Abstract The physical process constraining the most significant quasi-periodicity in cosmogenic radionuclide records and in reconstructed sunspot-number records, the sharply-defined ≈ 210-yr de Vries cycle, is unknown. It is found here to coincide within the measurement errors with the beat of the Schwabe cycle’s ≈ 1/10.0 yr Fourier-spectrum secondary frequency-peak, with the cycle’s ≈ 1/10.5-yr central Fourier peak which is also its autocorrelation period. The lesser-known, but just as significant de Vries companion-oscillation of ≈ 230-yr coincides with the beat of the ≈ 1/11.0 yr main frequency peak with the ≈ 1/10.5 yr period. In the classical solar-dynamo mode-typology based on symmetries, the three beating quasi-decadal periods would correspond to the dipole mode (≈ 11.0 yr), the quadrupole mode (putatively ≈ 10.0 yr in historical sunspot data), and the composite “mixed-parity” mode (≈ 10.5 yr). The secondary beat of the observed ≈ 210-yr and ≈ 230-yr bicentennial oscillations yields a 2310 yr period. This value is consistent with the prominent Hallstatt cycle’s estimated length of 2300 yr in the Holocene.
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