Abstract
AbstractIt is shown that the growth and decay rates of sunspot groups are both a function of the axial tilt angles of the groups. Both of these rates reach a maximum near the average tilt angle, which is around +5°, not 0°. (A positive tilt angle represents the situation where the leading spots are equatorward of the following spots). The fact that this peak angle is close to the average tilt and not 0° suggests that this may be the orientation of the subsurface toroidal flux tube(s) from which the sunspot groups form. In general, spot groups with positive tilt angles show faster average percentage growth rates for greater positive tilt angles. For negative tilt angles (following spots equatorward of leading spots) the situation is quite different. Within certain well-defined ranges of tilt angles the average group percentage area growth rates are quite high and the dispersion in this quantity is large. In other well-defined tilt angle ranges the percentage growth rates are uniformly (relatively) small. The difference between these average growth rates is more than a factor two. The reality of this result can be demonstrated by selecting for analysis various subsets of the data. It may be hypothesized that in general the dispersion measured in group tilt angles results from the random action of large-scale convection on the rising magnetic flux loops. Both growth and decay are facilitated in the case of flux loops that have a minimum twist. The percentage growth and decay rates are affected by the fact that groups with tilt angles near the average value tend to be larger than groups with different tilt angles. The ‘quantized’ behavior of the percentage growth rates for negative tilt angles is difficult to fit into any rough model of flux emergence. The more or less ‘quantized’ nature of the growth-tilt angle relationship for negative tilt angles is quite puzzling and may reflect some characteristic of the subsurface flux orientation or development which is at present unknown.
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