On nonshearing magnetic configurations in differentially rotating disks

A new class of disk MHD equilibrium solutions is described, which is valid within the standard local (shearing sheet) approximation scheme. These solu tions have the following remarkable property: velocity streamlines and magn etic lines of force rotate rigidly, even in the presence of differential ro tation. This situation comes about because the Lorentz forces acting upon m odified epicycles compel fluid elements to follow magnetic lines of force. Field line land streamline) configurations may be elliptical or hyperbolic, prograde or retrograde. These structures have previously known hydrodynami cal analogs: the planet solutions described by Goodman, Narayan, & Goldreic h. The primary focus of this investigation is configurations in the disk pl ane. A related family of solutions lying in a vertical plane is briefly dis cussed; other families of solutions may exist. Whether these MHD structures are stable is not yet known, but could readily be determined by three-dime nsional simulations. If stable or quasi-stable, these simple structures may find important applications in both accretion and galactic disks.