  The standard magnetorotational instability (SMRI) is a promising mechanism
for turbulence and rapid accretion in astrophysical disks. It is a
magnetohydrodynamic (MHD) instability that destabilizes otherwise
hydrodynamically stable disk flow. Due to its microscopic nature at
astronomical distances and stringent requirements in laboratory experiments,
SMRI has remained unconfirmed since its proposal, despite its astrophysical
importance. Here we report a nonaxisymmetric MHD instability in a modified
Taylor-Couette experiment. To search for SMRI, a uniform magnetic field is
imposed along the rotation axis of a swirling liquid-metal flow. The
instability initially grows exponentially, becoming prominent only for
sufficient flow shear and moderate magnetic field. These conditions for
instability are qualitatively consistent with SMRI, but at magnetic Reynolds
numbers below the predictions of linear analyses with periodic axial
boundaries. Three-dimensional numerical simulations, however, reproduce the
observed instability, indicating that it grows linearly from the primary
axisymmetric flow modified by the applied magnetic field.
