Spatio-temporal analysis of solar activity: main periodicities and period length variations

Aims. The spatio-temporal dynamics of solar activity has been investigated by studying the main oscillations and the time evolution of the basic periods. Methods. The spatio-temporal behavior of the green coronal emission line at 530.3 nm, recorded from 1939 to 2005, has been analyzed by using proper orthogonal decomposition, to extract the main components of the system, and the wavelet analysis to further investigate their time behaviour. Results. In addition to the main 11-year periodicity, a high-frequency component has been recorded mainly on the polar regions of the Sun, thus indicating a different origin from the emergence of active regions. Evidence for variations in the period lengths of this component has been found for the first time. The calculated period length varies between 1.5 years and 4 years, in good agreement with frequencies attributed to the quasi-biennial cycle extracted by different solar activity indicators. Our analysis shows that, unlike the main periodicity, the high-frequency component does not show the typical properties of a true mode of oscillation but seems to originate, in a narrow band of frequencies, from a stochastic superposition of different oscillators. Conclusions. The observed solar cycle frequencies in the range 1.5-4 years, commonly considered as independent modes of oscillation, could be the manifestation of the temporal modulation of a unique quasi-biennial periodicity. Our findings can provide more constraints on dynamo models introduced to describe the different components of the solar cycle. The calculated period length variations could also be helpful to improve our knowledge of the relationship between solar activity, neutrino flux variations and cosmic ray modulation.