Abstract

Numerous analyses suggest the existence of various quasi-periodicities in solar activity. The power spectrum of solar activity recorded in sunspot data is dominated by the ∼11-yr quasi-periodicity, known as the Schwabe cycle. In the mid-term range (1 month–11 yr) a pronounced variability known as a quasi-biennial oscillation is widely discussed. In the shorter time-scale a pronounced peak, corresponding to the synodic solar rotation period (∼27 d), is observed. Here we revisit the mid-term solar variability in terms of statistical dynamics of fully turbulent systems, where solid arguments are required to accept an isolated dominant frequency in a continuous (smooth) spectrum. For this, we first undertook an unbiased analysis of the standard solar data, sunspot numbers and the F10.7 solar radio flux index, by applying a wavelet tool, which allows one to perform a frequency–time analysis of the signal. Considering the spectral dynamics of solar activity cycle by cycle, we showed that no single periodicity can be separated, in a statistically significant manner, in the specified range of periods. We examine whether a model of the solar dynamo can reproduce the mid-term oscillation pattern observed in solar data. We found that a realistically observed spectrum can be explained if small spatial (but not temporal) scales are effectively smoothed. This result is important because solar activity is a global feature, although monitored via small-scale tracers like sunspots.