Aeronomy of Titan and Saturn: Recent Advances From Cassini/Huygens Observations I Posters
Presiding: I Mueller-Wodarg, Imperial College London; R Yelle, Lunar and Planetary Laboratory, University of Arizona
P21B-01 0830h
Saturn's Auroras from the Cassini Ultraviolet Imaging Spectrograph
Cassini's Ultraviolet Imaging Spectrograph (UVIS) has begun making detailed studies of Saturn's auroras. Two long slit spectral channels are used to obtain EUV data from 56.3-118.2 nm and FUV data from 111.5-191.3 nm. 64 spatial pixels along each slit are combined with slit motion to build up spectral images of Saturn, with sufficient spatial resolution to reveal Saturn's auroral oval. Observed emissions include H Lyman-alpha and H2 bands from Saturn's auroras and dayglow. The auroral spectrum is remarkably similar to that of Jupiter, showing short-wavelength FUV absorption due to methane, CH4. Saturn's auroral and dayglow spectra show significant differences. Saturn's aurora is observed to vary in brightness by at least a factor of four. The brightest auroral emissions seen so far occurred after 2004 day 207 19:30 when Cassini CAPS recorded passage of a solar wind shock. The enhanced auroral brightness persisted for days, and is seen at both poles of Saturn. Cassini RPWS observed enhanced auroral kilometric emissions during several auroral brightening events seen by UVIS. A campaign of Hubble Space Telescope UV imaging with ACS (Advanced Camera for Surveys) of Saturn's dayside southern auroral zone is planned for 2005 February 17. Cassini UVIS and VIMS will be observing Saturn's nightside northern aurora during this period. The UVIS long slit will be aligned with lines of latitude on Saturn, providing information about intensity and spectral variations along the auroral oval.
P21B-02 0830h
Ring Ion Dynamics at Saturn and Their Potential Consequences
The Cassini Orbiter CAPS and INMS detections of an ionosphere of Saturn's rings during SOI, coupled with the internal dipole field offset detected on previous missions, motivated the present numerical analysis of mass 32 and 16 singly charged ions in Saturn's inner magnetosphere. The combination of the source of heavy pickup ions in the plane of the rotational equator, at radial distances of ~1.4-2.4 Saturn radii, and the dipole offset combine to produce a heavy ion torus with suggested consequences yet to be observationally confirmed. Although the neutral atmosphere is expected to be roughly symmetric about the ring plane (Johnson et al. 2005), the modeled ion spatial distribution exhibits major North-South asymmetries due to asymmetric photo-ionization and the dipole offset, and also due to the fact that the ring ion source spans the radial distance where the Keplerian velocity is equal to the ion corotation velocity, Rc. We find that for R<Rc there is an implied zone of ring ion precipitation into Saturn's southern midlatitude hemisphere and, at the outer edge, the ring atmosphere can contribute heavy ion species to the inner magnetosphere. While the Cassini Orbiter is not destined to provide any further in-situ sampling of the ring ionosphere during its prime mission, its remote sensing capabilities in the UV and in energetic neutral atoms may yield some evidence for (or against) the presented modeling results.
P21B-03 0830h
Effects of Titan's Induced Magnetosphere on ENA Production
Titan possesses an extensive atmosphere and ionosphere that interact directly with the surrounding plasma environment (usually Saturn's magnetospheric plasma) forming an induced magnetosphere. Energetic Neutral Atoms (ENAs) are formed during this interaction when singly charged magnetospheric ions collide with exospheric neutral atoms and undergo charge-exchange. The energy of the incident ions is almost entirely transferred to the charge exchange produced ENAs, which then propagate along nearly rectilinear ballistic trajectories. ENA images are used to remotely sense the ion fluxes and spectra at the time of the charge exchange collision. They also give information about the magnetic field in the vicinity of Titan and thus to Titan's interaction with the magnetosphere of Saturn. We perform Monte Carlo test-particle simulations using the electric and magnetic fields from a 3-d MHD simulation of Titan's interaction to study the effects that Titan's induced magnetosphere has on the source region of ENAs. We examine the resulting location of ENA production and the corresponding fluxes that the Cassini spacecraft may observe. We also compare ENA results for different exospheric models.
P21B-04 0830h
Theory of "Optically-Thick" ENA Emission from Titan's Exosphere
The brightest energetic neutral atom (ENA) emission from Titan's inner exosphere is produced by energetic singly-charged ions undergoing multiple atomic collisions with the cold exospheric atoms at altitudes above 1500 km. Any proper transport theory of this "optically-thick" ENA emission must include all categories of atomic collisions (charge-exchange, stripping, ionization, and excitation) for both energetic ions and ENAs. The energies of the hydrogen and oxygen ENAs imaged by the Cassini/MIMI/INCA camera are above 10 keV/nuc, so the gyroradii of the ions producing the ENAs imaged by INCA are comparable to, or exceed the radius of Titan (2575 km) in a 5nT magnetic field. In this regime, the cross-sections for all collisions are very strongly peaked in the forward direction. These two afore-mentioned facts allow an analytic solution to the "optically-thick" coupled transport equations for ions and ENAs in terms of simple (exponential) functions. The resulting formulas for the ENA intensity as a function of the energetic ion intensity (spectrum and spatial distribution) are presented along with some simulated images. Relevant INCA observations are presented by Dandouras et al. and Brandt et al. elsewhere in this Special Session.
P21B-05 0830h
Solar Heating of Titan's Ionosphere
Titan's flybys by the Cassini orbiter provide a crucial and unique dataset on composition and temperature of Titan's ionosphere. Solar irradiance is a major energy source for Titan's upper atmosphere. The suprathermal electrons induced by photoionization are a significant heating source of Titan's ionosphere. In order to estimate the photoelectron heating on the ambient electron population, we have coupled a multi-stream electron transport model with an ionospheric model. The latter solves the continuity equation for the ionospheric and minor neutral species as well as the electron energy equation. We will assess the effect of photoelectrons on the ionospheric electron temperature within the context of the first Titan flyby by the Cassini orbiter.
P21B-06 0830h
A Review of Titan's EUV Airglow Leading up to Cassini
EUV airglow observations of Titan by the Voyager 1 Ultraviolet Spectrometer (UVS) in 1980 indicated that the spectrum between 52-110 nm was excited primarily by electron impact on N2. Since then, our understanding of the various emission features contributing to the observed signal has substantially evolved. High resolution electron impact emission laboratory measurements of the important N2 Carroll-Yoshino (CY) c4'-X (0,v") bands provided constraints on the predissociation yield of the excited c4'(0) state. CY(0,v") is radiatively trapped in Titan's atmosphere so that c4'(0) predissociation is compounded over its optically thin value, increasing the loss to the system. Model results have shown that the depleted CY(0,0) emission near 95.8 nm is redistributed to the more optically thin (0,v">0) bands with some additional loss to the system through extinction. More recently, models of Titan's airglow and observations of the Earth's airglow at high spectral resolution showed the possible significance of many weaker N2 and N I emissions that were unresolved by the UVS at Titan. In this review, we summarize the current understanding of the Titan EUV airglow up until the arrival of Cassini. We outline some questions that remain unanswered, which include, but are not limited to, a possible misidentification of N2 CY(0,0) in the Voyager 1 UVS spectra and a possible auroral contribution to the EUV airglow.
P21B-07 0830h
Laboratory Measurements of Molecular Nitrogen Vacuum Ultraviolet Photoabsorption Cross Sections and Line Widths in Support of Analyses of Titan's Atmosphere
The analyses of upcoming VUV occultation measurements of the N2-rich atmosphere of Titan will require reliable photoabsorption cross sections and line widths for the 100 electronic bands of N2 in the 80 to 100 nm wavelength region. We report measurements of these fundamental parameters of the absorption spectrum of 14N2 in the 93.5 - 100 nm spectral region. The room temperature absorption measurements were performed with the 6.65-meter vuv spectrometer at the Photon Factory synchrotron facility with a resolving power of approximately 125,000. A line-shape fitting routine is used to extract individual rotational line f-values and predissociation-broadened line widths within the fifteen bands reported in this study. Within individual bands, we find significant departures from the predicted line strength distributions based on isolated band models. Line width analyses within each band indicate that predissociation-broadening is often highly dependent on the rotational quantum number. We illustrate the importance of N2 line widths in the analysis of Cassini occultation measurements with sample N2 transmission models over selected wavelength regions.
P21B-08 0830h
The Michigan Titan Thermospheric General Circulation Model (TTGCM)
The Cassini flybys of Titan since late October, 2004 have provided data critical to better understanding its chemical and thermal structures. With this in mind, a 3-D TGCM of Titan's atmosphere from 600km to the exobase (~1450km) has been developed. This paper presents the first results from the partially operational code. Currently, the TTGCM includes static background chemistry (Lebonnois et al 2001, Vervack et al 2004) coupled with thermal conduction routines. The thermosphere remains dominated by solar EUV forcing and HCN rotational cooling, which is calculated by a full line-by-line radiative transfer routine along the lines of Yelle (1991) and Mueller-Wodarg (2000, 2002). In addition, an approximate treatment of magnetospheric heating is explored. This paper illustrates the model's capabilities as well as some initial results from the Titan Thermospheric General Circulation model that will be compared with both the Cassini INMS data and the model of Mueller-Wodarg (2000,2002).
http://data.engin.umich.edu/tgcm_planets_archive/thermo.html