P41B-01
Ray Tracing Jupiter`s HOM Radio Emission
Cassini, Galileo, and Voyager spacecraft observations show well-defined attenuation features in the hectometer (HOM) spectrum of Jupiter's radio emission. The features are best displayed as frequency versus time spectrograms of HOM intensity between 500 - 3000 kHz. The bands have been shown by Gurnett et al. (1998) to be the result of propagation processes involving these emissions from opposite hemispheres. Enhancements in the HOM intensity and occurrence are seen along the edges of the observed attenuation features which may indicate caustic surfaces due to refraction along the propagation path. Using magnetic field and density models of the Jovian magnetosphere, we present some ray tracing analyses to show that radio wave refraction from density enhancements in the Io flux tube can produce the attenuation structures seen in the observations. This can provide boundaries to the electron density within the Io flux tube.
P41B-02
Cassini Measurements of Satellite Influence on Jupiter's Radio Emission
Jupiter decameter wavelength emissions (DAM) have a known correlation with the orbital phases of the four Galilean satellites. In particular, the 2.0-5.6 MHz band of the DAM emission shows a small but significant enhancement in occurrence probability at specific orbital phases of the satellites. We performed a similar study using Cassini data that includes rotation period averaging and removal of the known satellite influences. We confirm the Galileo and Voyager studies in the 2.0-5.6 MHz band of the DAM emission and investigate the higher frequency bands 6-16 MHz. These analyses give further evidence for Alfvénic interactions between the satellites and Jupiter.
P41B-03
New Horizons at Jupiter: Energetic Particle Observations from PEPSSI
The Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument on the New Horizons spacecraft began making energetic particle measurements ~1000 Jovian radii ({R}J) upstream of Jupiter beginning in January 2007. Measurements are set to continue through May 2007 when the spacecraft will be ~2000 {R}J downstream of Jupiter. Following its gravity assist en route to Pluto, New Horizon's trajectory remains very near the Sun-Jupiter line, spending the first 40 days after closest approach (~32 {R}J, 28 February) within 100 {R}J of the Sun-Jupiter line and remaining within 300 {R}J through May. This trajectory provides the best opportunity for continuous Jovian magnetotail measurements to date. PEPSSI is a time-of-flight and energy spectrometer with angular coverage in a 25°x160° swath in six angular sectors. The instrument covers ions and electrons within the ~10 to 1000 keV range. A series of upstream events associated with interplanetary activity were measured prior to entry into the Jovian magnetosphere. Increased fluxes of energetic ions and electrons were detected near 70 {R}J, consistent with an inbound crossing of the magnetopause and consistent with similar observations at Jupiter by Voyager 1 and Voyager 2. Preliminary analysis reveals distinct signatures of H, He, O, and S ions, as well as of electrons, and shows highly structured temporal variations. We report on these first results from the PEPSSI instrument during its approach to, and recession from, Jupiter
P41B-04
Jovian Substorms and their Effects on the Magnetotail
The apojove of the Galileo Spacecraft spent several orbits in the near-jovian magnetotail. Strong southward or northward field turnings were observed and reported by Russell et al (1998) during these passes. These sudden dipolarizations were interpreted as signatures of tail reconnection similar to that seen in terrestrial substorms. In this paper, we examined all Jovian tail magnetic-field observations, identified all southward and northward turnings indicative of tail reconnection and determined the occurrence rate and location of reconnection events. In addition, we examined the strength of the magnetic field in the tail. We find the Jovian tail field strength increases before reconnection events and decreases after them. This behavior of the tail field around reconnection events is similar to the growth phase and expansion phase of terrestrial substorms but is much longer in duration, lasting several days. However, this growth phase must be due to endogenic causes, the storage of mass loaded flux tubes in the magnetotail. Thus substorms in the Jovian tail both resemble terrestrial substorms but at the same time are quite different.
P41B-05
Latitudinal Structure of Saturn's Ion Cyclotron Wave Belt from High-Inclination Orbits
In late 2006, the inclination of the Cassini spacecraft's orbits rose from nearly zero to about fifty-five degrees. Consequentially, new understanding of the latitudinal structure of the ion cyclotron waves was obtained when the spacecraft passed through the equatorial plane. The wave amplitudes peaks not at the equator, but at a height of +/- 0.2 Rs, with a slight bias to the southern hemisphere and decreases rapidly after that, disappearing by about 0.3 Rs. Surprisingly, the wave frequencies are different from those we noted on the equatorial passes, with the frequency being higher as the spacecraft approaches the equator and lower as it leaves. This velocity shift is best explained by Doppler shifting due to spacecraft motion. With Cassini's velocity of 12 km/s and the observed frequencies, we calculate that these waves have phase velocities near 35 km/s, around one third of the Alfven speed. We also find that within 0.1 Rs of the equatorial plane, the waves are propagating in both directions, but outside of that region, the waves propagate primarily away from the equatorial plane.
P41B-06
1D Hybrid Simulations of Ion Cyclotron Waves Generation by Mass Loading at Io
Ion cyclotron waves were detected by the Galileo spacecraft as it flew by the Jovian moon, Io, in 1995-2002. These waves are generated by populations of newborn sulfur monoxide and sulfur dioxide ions mass loaded from Io's atmosphere. Because the wave amplitudes are proportional to the number of ions generating them, the observed waves can be used to estimate the mass loading rate at Io. Using a 1D hybrid technique (kinetic ions, fluid electrons), we simulate the addition of newborn molecular sulfur ions to an Io torus-like plasma and examine the generated waves. The generated waves are narrowly peaked just below the newborn ion gyrofrequency and show little change over time for a uniform mass loading rate. Typically, the newborn ions lose ~30% of their initial energy as they scatter on the self-generated waves. Some of this energy is lost by transferring it other ion components in the plasma and some resides in the ion cyclotron waves. This disagrees with previous estimates of mass loading rates at Io assumed that the newborn ions would lose 50% of their initial energy to wave growth. Because the ions lose less energy than this, more ions are necessary in order to generate the observed wave amplitudes, so the previous mass loading rate estimates are too low.
P41B-07
Characteristics and Evolution of ion cyclotron waves in Saturn's magnetosphere associated with Cassini's engine exhaust.
On the day of Cassini's insertion on Saturn's magnetosphere (July 1, 2004) a long (90 minute) burst of ion cyclotron waves was observed. These waves were left-hand, elliptically polarized, and propagated at relatively large angles to the ambient magnetic field. Previous work (Russell et al., 2005) showed that the waves' frequency band was moderately broad and consistent with the singly ionized components of the engine exhaust gases: CO2, N2, CO, and H2O. Thus, wave origin has been explained in terms of an ion instability generated by ring- beam ions produced from the engine exhaust gases. In this work we analyze in further detail waves' characteristics and evolution.
P41B-08
Intrinsic magnetic Field of Titan Upper limit to the dipole moment
Titan is embedded in the Saturnian corotating magnetosphere with its orbit lying in the equatorial plane at radius of 20 Saturn radii. The interaction of Titan with this flowing magnetospheric plasma is complex due to the time variation of external conditions and the physics of the interaction with the neutral atmosphere and the ionosphere. A search for any intrinsic magnetic field of Titan must be carried out at low altitudes, where the complexities of the interaction are minimized. Large scale magnetic fields are observed below Titan's ionopause and these fields change both magnitude and orientation at low altitudes. The variability of these fields suggests that external fields and ionospheric currents contribute to the field seen at low altitudes, but if the interior of Titan is electrically conducting the radial component of the field should be least affected by them. Thus the radial component of the magnetometer data between 1100 km and 950 km (periapsis) are studied to determine how much of these fields may be due to an intrinsic dipole moment. By combining data at polar regions and equatorial regions, the dipole moments aligned in three Cartesian axes (g10, g11, h11) are inverted from the data. Pass to pass consistencies of the dipole moments allow us to estimate an upper limit of the intrinsic field. The probable error of the mean of the g10 term is sufficiently high that this value is consistent with zero, but the other two components may be statistically different than zero. The upper limit to the magnetic moment from these data is about 1.2 nT Rt 3.
P41B-09
3D Hybrid Simulation of the Titan's Plasma Environment
Titan plays an important role as a simulation laboratory for multiscale kinetic plasma processes which are key processes in space and laboratory plasmas. A development of multiscale combined numerical methods allows us to use more realistic plasma models at Titan. In this report, we describe a Particle-Ion--Fluid-Ion--Fluid- Electron method of kinetic ion-neutral simulation code (see, e.g. [Lipatov, 2002]). This method takes into account charge-exchange and photoionization processes. The model of atmosphere of Titan was based on a paper by Sittler, Hartle, Vinas et al., [2005]. The background ions H+, O+ and pickup ions H2+, CH4+ and N2+ are described in a kinetic approximation, where the electrons are approximated as a fluid. In this report we study the coupling between background ions and pickup ions on the multiple space scales determined by the ion gyroradiis. The first results of such a simulation of the dynamics of ions near Titan are discussed in this report and compared with recent measurements made by the Cassini Plasma Spectrometer (CAPS, [Hartle, Sittler et al., 2006]). E C Sittler Jr., R E Hartle, A F Vinas, R E Johnson, H T Smith and I Mueller-Wodarg, J. Geophys. Res., 110, A09302, 2005. R. E. Hartle, E. C. Sittler, F. M. Neubauer, R. E. Johnson, et al., Planet. Space Sci., 54, 1211, 2006. A S Lipatov, The Hybrid Multiscale Simulation Technology. An Introduction with Application to Astrophysical and Laboratory Plasmas, Springer-Verlag, Berlin, Heidelberg, New York, 2002, p.p. 1-403.
P41B-10
O+ at Titan's Exobase from a Hybrid Simulation
Titan's extensive atmosphere and exosphere interact with Saturn's rotating magnetospheric flow. We have studied this interaction with an advanced hybrid simulation model and present results for both the general characteristics of the interaction and the behavior of the O+ flow ions near the exobase for the conditions during the Voyager 1 flyby. Our model, that employs improved resolution and a realistic ion composition in the upstream flow (O+ and H+), shows that the ionotail and the magnetotail bend in a direction opposite to the convection electric field. We also present a simulated impact map for the oxygen ions of the flow with total O+ precipitation rate 1.3 · 1024, s-1 onto Titan's exobase. Although this rate indicates that the plasma conditions above the exobase do not inhibit O+ precipitation on the exobase, the direction of the O+ flow, however, is shown to turn drastically near the exobase. However, the light flow ions (H+) are effectively deflected by the magnetic barrier formed around Titan.
P41B-11
Conductivity and Electron Density in Titan's Atmosphere as Deduced From Mutual Impedance Measurements by the PWA-HASI Instrument on HUYGENS
During the descent of the HUYGENS Probe through the atmosphere of Titan, January 14th, 2005, the Permittivity, Waves and Altimetry (PWA), a subsystem of the HUYGENS Atmospheric Structure Instrument (HASI) detected an ionized layer around the altitude of 60 km with two different instruments, the Relaxation Probe (RP) and the Mutual Impedance Probe (MIP). For both instruments a more detailed data analysis, including unforeseen environmental effects, was necessary to reach precise quantitative estimations of the electrical conductivity. The present work is dedicated to the MIP data analysis. New laboratory tests have been performed to correct and validate the calibration data. Temperature effects have been included and two different numerical models of MIP electrodes and circuits in the HUYGENS body have been used. The effect of motion of HUYGENS in the ionized atmosphere that was neglected in the first analysis is revealed and analytical estimations of this effect allow to derive more accurate values of the conductivity profile. The final results are discussed and compared to the RP results. Electron density profiles are deduced showing more precisely the lower ionized layer and its surprisingly steep lower and higher boundaries, not predicted by any model. We conclude that the spatial variability of the free electron density is probably due to aerosol layers that can capture the electrons created in the medium.
P41B-12
Projects To Probe Titan's Surface Composition and Development of Atmospheric Removal Models for Cassini VIMS Data
In this work, we will describe recent projects performed by our group at the Jet Propulsion Laboratory, California Institute of Technology involving I/F data of Titan's surface acquired by Cassini's Visual and Infrared Mapping Spectrometer (VIMS), including the next stage of development of methods to de-gas and de-fog VIMS images. VIMS I/F spectra include contributions from both surface and atmospheric signal; therefore, current spectral data analysis necessarily focuses on portions of VIMS I/F spectra where atmospheric methane and scattering by haze is at a minimum. However, atmospheric opacity clears enough between wavelengths of 1 and 2 microns to provide strong potential to view complex landforms if a proper atmospheric correction can be applied. Plane- parallel radiative transfer (RT) correction methods have been used successfully in surface-atmospheric separation retrievals for Mars and offer some utility for VIMS observations of Titan that are away from the limb. In a previous work (Pitman et al. 2007, LPSC XXXVIII, p. 1164), we determined which inputs to radiative transfer models must be updated, given results from recent meetings and literature, and evaluated two plane-parallel RT models (adding-doubling, discrete ordinates) to determine which is more easily customized for surface- atmospheric separation of Titan. In this work, we report our progress on replacing Voyager with Cassini-Huygens inputs and how these models currently compare. Work performed under contract to NASA and by appointment to the NASA Postdoctoral Program (ORAU).
P41B-13
Measurement of the Dielectric Properties Performed by HASI-PWA at the Huygens Landing Site: Implications for Titan Surface Characterization
The safe landing of the Huygens Probe on Titan allowed the measurement of the dielectric properties of the surface. The Permittivity, Waves, and Altimetry (PWA) analyser, a subunit of the Huygens Atmospheric Structure Instrument (HASI), included a mutual impedance probe mounted on two booms that measured the dielectric properties of the surface during approximately 30 min. The array configuration on the booms, together with the coupling between the soil and the atmosphere, requires a careful evaluation of the attitude of the Huygens Probe on the surface for assessing the dielectric properties with good accuracy. In this paper we present the calibrated data set of PWA, including reference values to evaluate the permittivity and the conductivity of the surface. The dielectric properties contribute to the characterization of the medium and provide unique constraints of soil composition. Furthermore, surface measurements made with PWA at the landing site are useful for comparison with Cassini radar data, which evaluates surface properties at much larger scale.
P41B-14
Titan's northern lakes and terrains from SAR, high-SAR and high-resolution radiometry
High resolution radiometry obtained along with the Synthetic Aperture Radar (SAR) imaging of the latest T16, T17, T18, T19, T21, T23, T25 flybys and high-SAR T25 (SAR from approximately 20,000 km altitude) covering Titan's Northern hemisphere at latitude above 75 degree has shown evidence of hydrocarbon lakes and seas on Titan. A radiometric characterization based on calibrated data of selected lakes (with surface greater than 200 square km) and adjacent terrains is presented here by mean correlation of normalized radar cross-section (RCS) vs brightness temperature Tb and prospective models. Results show on average a 3-4 K brightness temperature increase in the lakes with respect to the surrounding terrains and seem somewhat consistent with a Kirchoff dielectric surface whose thermal emission properties are determined by Fresnel coefficients. These results seem to be consistent also with RCS incidence angle independency suggesting smooth surfaces of solid or liquid hydrocarbons (e=1.6-1.9; methane-ethane) or plains of solid non-water-ice materials (solid organics and sludge with dielectric constant of approximately 2.0). The terrains surrounding the lakes define a class more likely represented by a higher dielectric i.e. e=3.1 (water ice), in which more complex behavior is observed with respect to RCS and brightness temperature variations.
P41B-15
Clumps and Moonlets in Saturn's F Ring
Stellar occultations of Saturn's F ring detect nine statistically significant events, ranging in width from 27m to 9km, that indicate temporary aggregation of ring material. One event is also seen by VIMS. At least one event is elongated: these structures may cause temporary brightenings as seen by Voyager, Hubble, and Cassini. At least one, and perhaps two of the structures are opaque to starlight, indicating a number of small moons, as predicted by models that match Pioneer and Voyager data. These moons may be accompanied by long streaks of trailing debris, as shown in simulations by Lewis and Stewart. Even if these temporary aggregations are "seeded" by larger particles that are fragments of shattered moons, such objects have very short lifetimes against disruption. Thus, unless Saturn's F ring was created in the last few million years, a competing process of re- accretion must balance the destruction. This hypothesized "recycling" allows the F ring to be much more ancient. Similar features of propellers and under-dense moons in and around the main rings also require similar recycling, or a very recent formation. The range of ages indicated by Cassini observations are much easier explained by ongoing fragmentation and recycling through accretion, than by a single recent formation event. The high fraction of ice evident in ring spectra cannot be explained purely by coating their surfaces: CRAND observations and Mimas surface contradict this hypothesis. This purity of the rings against meteoritic infall can be consistent with the model of ancient rings if the meteoritic influx is overestimated or the ring mass has been underestimated. In that case, the rings could be primordial. Now they are continually recycling material, and would likely persist long into the future, but not exactly as we see them now.