One reason we can suppose why Li et al. (2003) have concluded the latitudinal distribution of sunspots follows the C function is that the bin size is too large to show details. In fact, the C function looks quite similar to the double Gaussian function. What the bimodality could mean is that while the sunspot formation sites move from high latitudes to low latitudes as the solar cycle proceeds there are two preferential sunspot formation sites. That is, the butterfly diagram consists of two pairs of wings rather than one pair of wings. This is somewhat surprising in the sense that the accepted paradigm has pretended that sunspots are scattered around a mean latitude, which steadily drifts equatorward. Is this
telling us about ’the active latitude’ or ’knots’ (e.g., Li et al., 2003; Solanki et al., 2008; Ternullo, 2010)? The relation between the
bimodality and them is worth further pursuing (e.g., Major, 2004).