6.4 How constraining is the sunspot butterfly diagram?
The shape of the sunspot butterfly diagram (see Figure 3) continues to play a dominant constraining
role in many dynamo models of the solar cycle. Yet caution is in order on this front. Calculations
of the stability of toroidal flux ropes stored in the overshoot region immediately beneath the
core-envelope interface indicate that instability is much harder to produce at high latitudes,
primarily because of the stabilizing effect of the magnetic tension force; thus strong fields at high
latitudes may well be there, but not produce sunspots. Likewise, the process of flux rope formation
from the dynamo-generated mean magnetic field is currently not understood. Are flux ropes
forming preferentially in regions of most intense magnetic fields, in regions of strongest magnetic
helicity, or in regions of strongest hydrodynamical shear? Is a stronger diffuse toroidal field
forming more strongly magnetized flux ropes, or a larger number of flux ropes always of the same
These are all crucial questions from the point of view of comparing results from dynamo models to
sunspot data. Until they have been answered, uncertainty remains as to the degree to which the sunspot
butterfly diagram can be compared in all details to time-latitude diagrams of the toroidal field, as produced
by this or that dynamo model.