ENA Emissions at Saturn
Composite images of hydrogen atoms (30-50 keV) and oxygen atoms (64-144 keV) have been combined for the first ~150 days of 2007 and 2008 when the Cassini spacecraft viewed the equatorial plane of Saturn from high latitudes (>35°N). Observation from these elevated latitudes allows projection of the emissions onto the equatorial plane of Saturn, from which the gross morphology of the emissions can be evaluated by combining many days' worth of ENA images. The projected images were corrected for slant viewing geometry and for Compton-Getting effects in the energetic protons and oxygen ions that generated the ENA emissions. In sun-fixed coordinates, the H emissions tend of concentrate on the nightside of Saturn just outside the orbit of Rhea (8.7 RS), while the oxygen emissions also concentrate on the nightside but between the orbits of Rhea and Dione (6.3 RS). In rotating coordinates (SLS longitude), single or double sources exist in both the H and O emissions. These sources may extend over 90° in longitude and appear at different longitudes at different times for different species. These sources persist for weeks or longer. There is no evidence for spiral patterns in the long-term ENA averages. The appearance and disappearance of individual ENA sources in SLS longitude suggests different sources rotate at different speeds, which may be related to the double sources observed in SKR periodicities.
Examination of Enceladus plume variability and the impact on magnetospheric neutral gas distribution
The neutral particle dominance over charged particles in Saturn's magnetosphere was evident prior to Cassini arrival at Saturn in 2004. However, the observation of active plumes emanating from the southern pole of the small icy moon, Enceladus provided key information for understanding particle sources and dynamics in the magnetospheric region. Estimates show this 252 km radius moon is likely the dominant source of particles in the magnetosphere producing 1027 to 1028 neutral water molecules per second. Cassini has flown through the plumes on several occasions which appear to indicate variability in this source rate. For this research, we use Cassini CAPS, MIMI and INMS observations during these encounters to constrain our 3-D multi-species neutral particle model to estimate the plume source rate for these encounters and also examine the impact of potential variability on magnetospheric neutral gas density.
Cassini/RPWS Dust Measurements During the Enceladus Flybys in 2008
The Cassini spacecraft completed four very close flybys of Enceladus in 2008 on days 072, 224, 283, and 305. All flybys were targeted to fly the spacecraft through the water vapor plume near the south pole of this tiny but energetic moon. At closest approach, the spacecraft was merely several tens of kilometers away from the moon's surface, providing good opportunities for the RPWS instrumentation to measure the parameters of micron-sized particles in the water plume. As the spacecraft approached the water plume, the RPWS dipole antenna started recording intense impulsive noise indicating substantially increased dust impacts. A Gaussian distribution can be fit to the impact rate. For all four flybys, the peak impact rate is around 600 to 800 per second, the corresponding peak number density is about 7.2 × 10-2m-3 to 9.6 × 10-2m-3, and the half thickness of the Gaussian distribution is around 1600 km. The dust particles in the plume are thought to have radii less than 10 micrometers. Due to the limitation of the RPWS lower threshold on detecting dust hits, only dust particles larger than one micrometer can be precisely measured. An RPWS frequency-time spectrogram on DOY 224 indicates that the peak of the intensity of dust impacts occurred five minutes later than the peak of the electron density. In this presentation, we will discuss the variation of dust flux as a function of distance from the south pole of Enceladus, the mass and size distribution, and comparisons of these optical depth measurements.
Why is Radial Transport of Plasma at Saturn Asymmetric?
Since Cassini' arrival at Saturn we have learnt a great deal about plasma transport in the magnetosphere. The Saturn system rotates rapidly and has a heavy, equatorially confined plasma disc - leading to centrifugally driven outflow. In the current picture of Saturn plasma transport in the middle magnetosphere (5 < L < 15) the outflow and inflow sectors are not symmetric - large scale slow outward transport is balanced by small scale rapid inward 'injections'. We seek to address the asymmetry by analysis of the current systems associated with pressure and mass density gradients at Saturn.
Evidence for Long-Duration Magnetic Reconnection in Saturn's Magnetotail
On 20 August 2006 the Cassini spacecraft was 32 Rs from Saturn in the northern magnetotail at a latitude of 13 deg and a local time of 21.9. At this time, a fast, sunward, magnetic-field-aligned beam of ions was observed, lasting approximately 1 hour. The beam was observed as the spacecraft crossed from the tail lobe into the plasma sheet, during an interval characterized by a slight northward turning of the magnetic field, followed by a strong dipolarization. CAPS observations show that the bulk of the ions, from a few to around 10 keV were almost exclusively H+, whereas at higher energies MIMI observed both H+ and O+. In analogy to the Earth's plasma-sheet boundary layer, the likeliest source of such ion beams traveling from the more distant tail toward Saturn is a region of magnetic reconnection tailward of the spacecraft. The persistence of the beam for an hour demonstrates that such reconnection can be of long duration in Saturn's magnetotail. The sequence of plasma and field observations for this event will be presented, and the implications for Saturn's magnetotail dynamics will be examined.
Second Harmonics of Saturn Narrowband Radio Emissions
We report second harmonic emissions of the 20 kHz Saturn narrowband emissions observed by the Cassini Radio and Plasma Wave Science (RPWS) instrument. These emissions are similar in many ways to the second harmonic emissions observed in both solar radio bursts and planetary radio emissions. Polarization measurements show that the fundamental emissions are polarized as L-O mode whereas the second harmonics are polarized as R-X mode. The occurrence probability of the second harmonic emissions will be shown. The spectra of the fundamental and second harmonic bands will be compared in detail to find out the correlation between their power fluxes. We propose the nonlinear wave-wave interactions of upper hybrid waves as the generation mechanism of second harmonic narrowband emissions.