Solar nebula magnetohydrodynamics

The dynamical state of the solar nebula depends critically upon whether or not the gas is magnetically coupled. The presence of a subthermal field wil l cause laminar flow to break down into turbulence. Magnetic coupling, in t urn, depends upon the ionization fraction of the gas. The inner most region of the nebula (less than or similar to 0.1 AU) is magnetically well-couple d, as is the outermost region (greater than or similar to 10 AU). The magne tic status of intermediate scales (similar to 1 AU) is less certain. It is plausible that there is a zone adjacent to the inner disk in which turbulen t heating self-consistently maintains the requisite ionization levels. But the region adjacent to the active outer disk is likely to be magnetically " dead.'' Hall currents play a significant role in nebular magnetohydrodynami cs. Though still occasionally argued in the literature, there is simply no evid ence to support the once standard claim that differential rotation in a Kep lerian disk is prone to break down into shear turbulence by nonlinear insta bilities. There is abundant evidence-numerical, experimental, and analytic- in support of the stabilizing role of Coriolis forces. Hydrodynamical turbu lence is almost certainly not a source of enhanced turbulence in the solar nebula, or in any other astrophysical accretion disk.