  Magnetorotational instability (MRI) in a convectively-stable layer around the
neutrinosphere is simulated by a three-dimensional model of supernova core. To
resolve MRI-unstable modes, a thin layer approximation considering only the
radial global stratification is adopted. Our intriguing finding is that the
convectively-stable layer around the neutrinosphere becomes fully-turbulent due
to the MRI and its nonlinear penetration into the strongly-stratified
MRI-stable region. The intensity of the MRI-driven turbulence increases with
magnetic flux threading the core, but is limited by a free energy stored in the
differential rotation. The turbulent neutrinosphere is a natural consequence of
rotating core-collapse and could exert a positive impact on the supernova
mechanism.
