However, to characterize this point, we analyzed the trace
of the spectral matrix S i j(f) of the PCs (Tr S =
i S ii(f)) for
the two different regimes (i = 1, 2 and i = [3, 18]). The results
are reported in Fig. 13. The trace of the spectral matrix for the
first dynamical regime is characterized mainly by a pronounced
peak at the typical 11-yr periodicity and other peaks close to
its harmonics (up to 6), while the trace of the spectral matrix
for the second dynamical regime shows a very peculiar peak at
fb ∼ 1
2 yr−1 along with a nearly 11-yr modulation. The observed
peculiar periodicity of Tb = f −1
b ∼ 2 yr is in the same range of
timescales associated with the quasi-biennial oscillation (∼2 yr),
observed in many solar spectral lines and other activity indicators
(see e.g., Benevolelenskaya 1998; Polygiannakis et al. 2003;
Cadavid et al. 2005; Penza et al. 2006; Laurenza & Storini 2008).