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On the Origin of the Double Magnetic Cycle of the Sun

The magnetic solar cycle consists of two components: the well-known main cycle with period T0 = 11 yr, and a high-frequency component with period close to 2 yr, whose origin is not yet understood. Here we use proper orthogonal decomposition (POD) analysis on the green coronal emission line at 530.3 nm to investigate the spatio-temporal behavior of the solar cycle. We find that very few POD modes suffice to describe the cycle. In particular, the first most energetic mode describes the main 11 yr cycle, while a 2 yr periodic signal dominates the second POD mode only for high solar latitudes. The quasi-biennial signal is confined to the fourth mode for equatorial data, thus suggesting that, within the dynamo effect, the high-frequency cycle should be due to some phenomenon that is different from the emergence of solar active regions.