.5 degrees S. latitude. From 88 degrees to 76 degrees S. latitude, the zonal wind structure as a function of radius/latitude is close to that expected for flows which maintain constant angular momentum. The picture that emerges is that this system is a giant polar vortex, spanning more than 15,000 km in diameter and at least 40 km in depth. Two distinct types of reflective, discrete clouds are observed interspersed throughout the region: bright clouds at continuum wavelengths from 0.6 to 2.7 microns characterized in our preliminary modeling as having imaginary indices of refraction near 0.002 at 0.7 micron, and spectrally dark clouds with twice that value. This suggests that two types of discrete clouds, colored by two distinct chemical compositions, reside in the south polar region. This is perhaps indicative of upwellings of materials from two distinct altitude regions in the depths of the south pole.
P23C-03
Turbulent Deep Convection at low Rossby Number: A Model for Zonal Flow and Thermal Emissions of Jupiter and Saturn
We use numerical models to show that deep convection can result in the observed surface fluid flow and thermal emission patterns of Jupiter and Saturn. The simulations of Boussinesq convection in a spherical shell are dynamically self-consistent and generate large-scale zonal jets that interact with thermal plumes to produce the surface heat flow pattern. The surface fluid flow is dominantly zonal with a prograde equatorial jet and multiple alternating jets at higher latitudes. The zonal jet widths in our numerical models, and of Jupiter and Saturn, follow Rhines scaling. The scaling for zonal flow in a spherical shell is distinguished from that in a full sphere or a shallow layer by the effect of the tangent cylinder, which marks a reversal in the sign of the planetary β - parameter and a jump in the Rhines length. This jump is present in the numerical simulations as a sharp equator-ward increase in jet widths — a transition that is also apparent on Jupiter and Saturn. Our models generate a surface heat flow pattern with a broad minimum at the equator and peaks at the poles. The zonal jets modulate this pattern at smaller latitudinal scales. Superposing the model heat flow pattern with incoming solar radiation results in global heat flow that, similar to Jupiter and Saturn, is roughly constant in latitude. Our results support the hypothesis that the large-scale patterns of heat and zonal flow originate deep within the molecular hydrogen envelopes of the giant planets.
P23C-04 INVITED
Eddies, Convection, and Maintenance of the Saturn General Circulation: Results from Cassini Imaging
Cassini ISS imaging of Saturn's southern hemisphere during approach and in the months after orbit insertion have shown the mean zonal wind pattern to be relatively invariant with time outside the equatorial region. A major objective of the Cassini imaging investigation is to deduce the physical processes responsible for maintaining the momentum balance of the Saturn jets and to thereby constrain theories of jovian planet circulation. A related question is whether the mean meridional overturning circulation on Saturn more closely resembles Earth's tropical Hadley cell, with convective heating driving a thermally direct circulation, or its midlatitude Ferrel cell, with larger-scale eddies mechanically driving a thermally indirect overturning. Voyager images provided too small a sample at too low resolution to address this issue. For Cassini, we applied an automated cloud tracking algorithm to pairs of images over 4 Saturn rotations to estimate eddy fluxes of zonal momentum. The resulting dataset produces an order of magnitude more wind vectors than were the case for Voyager, with relatively unbiased sampling. The results show clear patterns of eddy momentum flux that are positively correlated with the latitudinal shear of the mean zonal wind, indicating that eddies are supplying the kinetic energy of the mean flow. We also detect moist convective storms by monitoring the evolution of anomalous bright features in continuum images, with supporting evidence of high cloud tops in weak methane band images when available. These show a tendency for convection to occur preferentially in cyclonic shear zones and not at all in anti-cyclonic shear regions. This implies that air rises in the cyclonic regions, drifts equatorward (poleward) across eastward (westward) jets, and sinks in the anti-cyclonic regions. The Coriolis force on the implied mean meridional flow decelerates (accelerates) these jets, potentially balancing the acceleration (deceleration) due to the eddy fluxes. This picture is consistent with recent Cassini and Galileo inferences about Jupiter and supports the Ferrel cell picture. Recent models that simulate baroclinic instability on jovian planets produce similar momentum balances. Latent heating due to moist convection in this picture may create the temperature contrasts that drive the baroclinic eddies that perform most of the eddy momentum transport. Recent images providing our first look at Saturn's northern hemisphere during the Cassini era show that the westward (eastward) jets at 34 (42) degrees North planetocentric latitude are still present, and that eddy fluxes are positively correlated with the mean zonal wind shear for this jet pair as well.
P23C-05
The rotation of Saturn's auroral oval
Near-planetary period oscillations have been observed in all explored regions of Saturn's magnetosphere, despite the fact that Saturn's magnetic and spin axes are closely co-aligned. In addition, the traditional measure of Saturn's rotation period, Saturn kilometric radiation (SKR), has been shown to vary over the timescale of months. We report that Saturn's southern auroral oval rotates with a radius of ~1° and period ~10.7 h, an unexpected result in a supposedly axisymmetric system. The implication of this new observation is either that it reveals the true dipole tilt of Saturn, in which case it represents the first direct observation of the rotation of Saturn's deep interior, or it indicates the presence of an external current system that produces an effect similar to a dipole tilt. Until we observe both poles simultaneously this will remain an ambiguity of the result. In either case, this work places an independent constraint on the dipole tilt of Saturn.
P23C-06 INVITED
Saturn's Variable Radio Period
Latest SKR measurements by Cassini/RPWS confirm the observed, slow and regular increase of its rotational modulation period (~10.9 hr), at the scale of about 0.5 percent over the last three years. In the meantime, similar drifts could also be find in some other observed magnetospheric phenomena (e.g. magnetic field, UV auroras, etc…), indicating that the inner magnetosphere of Saturn is globally changing at the 30-year scale of its revolution around the Sun. Refined analyses of individual SKR radio components (Kurth et al. (2007), Gurnett et al. (this session)) suggest that different regions of the inner magnetosphere might correspond to different apparent periods, all of them being substantially larger than the internal rotation period of 10.543 hr, determined from the measured planetary gravity field (Anderson and Schubert (2007)). We further discuss the constraints brought by SKR observations on spin modulated phenomena in Saturn's environment and, more particularly, address the question of the nature of the SKR rotational modulation, i.e. a pulsing source of radio emission (blinking light) versus a rotating disturbance (searchlight).
P23C-07
An Overview of the Rotational Modulation of Three Types of Saturnian Radio Emissions
In this paper we analyze and compare the rotational modulation of three types of Saturnian radio emissions detected by Cassini. These are (1) Saturn kilometric radiation (SKR), which is an intense radio emission generated by the cyclotron maser mechanism along the high latitude auroral magnetic field lines at typical frequencies ranging from about 50 to 500 kHz; (2) narrowband Saturn myriametric radiation (nSMR), which is a narrowband radio emission generated in the inner region of the magnetosphere by mode conversion from electrostatic upper-hybrid emissions at frequencies ranging from about 5 to 20 kHz; and (3) auroral hiss, which is a whistler-mode emission generated at frequencies below about 100 Hz by electron beams associated with the auroral field-aligned current system. Spectrum analyses of the rotational modulation of the SKR shows that this radiation is made up of at least two components. The first component, which is believed to originate from a pulsing clock-like source on the dayside of Saturn, has a modulation period that has varied slowly from about 10.76 to 10.81 hours over the roughly three-year period since Cassini arrived at Saturn. This period is substantially greater than the rotation period of 10.543 hours recently reported by Anderson and Schubert (2007) for the interior of Saturn, implying a significant slippage relative to the rotation of the internal magnetic field of Saturn. The second component of the SKR, which has only recently been clearly identified from passes at high latitudes, has a much less variable period ranging from about 10.54 to 10.60 hours. This period is very close to the internal rotation period reported by Anderson and Schubert. The close proximity of this period to the internal rotation period suggests that the source may be associated with the rotating features commonly observed in auroral images, possibly indicating the presence of a rotating anomaly associated with the internal magnetic field at high latitudes. Another possibility is that the second component may be generated by a periodic phase modulation of the first slowly varying (clock-like) component by long period variations in the solar wind velocity, as has been suggested previously by Cecconi and Zarka (2005). Because the nSMR tends to occur in association with magnetospheric storms and is not continuously present, the modulation period of this radio emission cannot be determined as accurately as for the SKR. To within the available accuracy the modulation period of the nSMR is consistent with the period of the first (clock-like) component of the SKR, which is known to be locked to the rotation rate of the magnetic field (and possibly of the plasma) in the inner region of Saturn's magnetosphere. This implies that the source of the nSMR is slipping relative to the internal rotation of Saturn at a rate that is very similar to the first component of the SKR. Because of the limited amount of data that is available in the high latitude regions where the auroral hiss is observed, the modulation period of the auroral hiss is even more difficult to determine with useful accuracy. However, our preliminary measurements show that the modulation period of the auroral hiss is close to the internal rotation period reported by Anderson and Schubert, which would be consistent with a rotating auroral source.
P23C-08 INVITED
Rotational Modulations of Saturn's Magnetosphere and Synchronous Regeneration of its Asymmetry
We show that Saturn's magnetosphere displays two types of rotational modulations in the fluxes of energetic charged particles and field strength. In the "in-phase" modulations which are observed outside of the current sheet, the magnetic field strength and charged particle fluxes are observed to vary in phase. In the second type of rotational modulations, which are observed during current sheet crossings, the particle fluxes and the field strength are seen to be anti-correlated. In this work we will show that Saturn's magnetosphere contains semi-permanent azimuthal variations in fluxes of energetic particles and plasma density. Using arguments based on stress balance, we show that such plasma variations would create the observed in-phase magnetic field variations above the current sheet. Next, we demonstrate that when the solar elevation angle is large, the solar wind dynamic pressure generates a tilt in the current sheet of Saturn because of magnetosphere's asymmetric "rigidity". In this process, the "light" azimuthal sectors of the current sheet are pushed further away from the rotational equator than are the "heavy" sectors which remain close to the rotational equator. This current sheet tilt is the source of "out-of-phase" periodicities observed at low latitudes in Saturn's magnetosphere. Finally, we present a heuristic model of synchronous regeneration of azimuthal asymmetry in Saturn's magnetosphere by a process which maintains and reinforces the existing azimuthal variations in the charged particle fluxes. We postulate that when the heavy and distended portions of the m = 1 semi-permanent azimuthal structure face the magnetotail, field reconfiguration and reconnection events are maximized. The resulting bursty bulk flows and plasma injections into the middle and inner magnetosphere then reinforce the existing azimuthal enhancement of particle fluxes, perpetuating the azimuthal asymmetries of the magnetosphere.