Magnetorotational Transport in the Early Sun

Angular momentum transport must have occurred in the Sun's radiative zone toexplain its current solid body rotation. We survey the stability of the earlySun's radiative zone with respect to diffusive rotational instabilities, for avariety of plausible past configurations. We find that the (faster rotating)early Sun was prone to rotational instabilities even if only weak levels ofradial differential rotation were present, while the current Sun is not.Stability domains are determined by approximate balance between dynamical anddiffusive timescales, allowing generalizations to other stellar contexts.Depending on the strength and geometry of the weak magnetic field present, thefastest growing unstable mode can be hydrodynamic or magneto-hydrodynamic (MHD)in nature. Our results suggest that diffusive MHD modes may be more efficientat transporting angular momentum than their hydrodynamic(``Goldreich-Schubert-Fricke'') counterparts because the minimum spatial scalerequired for magnetic tension to be destabilizing limits the otherwise verysmall scales favored by double-diffusive instabilities. Diffusivemagneto-rotational instabilities are thus attractive candidates for angularmomentum transport in the early Sun's radiative zone.Comment: 23 pages, 4 figures, accepted for publication in Ap