The Butterfly Diagram Leopard Skin Pattern

Ternullo Maurizio, mternullo@oact.inaf.it, INAF - Osservatorio Astrofisico di Catania, Italy

Abstract
This work deals with the photospheric magnetic flux large scale distribution in the course of the solar cycle. The subject is approached by means of a Butterfly Diagram (BD), constructed using the sunspot group area as a proxy for the magnetic flux. A smoothing algorythm has been applied to the input data to suppress fluctuations with periods shorter than one year and to give visibility to the long period behavior. The magnetic flux is distributed very dishomogeneously, most of it being concentrated in few small portions (knots) of the BD, strictly limited to both latitude and duration (up to six months). The BD may be said to be but a cluster of knots; its aspect accounts for the expression ``leopard skin pattern'', provided that dark colors are used to represent high spot concentrations. Knots appear in a seemingly random way, at latitudes either lower or higher than previous ones, challenging our predictive capability. As an effect, we observe the spot zone centroid inverting its familiar behavior and drifting poleward two or three times per cycle in either hemisphere. This picture of the BD internal structure is incompatible with the popular description according to which spots are scattered on a continuous curve (usually, referred to as the spot mean latitude), steadily approaching the equator. It appears more correct to state that spots are scattered at as many latitudes as knots. The spot mean latitude should be regarded as a mere arithmetic artifact. Moreover, in a relevant number of cycles, knots avoid an oblique strip running among them (approximately, where one would expect to find the spot mean latitude) so as to divide the butterfly wing into two roughly equivalent parts. This amounts to saying that two (or even more) activity streams run equatorward throughout either hemisphere, at a rate higher than the spot zone as a whole and with a time lag of a couple of years between them. In the light of this description, the aforementioned zigzag path of the spot zone centroid is the consequence of the simultaneous presence of two streams, active at different latitudes, during most of the cycle.