Track analysis of energetic neutral atom blobs at Saturn

Abstract

A statistical analysis of observations of the Magnetospheric Imaging Instrument (MIMI) on the Cassini spacecraft at Saturn reveals the morphology and dynamics of energetic neutral atoms (ENA). The ENA (20-50 keV hydrogens) images are smoothed and projected onto Saturn's equatorial plane, and the intensities are slant-corrected using a thin-disk approximation. The resulting images exhibit intensity "blobs" that move in a prograde (corotational) sense about Saturn. Blob centroids are determined and tracked for as long as they persist as recognizable objects. The radial distribution of the centroids maximizes near the orbit of Rhea (∼9 RS, 1 RS = 60,268 km). The angular speeds of all blobs lasting longer than 5 h vary from near zero to nearly twice the rigid corotation speed. However, the longer-lived blobs (those lasting longer than 10 h) exhibit a sharp cutoff at the corotation speed. Inside the orbit of Rhea, the angular speeds decrease linearly with radial distance with a slope of -11.4°/h/RS, while outside the orbit of Rhea, the speeds are essentially constant at 13.1 ± 1.6°/h. Generally, the angular speeds differ from corotation (33.4°/h). The angular speeds tend to be somewhat slower near midnight than at other local times. The radial speed distribution peaks at the slightly inward value of -0.26 RS/h. Radial speeds exhibit a slight outflow from 6 to 10 h local time and a slight inflow from 16 to 20 h. Copyright 2008 by the American Geophysical Union.