The dynamic evolution of twisted magnetic flux tubes in a three-dimensional convecting flow.: II.: Turbulent pumping and the cohesion of Ω-loops

We present a set of three-dimensional MHD simulations using the anelastic approximation of active region scale flux ropes embedded in a turbulent, stratified model convection zone. We simulate the evolution of Omega-loops and other magnetic structures of varying field strengths, helicities, and morphologies in both rotating and nonrotating background states. We show that if the magnetic energy of a flux tube is weak relative to the kinetic energy density of strong downdrafts, convective flows dominate the evolution, flux tubes of any shape rapidly lose cohesion, and the magnetic field redistributes itself throughout the domain over timescales characteristic of convective turnover. We determine the conditions under which magnetic tension resulting from field line twist can provide the force necessary to prevent a relatively weak flux tube from losing cohesion during its ascent through the turbulent convection zone. Our simulations show that there is no initial tendency for a horizontal magnetic flux tube or layer to be preferentially transported in one vertical direction over the other solely as a result of the presence of an asymmetric vertical flow field. However, as the simulations progress, there is a transient net transport of magnetic flux into the lower half of the computational domain as the distribution of the magnetic field changes and flux is expelled from cell centers into converging downflows and intergranular lanes. This pumping mechanism is weak and uncorrelated with the degree of vertical flow asymmetry. We find that the strong turbulent pumping evident in simulations of penetrative convection-the efficient transport of magnetic flux to the base of the convection zone over several local turnover times-does not manifest itself in a closed domain in the absence of a convective overshoot layer. Thus, we suggest that this rapid redistribution of flux is primarily due to the penetration of magnetic flux into the stable layer where it remains over a timescale that far exceeds that of convective turnover. We also find that different treatments of the viscosity of a Newtonian fluid in which the coefficient of either kinematic or dynamic viscosity is held constant throughout the domain-do not affect the global average evolution of embedded magnetic structures, although the details of the evolution may differ between models.