CAN EQUIPARTITION FIELDS PRODUCE THE TILTS OF BIPOLAR MAGNETIC REGIONS

Bipolar magnetic regions (BMRs) are believed to be formed by magnetic flux loops emerging from the bottom of the convection zone. BMRs obey Joy's law, which states that the line joining the two spots makes an angle with the latitude, called tilt, which increases with increase in latitude. D'Silva & Choudhuri (1992) show that if these BMRs have to obey the observed Joy's law, fields at the bottom of the convection zone should lie between 60 and 160 kG. Then, Coriolis force has just the right value compared to magnetic buoyancy so as to give these fields the right tilt. A dynamo operating at the bottom of the convection zone cannot produce such strong fields. The fields can at most be in equipartition with the velocities there and hence should be weaker than 10 kG. Here we study the effect of turbulence on the nonaxisymmetric flux rings of equipartition field strength. We employ the small-scale momentum exchange mechanism (introduced for axisymmetric rings by Choudhuri & D'Silva 1990) and the giant cell drag combined with Kelvin-Helmholtz drag mechanism (used by D'Silva & Choudhuri 1991 for axisymmetric rings), and we show that the giant cell drag and small-scale momentum exchange mechanism can make equipartition flux loops emerge at low latitudes, in addition to making them exhibit the observed tilts. However, the sizes of flux tubes have to be restricted to a couple of hundred kilometers. We also introduce an ad hoc constraint on the footpoints of the flux loops, by not letting them move in the phi direction, and find that equipartition fields of any size can be made to emerge at sunspot latitudes with the observed tilts by suitably adjusting the footpoint separations.