  By performing ideal magnetohydrodynamical (MHD) simulations with weak
vertical magnetic fields in unstratified cylindrical shearing boxes with
modified boundary treatment, we investigate MHD turbulence excited by
magnetorotational instability. The cylindrical simulation exhibits extremely
large temporal variation in the magnetic activity compared to the simulation in
a normal Cartesian shearing box, although the time-averaged field strengths are
comparable in the cylindrical and Cartesian setups. Detailed analysis of the
terms describing magnetic-energy evolution with "triangle diagrams"
surprisingly reveals that in the cylindrical simulation the compression of
toroidal magnetic field is unexpectedly as important as the winding due to
differential rotation in amplifying magnetic fields and triggering intermittent
magnetic bursts, which are not seen in the Cartesian simulation. The importance
of the compressible amplification is also true for a cylindrical simulation
with tiny curvature; the evolution of magnetic fields in the nearly Cartesian
shearing box simulation is fundamentally different from that in the exact
Cartesian counterpart. The radial gradient of epicycle frequency, $\kappa$,
which cannot be considered in the normal Cartesian shearing box model, is the
cause of this fundamental difference. An additional consequence of the spatial
variation of $\kappa$ is continuous and ubiquitous formation of narrow
high(low)-density and weak(strong)-field localized structures; seeds of these
ring-gap structures are created by the compressible effect and subsequently
amplified and maintained under the marginally unstable condition regarding
"viscous-type" instability.
