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By performing ideal magnetohydrodynamical (MHD) simulations with weakvertical magnetic fields in unstratified cylindrical shearing boxes withmodified boundary treatment, we investigate MHD turbulence excited bymagnetorotational instability. The cylindrical simulation exhibits extremelylarge temporal variation in the magnetic activity compared to the simulation ina normal Cartesian shearing box, although the time-averaged field strengths arecomparable in the cylindrical and Cartesian setups. Detailed analysis of theterms describing magnetic-energy evolution with "triangle diagrams"surprisingly reveals that in the cylindrical simulation the compression oftoroidal magnetic field is unexpectedly as important as the winding due todifferential rotation in amplifying magnetic fields and triggering intermittentmagnetic bursts, which are not seen in the Cartesian simulation. The importanceof the compressible amplification is also true for a cylindrical simulationwith tiny curvature; the evolution of magnetic fields in the nearly Cartesianshearing box simulation is fundamentally different from that in the exactCartesian counterpart. The radial gradient of epicycle frequency, $\kappa$,which cannot be considered in the normal Cartesian shearing box model, is thecause of this fundamental difference. An additional consequence of the spatialvariation of $\kappa$ is continuous and ubiquitous formation of narrowhigh(low)-density and weak(strong)-field localized structures; seeds of thesering-gap structures are created by the compressible effect and subsequentlyamplified and maintained under the marginally unstable condition regarding"viscous-type" instability.

MHD in a cylindrical shearing box II: Intermittent Bursts and Substructures in MRI Turbulence