  The evolution of current sheets in accretion flows undergoing
magnetorotational instability (MRI) is examined through two and three
dimensional numerical modelling of the resistive MHD equations in global
cylindrical geometry. With an initial uniform magnetic field aligned in the
vertical ($z$) direction, MRI produces radially extended toroidal (azimuthal)
current sheets. In both 2D and 3D when axisymmetric modes dominate, these
current sheets attract each other and merge in the poloidal ($rz$) plane,
driving magnetic reconnection when the Lundquist number $S > 3 \times 10^2$,
making it a possible source of plasmoids (closed magnetic loops) in accretion
disks. At high Lundquist numbers in the 2D regime, starting at $S = 5 \times
10^3$, self-consistent MRI-generated current sheets become thin and subject to
plasmoid instability, and therefore spontaneous magnetic reconnection. When
non-axisymmetric 3D modes dominate, turbulence makes the azimuthal current
sheets further unstable, and stretch vertically. Toroidally extended vertical
current sheets in the inner region, as well as larger 3D magnetic islands in
the outer regions of the disks are also formed. These findings have strong
ramifications for astrophysical disks as potential sources of plasmoids that
could cause local heating, particle acceleration, and high energy EM radiation.
