A THEORETICAL-MODEL FOR TILTS OF BIPOLAR MAGNETIC REGIONS

Joy's law (Hale et al. 1919) states that bipolar magnetic regions (BMR s) are inclined to the latitudinal line, with the p-spot (preceding sp ot) of the BMR closer to the equator and the tilt angle increasing wit h latitude. It is believed that the solar dynamo operates in the overs hoot region just below the convection zone and the BMRs are produced b y the flux loops rising from there due to magnetic buoyancy. These ris ing loops are expected to be twisted by the Coriolis force so that the y eventually emerge on the solar surface with a tilt. We extend the nu merical calculations of Choudhuri (1989) to study the tilts produced o n the rising flux loops by the Coriolis force. We find that the theore tically calculated tilts match the observations only if the magnetic f ield of the flux loops lies in the range between 60 and 160 kG. For su ch flux loops, the tilt has the correct magnitude and also varies corr ectly with the latitude. If the magnetic fields were stronger than 160 kG, then Coriolis force is much weaker than magnetic buoyancy and is only able to produce tilts which are very small in overall magnitude, though they still vary correctly with latitude. On the other hand, if the fields were smaller than 60 kG, then the Coriolis force would have been so overpowering that the flux loops would move parallel to the r otation axis as found earlier (Choudhuri 1989). Such flux loops appear only in high latitudes and do not obey Joy's law. On changing the dra g on the flux tube, these conclusions are not changed. If we change th e footpoint separation of the flux loop, then we find that magnetic te nsion may halt the rise of the flux loop if the footpoint separation i s below a critical value. However, for flux tubes which are able to re ach the surface, the range from 60 to 160 kG for the magnetic field st ill approximately holds. Thus our calculations seem to rule out either equipartition fields (about 10 kG) or very strong megagauss fields.