  We analyze several thousand Cassini ISS images in order to study the inner
edge of the Keeler gap in Saturn's outer A ring. We find strong evidence for an
m=32 perturbation with a mean amplitude of radial variation of 4.5 km. Phase
analysis yields a pattern speed consistent with the mean motion of Prometheus,
indicating that this pattern is generated by the 32:31 Inner Lindblad resonance
with Prometheus. In addition, we find evidence of 18-lobed and 20-lobed
patterns with amplitudes of ~1.5 km. These patterns, whose rotation rates
correspond to resonance locations ~4 km interior to the gap edge, are believed
to be normal modes. The former is probably related to the nearby 18:17 (m=18)
resonance with Pandora. In addition to these resonant and normal mode patterns,
we also observe multiple localized features that appear to move at the local
keplerian rate and that persist for only a few months. One hypothesis is that
different groups of ring particles at the inner edge of the gap may be reacting
differently to the resonance with Prometheus, with local variations in the
forced eccentricity and/or pericenter; an alternative hypothesis is the
existence of several unseen objects embedded at or near the inner edge of the
Keeler gap, similar to those suspected to exist at the outer edges of the A and
B rings (Spitale and Porco 2009, 2010). In either case, observations of the
ring edge at opposite ansae demonstrate that the localized features must be on
eccentric orbits.
