Using only physical mechanisms , i.e. , 3D MHD with no phenomenological viscosity , we have simulated the dynamics of a moderately thin accretion disk subject to torques whose radial scaling mimics those produced by lowest-order post-Newtonian gravitomagnetism .
In this simulation , we have shown how , in the presence of MHD turbulence , a time-steady transition can be achieved between an inner disk region aligned with the equatorial plane of the central mass ’ s spin and an outer region orbiting in a different plane .
The position of the equilibrium orientation transition is determined by a balance between gravitomagnetic torque and warp-induced inward mixing of misaligned angular momentum from the outer disk .
If the mixing is interpreted in terms of diffusive transport , the implied diffusion coefficient is \simeq ( 0.6 – 0.8 ) c _ { s } ^ { 2 } / \Omega for sound speed c _ { s } and orbital frequency \Omega .
This calibration permits estimation of the orientation transition ’ s equilibrium location given the central mass , its spin parameter , and the disk ’ s surface density and scaleheight profiles .
However , the alignment front overshoots before settling into an equilibrium , signaling that a diffusive model does not fully represent the time-dependent properties of alignment fronts under these conditions .
Because the precessional torque on the disk at the alignment front is always comparable to the rate at which misaligned angular momentum is brought inward to the front by warp-driven radial motions , no break forms between the inner and outer portions of the disk in our simulation .
Our results also raise questions about the applicability to MHD warped disks of the traditional distinction between “ bending wave ” and “ diffusive ” regimes .