To investigate the occurrence of spatio-temporal complexity in
the 11-year solar activity cycle, we consider the total sunspot
area butterfly diagram (see Fig. 1), compiled from Royal
Greenwich Observatory and USAF Solar Optical Observing
Network observations and available at the NASA Solar Physics
Marshall Space Flight Center1. The butterfly diagram is, indeed,
the only spatio-temporal dataset that, spanning over more
than 12 solar cycles, can be considered sufficiently long to apply
the NOC analysis, which requires a set of temporal snapshots
(here 1759 Carrington rotations) larger in number than the
total number of spatial observations (here 50 latitudinal bins).
The butterfly diagram ψ(x, t) refers to the latitudinal (x) and
temporal (t) distribution of the sunspot areas in units of millionths
of a hemisphere since May, 1874. Data, representing
the total sunspot area, are organized in 50 latitude bins per
Carrington rotation (∼27.28 days). The latitude bin locations are
uniformly distributed with respect to sin(θ), where θ is the latitude
(i.e., xi = sin(θi)), while the temporal window spreads over
1759 Carrington rotations. The sunspot butterfly diagram data
used in this work was compiled following RGO/USAF/NOAA
instructions1 for data after 1976, which requires to increase
USAF/NOAA spot areas by a factor of 1.4.