THE ORIGIN OF MORPHOLOGICAL ASYMMETRIES IN BIPOLAR ACTIVE REGIONS

We have carried out a series of three-dimensional numerical simulations to study the dynamical evolution of emerging flux loops in the solar convective envelope. The innermost portions of the loops are anchored beneath the base of the convection zone by the subadiabatic temperature gradient of the underlying overshoot region. We find that, as the emerging loops approach the photosphere, the magnetic field strength of the leading side (toward the direction of rotation) of each rising loop is about twice as large as that of the following side at the same depth. This asymmetry in the field strength develops as a result of the combined action of the Coriolis force and the anchoring of the innermost portions of the emerging loops. As each loop rises, the Coriolis force induces a flow within the rising loop which evacuates the plasma out of the leading side through the apex, and deposits it in the following side. The evacuation of plasma out of the leading side of the rising loop results in an enhanced magnetic field strength there compared with the following side. We argue that this result offers a natural explanation for the observed fact that the preceding (leading) polarity of an active region tends to be in the form of large, well-formed sunspots, whereas the following polarity tends to have a less organized and more fragmented appearance, and that the preceding spots tend to be larger in area, fewer in numbers, and have a longer lifetime than the following spots. We also find, as a result of our simulations, that loops originally from latitudes less than 30-degrees, and with initial field strength ranging from 3 x 10(4) G to 9 x 10(4) G, all emerge at latitudes which are consistent with the observed sunspot zones.