Fig. 13. The trace of the spectral matrix for the two dynamical regimes,
i = 1, 2 (upper panel) and i = [3, 18] (lower panel), respectively. The
dashed lines represent the 95% confidence levels. Vertical dashed lines
refers to the principal solar cycle periodicity and its harmonics (up to
6). The arrow in the lower panel indicates a peculiar peak at ∼1
2 yr−1, a
frequency in good agreement with the well known quasi-biennal oscillation
(QBO).