Figure 1 shows the main features of the butterfly diagram: (1) the onset of the cycle at mid-latitudes; (2) the sunspot drift toward the equator and its slowdown represented by a change in the slope of the butterfly wing (Maunder 1904; Li et al. 2001); (3) the tail-like attachment over the minimum phase that is more prominent when the activity is stronger, which might lead to the overlap of successive cycles; (4) the length of the overlap varies within 1–2 years. It characterizes only the minimum phase and it is confined at latitudes ≤15° (Cliver 2014). This feature is also seen in torsional oscillations shown in the bottom panels of Figure 1 (Wilson et al. 1988; Howe et al. 2009). The rate of drift of sunspots toward the equator slows as the sunspot band approaches the equator, and halts at about 8° latitude (Hathaway et al. 2003). The end of the migration does not correspond to the end of the activity because it produces the tail-like attachment. When the new cycle at mid-latitudes starts before the end of the old cycle at low latitudes, it causes successive cycles to overlap. FTD models driven only by the Babcock–Leighton mechanism (Chatterjee et al. 2004), or along with the α-turbulent effect operating in the bulk of the convection zone, currently have the best agreement with observations (Passos et al. 2014), because the length of the simulated overlap is short and it occurs only during the minimum at low latitudes. Conversely, thin-shell dynamo wave models (Moss & Brooke 2000; Schüssler & Schmitt 2004; Bushby 2006) or the thin-shell flux transport dynamo (Dikpati & Gilman 2001) tend to produce dynamo waves with too short a wavelength, leading to excessive overlap between adjacent cycles because this involves a wider range of latitudes. Furthermore they also fail to reproduce the tail-like attachment over the minimum phase. Moreover the direction of the migration of activity could also provide information on the nature of the α mechanism. Both formalisms make strong assumptions to initiate the sunspot cycle at mid-latitudes. The Babcock–Leighton FTD models assume that the deep equatorward meridional flow penetrates slightly below the convection zone to a greater depth than usually believed (Nandy & Choudhuri 2002), in order to prevent the onset and occurrence of a sunspot cycle above 45° as well as any other kind of cyclic activity. The same result is achieved with the αΩ dynamo wave by inhibiting the α-turbulent effect at higher latitudes (Schüssler & Schmitt 2004). Based on these assumptions, the magnetic activity in any type of FTD model starts at higher latitudes and then propagates only equatorward, while in the thin-shell αΩ dynamo wave the magnetic activity can propagate poleward as well as equatorward (Bushby 2006). These two branches are also clearly seen in the torsional oscillation pattern (e.g., Howe et al. 2009). This results from the solar-like differential profile, which is characterized by a sign change in 





 in the tachocline at high and low latitudes (Ruediger & Brandenburg 1995). This sign change, however, has not yet been confirmed by helioseismic observations.
Citation: Chatterjee et al. 2004
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