Upstable nonaxisymmetric three-dimensional and isothermal perturbations are demonstrated in a stably stratified thin disk with a corotating corona. The third dimensionality of the perturbations in the vertical direction introduces a crucial corotation singularity which turns out to be an unstable source for wave emission. The emitted waves are symmetric about, and propagating away from, the corotation radius with conserved wave action. Existence of a corotating high-temperature, low-density atmosphere (i.e., a corona) is found to be essential for the particular unstable perturbations reported here to be vertically trapped in a resonant cavity near the corotation radius. In the short vertical wavelength limit, the unstable perturbations become surface waves pinned near the disk-corona interface, the growth rate of which can be unboundly large, scaled as the square-root of the vertical wavenumber. In addition, the unstable perturbations possess helicity, which may potentially produce the dynamo action in amplifying the disk magnetic field in the nonlinear regime.
