  Density waves in Saturn's rings are usually tightly wrapped spiral patterns
generated by resonances with either Saturn's moons or structures inside the
planet. However, between the Barnard and Bessel Gaps in the Cassini Division
(i.e. between 120,240 and 120,300 km from Saturn's spin axis), there are
density variations that appear to form an axisymmetric density wave consisting
of concentric zones of varying densities that propagate radially through the
rings. Axisymmetric waves cannot be generated directly by a satellite
resonance, but instead appear to be excited by interference between a nearby
satellite resonance and normal mode oscillations on the inner edge of the
Barnard Gap. Similar axisymmetric waves may exist just interior to other
resonantly confined edges that exhibit a large number of normal modes,
including the Dawes ringlet in the outer C ring and the outermost part of the B
ring.
