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tokens: and fast (up to 0.1 local sound speed) accretion happening at disk surface, which is similar to the flow pattern in global ideal MHD simulations (Zhu Stone 2018 ). A similar trend can be seen in the radial mass flux ( ρv r ) plot in the lower middle panel. The Rφ and zφ components of total stress are plotted in the rightmost two panels. The Rφ stress at both radii are close to zero at midplane, as already shown by very low midplane α in Figure 4 . Almost zero Rφ stress at different radii also suggests that midplane accretion or decretion is not driven by the Rφ component. As calculated in Section 3 , the vertical ( z ) gradient of also contributes to . In the zφ stress panel, maximizes at midplane, which is in agreement with the ρv r spike in the lower middle panel. At R = 1.33 (13.3 au), the midplane decretion is driven by zφ stress’s negative z gradient. The surface accretion is related to the two positive z gradient parts around disk surface. The differences of at these two radii also suggest very different magnetic field structure. Figure 5. Various quantities along the vertical direction at R = 1 (blue) and R = 1.33 (orange, local diffusion maximum at the CO 2 snow line). The quantities are averaged over both the azimuthal direction and time (from t = 49 to t = 50 T 0 with Δ t = 0.1 T 0 ). Top row from left to right: gas density, gas radial velocity to local sound speed ratio, R – φ stress; bottom row: inverse of plasma β , radial mass flux, and z – φ stress. The origin of the differences in stress structure can be understood from the 2D plot on the z – R plane, showing how magnetic field is evolved in Figure 6 . At the beginning of the simulation, the magnetic field only has a vertical ( z ) component, so B φ , B R = 0 everywhere. Apart from the dominant Keplerian shear ( dv φ / dR ) at midplane, the disk is also differentially rotating vertically ( dv φ / dz ), as midplane is the fastest and the atmosphere portion, which is mostly pressure gradient supported, rotates much slower. After the simulation starts, the field lines closer to midplane are dragged toward the rotation direction, making the B φ positive below midplane and negative above midplane. The motion along the r direction is even more complicated. From t = 0.5, at the snow line, the velocity shear between wind and surface accretion forms a vortex-like flow pattern that drives gas infall to the midplane. This gas motion drags field lines toward midplane, making a kinked form of magnetic field. Accretion at midplane also drags field lines inward, amplifying horizontal field component B R , and the polarity of B R changes sign in the accretion stream. Close to midplane, Keplerian shear stretches horizontal field lines, just like the growth of azimuthal fields in the linear phase of MRI. The process makes the field kink at midplane even sharper. The vertical gradient of the B φ component tends to get steeper with AD (Brandenburg Zweibel 1994 ). The strength of AD is