A SEARCH FOR ASYMMETRIC FLOWS IN YOUNG ACTIVE REGIONS

We have studied the temporal evolution of photospheric velocities in y oung active regions that show evidence of ongoing magnetic flux emerge nce. We searched for asymmetries in the vertical plasma flows between the leading and following legs of the magnetic flux tubes. Such asymme tries are predicted in models of flux tubes rising in the convection z one (see the recent work of Fan, Fisher, & DeLuca). These models, whic h successfully describe several aspects of active region formation, pr edict plasma flows from the leading to the following leg of a magnetic flux loop, driven by the Coriolis force acting on the rising loop. Th ese flows contribute to an excess of gas pressure in the following leg with respect to the leading one. Our results show a predominance of d ownflow in the leading part of three young regions with respect to the following part, contrary to the model predictions. The observed asymm etries, obtained by averaging over the totality of the magnetic struct ures, range from 60 to 150 m s(-1). Their real value, however, could b e higher if the age and effective magnetic filling factor were taken i nto account. The flow asymmetry seems to disappear when the active reg ions enter a phase of magnetic stability. We suggest two possible inte rpretations of these results in terms of the dynamics of emerging magn etic flux tubes as the most plausible ones. One possibility is that th e rising flux tube experiences severe fragmentation during the last st ages of emergence through the convection zone. After fragmentation, th e greater effect of aerodynamic drag strongly reduces the rise speed o f the smaller flux tubes and hence the Coriolis force that drives the flows from the leading to the following leg of the magnetic loop. Sinc e the higher gas pressure present in the following leg is no longer ba lanced, it will then drive a flow in the opposite direction, i.e., fro m the following to the leading side. Estimates of these pressure-drive n flow velocities are consistent with the observed values. A second po ssibility is that the asymmetric flows originate from a preexisting su perrotational velocity within high-field strength toroidal flux rings near the base of the solar convection zone. As pointed out in the rece nt work of Moreno-Insertis, Schussler, & Ferriz-Mas, such superrotatio nal velocities are required to maintain toroidal flux rings in dynamic al equilibrium.