  Images obtained by the Cassini spacecraft between 2012 and 2015 reveal a
periodic brightness variation in a region of Saturn's D ring that previously
appeared to be rather featureless. Furthermore, the intensity and radial
wavenumber of this pattern have decreased steadily with time since it was first
observed. Based on analogies with similar structures elsewhere in the D ring,
we propose that this structure was created by some event that disturbed the
orbital motions of the ring particles, giving them finite orbital
eccentricities and initially aligned pericenters. Differential orbital
precession then transformed this structure into a spiral pattern in the ring's
optical depth that became increasingly tightly wound over time. The observed
trends in the pattern's radial wavenumber are roughly consistent with this
basic model, and also indicate that the ring-disturbing event occurred in early
December 2011. Similar events in 1979 may have generated the periodic patterns
seen in this same region by the Voyager spacecraft. The 2011 event could have
been caused by debris striking the rings, or by a disturbance in the planet's
electromagnetic environment. The rapid reduction in the intensity of the
brightness variations over the course of just a few years indicates that some
process is either damping orbital eccentricities in this region or causing the
orbital pericenters of particles with the same semi-major axis to become
misaligned.
