In Figs. 9 and 10, we display the two plain projections of the
phase portrait as reconstructed using A2(t) versus A1(t), and A4(t)
versus A3(t), respectively. By smoothing the trajectories over a
time window ∼1
4 of solar cycle length, it is possible to differentiate
between the nature of the first two PCs from the others. While
no evidence of a regular temporal structure is found in the A4(t)
− A3(t) projection, the smoothed trajectory of A2(t) versus A1(t)
exhibits a certain degree of order, as shown by the existence of
cyclic oscillations. The spread in the smoothed trajectories in the
[A2(t), A1(t)] plane projection of the phase portrait suggests that