A search for asymmetric flows in young active regions

We have studied the temporal evolution of photospheric velocities in young active regions that show evidence of ongoing magnetic flux emergence. We searched for asymmetries in the vertical plasma flows between the leading and following legs of the magnetic flux tubes. Such asymmetries are predicted in models of flux tubes rising in the convection zone (see the recent work of Fan, Fisher, & DeLuca). These models, which successfully describe several aspects of active region formation, predict plasma flows from the leading to the following leg of a magnetic flux loop, driven by the Coriolis force acting on the rising loop. These flows contribute to an excess of gas pressure in the following leg with respect to the leading one. Our results show a predominance of downflow in the leading part of three young regions with respect to the following part, contrary to the model predictions. The observed asymmetries, obtained by averaging over the totality of the magnetic structures, range from 60 to 150 m s(-1). Their real value, however, could be higher if the age and effective magnetic filling factor were taken into account. The flow asymmetry seems to disappear when the active regions enter a phase of magnetic stability. We suggest two possible interpretations of these results in terms of the dynamics of emerging magnetic flux tubes as the most plausible ones. One possibility is that the rising flux tube experiences severe fragmentation during the last stages of emergence through the convection zone. After fragmentation, the greater effect of aerodynamic drag strongly reduces the rise speed of the smaller flux tubes and hence the Coriolis force that drives the flows from the leading to the following leg of the magnetic loop. Since the higher gas pressure present in the following leg is no longer balanced, it will then drive a flow in the opposite direction, i.e., from the following to the leading side. Estimates of these pressure-driven flow velocities are consistent with the observed values. A second possibility is that the asymmetric flows originate from a preexisting superrotational velocity within high-field strength toroidal flux rings near the base of the solar convection zone. As pointed out in the recent work of Moreno-Insertis, Schussler, & Ferriz-Mas, such superrotational velocities are required to maintain toroidal flux rings in dynamical equilibrium.