Small-scale azimuthal structure in Saturns rings

Stellar occultations provide the highest spatial resolution data of Saturns rings but only as one-dimensional transparency profiles. The Cassini Ultraviolet Imaging Spectrograph (UVIS) High Speed Photometer (HSP) measured 277 occultations of UV-bright stars by Saturns rings over a range of viewing geometries and with varying coverage of the rings. Almost all were observed with sampling rates of 1-2 ms and Fresnel scales (D) of ~10 m. For 119 of these stellar occultations, the line-of-sight from the star to Cassini traced a chord across the rings over the course of the observation. Near the minimum ring plane radius of each of these chords, the occultation point in the rings moved tangentially to the ring particles orbital direction for several hundreds of km in ring azimuth in the frame comoving with the ring particles. In the same time interval, the occultation point moved less than 10 m in ring radius. While studies of large-scale ring properties often treat the rings as azimuthally symmetric, azimuthally limited structures are observed frequently on scales ranging from hundreds of m to several tens of km in the local Keplerian frame. The largest of these mesoscale structures are observed in Cassini Imaging Subsystem (ISS) particle tracking images which slewed with the orbital motion of the particles with pixel scales as low as 300 m. In images these structures are referred to as streaky texture, feathery texture, and straw, and their formation mechanism is poorly understood. At each of the minimum ring radii of the 119 UVIS chord occultations we characterize the azimuthal ring structure by comparing occultation data with Cassini ISS images and n-body simulations in order to determine how the local ring particle size distribution and dynamical environment influence the formation of quasi-stable mesoscale structures in the rings. We find evidence of self-gravity wake formation in regions dominated by 150 m viscous over-stable waves in the A and B rings as well as azimuthally extended lobes of 10 20 m propeller moonlets in the C ring plateaus.