P54A-01 INVITED 16:00h
Implications of Cassini UVIS Observations of Neutral Gas in the Saturn Magnetosphere
\noindent Images of emission from atomic oxygen and hydrogen in the Saturn magnetosphere have been obtained in the Cassini pre orbit insertion period using the Cassini UVIS imaging spectrometers. Over the period December 25, 2003 - May 12, 2004, the atomic oxygen shows temporal variability in total abundance as well as spatial distribution on a $\sim$2 month time scale. The atomic oxygen is measurable to at least $\pm$ 10 R$_{S}$ in the orbital plane and $\pm$ 4 R$_{S}$ in the polar dimension. The atomic hydrogen is detectable to $\pm$ 45 R$_{S}$ in the orbital plane and projected to be measurable to $\pm$ 20 R$_{S}$ polar. The oxygen emission peaks in the 2 R$_{S}$ - 4 R$_{S}$ region in the orbital plane in a generally asymmetric distribution in local time, with greater abundance on the dark side of the planet. The hydrogen distribution is also asymmetric, showing an extensive anti-sunward component. The magnitude of the variation in oxygen abundance as well as the rate estimated from charge capture in the plasma sheet indicates $\sim$ 500 Mkg loss over a 2 month period. The generation of water product gas from the local solids at this rate implies that mass the equivalent of the E-ring micron sized population is removed on this time scale. A projection to the total mass of the E-ring indicates a characteristic time of $\sim$ 100 My. The erosion of available solids requires a large surface area, and rapid surface reaction rates. A proposed mechanism involves the formation of solid state cluster ions produced in the reaction of low energy plasma sheet ions with micron sized grains, followed by rapid dissociative recombination into the vacuum.
http://lasp.colorado.edu/cassini
P54A-02 INVITED 16:20h
An Overview of Cassini Radio, Plasma Wave, and Langmuir Probe Observations in the Vicinity of Saturn
The Cassini Radio and Plasma Wave Science (RPWS) instrument detected a wide variety of radio and plasma wave phenomena during the approach and first flyby of Saturn. These include Saturn kilometric radiation (SKR), Saturn electrostatic discharges (SEDs), upstream electrostatic waves, the bow shock, trapped continuum radiation, whistler-mode chorus emissions, electrostatic upper-hybrid emissions, impulsive signals from dust impacts, narrowband electromagnetic emissions in the low-density inner region of the magnetosphere, whistler-mode resonance-cone emissions similar to terrestrial auroral hiss, and many sporadic narrowband whistler-mode emissions apparently associated with the rings. In addition, electron density and temperature measurements were obtained from a Langmuir probe. In this paper we interpret these observations in terms of various key parameters and processes in the Saturnian system. These including new values for the radio rotation period of Saturn, long term variations in the occurrence of atmospheric lightning, the plasma density and temperature in the inner region of Saturn's magnetosphere, the distribution of micron-sized particle near the ring plane, evidence of an electrodynamic interaction between the rings and the co-rotating magnetosphere of Saturn, and the possible occurrence of meteoroid impacts on the rings.
P54A-03 16:40h
High Spectral and Temporal Resolution Observations of Saturn Kilometric Radiation
The Cassini Radio and Plasma Wave Science (RPWS) instrument is the most capable planetary radio astronomy instrument to have been flown to the outer planets. Among other attributes, it includes a wideband receiver with 10- and 75-kHz baseband modes as well as a high-frequency down-conversion mode tunable between 125 kHz and 16 MHz that provides very high resolution spectral and temporal observations of Saturn's radio emissions. This paper presents an initial survey of dynamic spectra of Saturn kilometric radiation acquired upon approach and the first orbits of Saturn. Based on these early observations of Saturn's kilometric observations, perhaps the most succinct description of the appearance of the emissions in dynamic spectra is highly variable. The emissions display upward and downward drifting features with bandwidths down to the few hundred Hz resolution of the instrument and drift rates up to a few kHz per second. At other times, the emissions are much more diffuse or continuous, showing little spectral structure on scales of 10 or 20 kHz. Some features show very sharp upper frequency cutoffs and indistinct lower frequency cutoffs, somewhat suggestive of a caustic effect. The dynamic spectral features provide insight into the sometimes highly nonlinear nature of the cyclotron maser instability believed to generate the emissions and various processes such as diffraction within and near the radio emission source which shape the radio spectrum.
P54A-04 INVITED 16:55h
Energetic Neutral Atom Emission During Cassini's First Orbits at Saturn: Source Strength and Dynamics
The Ion and Neutral Camera (INCA), one of three sensors that comprise the Magnetospheric Imaging Instrument (MIMI) on the Cassini/Huygens mission to Saturn and Titan, is designed to operate in one of two modes: either as a high geometry factor ion instrument with high angular resolution, or as an Energetic Neutral Atom (ENA) camera that images the ENA emission from various ion/gas interaction regions in the Saturnian magnetosphere. During Cassini's approach and the first two orbits about Saturn we have begun analyzing INCA data at great distances, where Saturn is an unresolved source. Our emphasis for most of this period is in characterizing variations in the source strength on time scales of hours to weeks (especially looking for periodicities at Saturn's rotation period) and correlating those variations with upstream ion activity as seen in the MIMI ion data, as well as with Saturn Kilometric Radiation as measured by the University of Iowa Radio and Plasma Wave Science instrument. In contrast, during Saturn Orbit Insertion with its traversal of the region inside the radius of Saturn's main rings, INCA had the opportunity to image the interaction between the inner magnetospheric trapped ion population and the inner magnetospheric cold gas population. During the portions of the second Cassini orbit when Cassini returns to within 20 Rs of Saturn, we expect to have additional resolved images of the inner magnetospheric ENA emission region that will add to our knowledge of the spatial distribution of the energetic ions and cold gas inside about 10 Rs, the region that dominates the Saturn output of ENA.
http://sd-www.jhuapl.edu/CASSINI/
P54A-05 17:15h
Low-Altitude ENA Emission from Energetic Ions Trapped in Saturn's Exosphere
The Ion and Neutral Camera (INCA), one three components of the MIMI experiment on the Cassini orbiter, viewed the low-latitude northern hemisphere of Saturn in a sequence of 16-minute images from an altitude of only 0.4-1.4 R$_{S}$ during two hours (0108-0316 UT on 1 July 2004) following the Saturn Orbit Insertion (SOI) engine burn. A low-altitude band of energetic neutral atom (ENA) emission appeared to be centered $\sim$N10$\deg$ and extended from post-noon to at least midnight in the TOF energy range $\sim$10-100 keV/nucleon. The feature was observed in both hydrogen and oxygen ENAs, and appeared to be eclipsed by the inner edge of the D-ring (r=1.11 R$_{S}$). It was $\sim$1/10 the brightness of the ENA emission from the ring current region (3-8 R$_{S}$), also visible in the same images. We interpret this ENA emission to be produced by double charge exchange. ENAs are generated by singly-charged energetic ions from the ring current region. Those that enter Saturn's molecular hydrogen (H$_{2}$) exosphere are stripped and thus become ions temporarily trapped on magnetic field lines several thousand kilometers above the 1-bar level. Subsequently, these ions undergo a second charge exchange collision and are emitted from the exosphere as ENAs once again. The brightness of the exospheric emission, relative to the ring current source region, implies that the ENA emission is optically thick. The double charge-exchange mechanism was identified at Earth as the source of a low-altitude (L=1.1) radiation belt in the early 1970s. The main difference is that the Earth's (atomic) hydrogen geocorona is optically thin to ENA, while Saturn's (molecular) hydrogen exosphere is optically thick.
P54A-06 17:30h
Structure of Saturn's Magnetosphere as Revealed by Energetic Particles
At the end of June/beginning of July 2004 Cassini entered the magnetosphere of Saturn, 24 years after the last encounter of a spacecraft with the planet in 1981. Cassini entered the magnetosphere at about 08:00 local time 15 degrees below the equatorial plane, crossed the ring plane at a distance of 158500 km from the planet and reached 80230 km during closest approach, much closer than the smallest distance of Voyager. We report on results from the Low Energy Magnetospheric Measurement System (LEMMS), one out of three detector systems of the Magnetospheric Imaging Instrument (MIMI). MIMI/LEMMS is designed to investigate the global configuration and topology of the Kronian magnetosphere and to study dynamical processes. Its double-ended sensor is able to detect energetic electrons (15 keV- 5 MeV) and ions with energies above 30 keV separately. Several different regions inside the magnetosphere could be identified through changes and differences in the energetic particle characteristics: A lobe region with low ion and electron intensities, a plasma sheet region with higher intensities and very abrupt changes when the spacecraft enters/leaves the plasma sheet, the ring current region close to the planet with the highest measured intensities, and the region above the rings with extremely low particle intensities near background. Besides the very high intensities in the main radiation belts close to the planet and the very low intensities above the rings around closest approach results of MIMI/LEMMS observed the Kronian magnetosphere in a highly dynamic state indicated by so-called injection events and quasi-periodic fluctuations of the intensities with a period of about an hour. These periodicities have also been observed in magnetic field and plasma wave data on board Cassini. Preliminary analysis correlate these fluctuations with the motion of the plasma sheet of Saturn which either has a wavy surface or corrugated sheet structure within this period.
P54A-07 17:45h
Charged Neutral Interactions Inside 3 Saturn Radii Using Cassini MIMI Data
The Cassini Magnetospheric Imaging Instrument (MIMI) has three separate sensors which, taken together, have the capacity to detect energetic charged particles and neutral atoms and molecules. During orbit insertion, Cassini reached a closest approach distance of about 1.3 Saturn radii, flying at low latitudes over the ring plane. Ion and electron data taken close to the planet reveal strong flux decreases at some radial distances associated with planetary rings. Charged particles at a wide range of energies are apparently lost to collisions with neutral material such as ring particulates. The sharp flux decrease provides us with an opportunity to constrain the total amount of neutral material as a function of planetary radius. Furthermore, the particularly clean region sampled over the rings will allow us to study several sources of contamination to the MIMI measurements. The differential decreases by species and energy allow us to quantify the contributions of trapped and non-trapped particles to the penetrating background on the MIMI instrument.