  Some circumstellar disks are observed to show prominent spiral arms in
infrared scattered light or (sub-)millimeter dust continuum. The spirals might
be formed from self-gravity, shadows, or planet-disk interactions. Recently, it
was hypothesized that massive vortices can drive spiral arms in protoplanetary
disks in a way analogous to planets. In this paper, we study the basic
properties of vortex-driven spirals by the Rossby Wave Instability in 2D
hydrodynamics simulations. We study how the surface density contrast, the
number, and the shape of vortex-driven spirals depend on the properties of the
vortex. We also compare vortex-driven spirals with those induced by planets.
The surface density contrast of vortex-driven spirals in our simulations are
comparable to those driven by a sub-thermal mass planet, typically a few to a
few tens of Earth masses. In addition, different from the latter, the former is
not sensitive to the mass of the vortex. Vortex-driven spiral arms are not
expected to be detectable in current scattered light observations, and the
prominent spirals observed in scattered light in a few protoplanetary disks,
such as SAO 206462 (HD 135344B), MWC 758, and LkHa 330, are unlikely to be
induced by the candidate vortices in them.
