MHD in a cylindrical shearing box

Speaker: Takeru Suzuki


By performing magnetohydrodynamical (MHD) simulations with weak vertical magnetic fields in unstratified cylindrical and Cartesian shearing boxes, we investigate basic properties of MHD turbulence excited by magnetorotational instability. While both cylindrical and Cartesian cases give a similar level of the time-averaged saturated magnetic field strengths, the cylindrical setup exhibits extremely large time-variability. Detailed analysis of the terms describing magnetic-energy evolution with “triangle diagrams” surprisingly reveals that in the cylindrical simulations, including a case with small curvature, the compression of toroidal magnetic field is unexpectedly as important as the winding due to differential rotation in the amplification of the magnetic field, which is not seen in the Cartesian simulation. This suggests that the physical properties of magnetic evolution in the Cartesian shearing box simulation is fundamentally different even from those in the nearly Cartesian simulation in the cylindrical setup. In addition, the compressible amplification plays a substantial role in triggering intermittent bursty enhancements in magnetic energy observed in the cylindrical simulations. We discuss that {\it the radial gradient of epicycle frequency}, which cannot be handled in the normal Cartesian shearing box model, significantly contributes to the bursty magnetic activity due to compressible amplification. We also introduce a modified prescription for the cylindrical shearing boundary condition.


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