2-DIMENSIONAL SIMULATIONS OF BUOYANTLY RISING, INTERACTING MAGNETIC-FLUX TUBES

We perform two-dimensional simulations of the buoyant rise of twisted horizontal magnetic flux tubes through an adiabatically stratified lay er representing the solar convection zone or other marginally stable a tmosphere. The numerical calculations employ the anelastic approximati on to the basic MI-ID equations. We confirm the results of recent comp ressible simulations by Moreno-Insertis & Emonet that the azimuthal co mponent of the tube magnetic field can prevent the splitting of the tu be into a vortex pair, and that most of the flux in the initial tube c ross section rises in the form of a rigid body that reaches a terminal speed similar to the prediction of the often-employed thin-flux-tube model. We also study the interaction between a pair of buoyant flux tu bes as they rise in proximity. In the case of two identical flux tubes that start from the same level, we iind that the wake behind each tub e interacts with the wake of the other; prompting mirror-symmetric vor tex shedding in each wake. As a result, each tube gains around it a ne t circulation of the opposite sign of the most recently shed eddy; thi s causes a periodic, horizontal lift force that makes the tubes oscill ate horizontally as they rise. The tube interactions in this case diff er substantially from the inviscid limit studied previously. For two i dentical flux tubes that start at different levels, the resulting inte ractions depend upon the details of the initial configuration of the t wo tubes and can be very different from the interactions seen in the s ymmetrical case. In the asymmetric case, it becomes possible for one f lux tube to be drawn into the wake of the other, leading eventually to a merger of the tubes.