  Circumstellar asymmetries such as central warps have recently been shown to
cast shadows on outer disks. We investigate the hydrodynamical consequences of
such variable illumination on the outer regions of a transition disk, and the
development of spiral arms. Using 2D simulations, we follow the evolution of a
gaseous disk passively heated by the central star, under the periodic forcing
of shadows with an opening angle of $\sim$28$^\circ$. With a lower pressure
under the shadows, each crossing results in a variable azimuthal acceleration,
which in time develops into spiral density waves. Their pitch angles evolve
from $\Pi \sim 15^\circ-22^\circ$ at the onset, to $\sim$11$^\circ$-14$^\circ$,
over $\sim$65~AU to 150~AU. Self-gravity enhances the density contrast of the
spiral waves, as also reported previously for spirals launched by planets. Our
control simulations with unshadowed irradiation do not develop structures,
except for a different form of spiral waves seen at later times only in the
gravitationally unstable control case. Scattered light predictions in the
$H$-band show that such illumination spirals should be observable. We suggest
that spiral arms in the case-study transition disk HD~142527 could be explained
as a result of shadowing from the tilted inner disk.
