Statistics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p12b..03c&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P12B-03
Statistics
[5759] Planetary Sciences: Fluid Planets / Rings And Dust, [6265] Planetary Sciences: Solar System Objects / Planetary Rings, [6275] Planetary Sciences: Solar System Objects / Saturn, [6297] Planetary Sciences: Solar System Objects / Instruments And Techniques
Scientific paper
The finite sizes of particles and clumps of particles in Saturn's rings cause fluctuations in the signal received in stellar occultation measurements. We analyze occultations observed by the Cassini Ultraviolet Imaging Spectrograph (UVIS) that have a characteristic integration area in the ring plane of 20 m on a side. When structures in the rings are not vanishingly small compared to this scale the photon counts measured by the UVIS High Speed Photometer show a variance in excess of Poisson counting statistics that would arise if observing a star through a uniform translucent medium. Roughly speaking, this excess variance is larger for larger particles or structures compared to the integration area in the ring plane from which starlight is received at each measurement. Because different occultations have both different viewing geometries with respect to the ring plane as well as different integration areas, each occultation provides a different perspective on the structure of particles in the rings. In the limits of very low and very high optical depth, the excess variance approaches zero as the photon count rates in these cases are due to either unobstructed starlight or background signal, respectively. Intermediate optical depths of a few tenths to ~1 provide the greatest sensitivity of occultation counting statistics on characteristic length scales of particles or structures in the rings. For a given particle size distribution, for example, the excess variance grows with optical depth to a peak at an optical depth of about 0.5 before declining toward zero at higher optical depths. Thus, by examining the distribution of excess variance with optical depth in different ring regions we can obtain new information about differences in particle populations and structures. We find that the plateau regions of the C ring have a significantly lower excess variance than the rest of the C ring indicating that the largest particles (or clumps) in the plateaus are smaller than those in the rest of the C ring. We find also evidence for different characteristic lengths between different regions of the B ring and even within regions of the B ring previously identified purely from their optical depth structure. Excess variance is largest in the A ring where the formation of self-gravity wakes is most prominent. In addition we examine the skewness of the occultation data which provides information on the relative abundance of regions of unusually low or high count rates corresponding to very high or low optical depths, respectively. We compare the observations with statistics from synthetic light curves derived from various model particle size distributions and will describe the different particle populations and structures derived from analysis of the occultation statistics.
Colwell Joshua E.
Cooney J.
Esposito W. L. W. L.
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