  Saturn's F ring is subject to dynamic structural changes over short periods.
Among the observed phenomena are diffuse extended bright clumps (ECs) ~ 3-40
degrees in longitudinal extent. These ECs appear, evolve, and disappear over a
span of days to months. ECs have been seen by the two Voyager spacecraft, the
Cassini orbiter, and various ground- and space-based telescopes. Showalter
(2004, Icarus, 171, 356-371) analyzed all Voyager images of the F ring and
found that there were 2-3 major and 20-40 minor ECs present in the ring at any
given time. We expand upon these results by comparing the ECs seen by Voyager
to those seen by Cassini in 2004-2010. We find that the number of minor ECs has
stayed roughly constant and the ECs have similar distributions of angular
width, absolute brightness, and semimajor axis. However, the common
exceptionally bright ECs seen by Voyager are now exceedingly rare, with only
two instances seen by Cassini in the six years, and they are now also much
dimmer relative to the mean ring background. We hypothesize that these bright
ECs are caused by the repeated impacts of small moonlets with the F ring core,
and that these moonlets have decreased in number in the 25 years between
missions.
