  In this work, we apply multichannel singular spectrum analysis (MSSA), a
data-adaptive, multivariate, non-parametric technique that simultaneously
exploits the spatial and temporal correlations of the input data to extract
common modes of variability, to investigate the intermediate
quasi-periodicities of the Fe XIV green coronal emission line at 530.3 nm for
the period between 1944 and 2008. Our analysis reveals several significant
mid-term periodicities in a range from about one to four years that are
consistent with the so-called quasi-biennial oscillations (QBOs), which have
been detected by several authors using different data sets and analysis
methods. These QBOs display amplitudes varying significantly with time and
latitude over the six solar cycles (18 to 23) covered by this study. A clear
North-South asymmetry is detected both in their intensity and period
distribution, with a net predominance of spectral power in the active-region
belt of the northern hemisphere. On the other hand, while the QBOs with periods
> 1.7 years are particularly intense around the polar regions and therefore
related to the global magnetic field, the ones with shorter periods are mainly
generated at mid-latitudes, in correspondence with the emergence of active
regions. Our findings indicate that the North-South asymmetry manifested in the
uneven latitudinal distribution of QBOs is a fundamental, albeit puzzling,
characteristic of solar activity.
