Appearance of co-existing spiral modes
The animation on the right shows the time evolution of the net
overdensity of two superposed steady-amplitude, bi-symmetric spiral
blobs (shown to the left) that turn at differing rates. The inner
blob has the higher pattern speed. The blobs are just drawings, with
no underlying dynamics.
The appearance of the time evolution of these two superposed steady
"waves" is of a shearing transient pattern. The peak amplitude rises
as the pattern "swings" from slighly leading to training and then
decreases as the spiral becomes more tightly wrapped, before finally
the pattern snaps and repeats. Thus simulations that exhibit
recurrent transient spiral patterns, as they always do, could still be
supporting long-lived underlying spirals.
I (Jerry Sellwood) first showed this animation at the "Stars Without
Borders" Workshop, held in Medana, Slovenia on May 21, 2012. Please
acknowledge the author if you download the animation.