6.2 Quasi-biennial variations and double peaked maxima

Another interesting periodicity is one found with a period of about two years (Benevolenskaya, 1995Mursula et al., 2003). Many sunspot cycles exhibit double peaks when the monthly International Sunspot Number is smoothed with the traditional 13-month running mean. This was noted by Gnevyshev (196319671977) and often referred to as the Gnevyshev Gap. Wang and Sheeley Jr (2003) found that the Sun’s dipole magnetic moment and open magnetic flux exhibits multiple peaks with quasi-periodicities of about 1.3 years that they attributed to the stochastic processes of active region emergence and a decay time of about 1 year set by the dynamical processes of differential rotation, meridional flow, and supergranule diffusion. These quasi-periodic variations are also evident in the wavelet spectrum shown in Figure 37View Image. Multiple significant peaks of power are seen intermittently at periods between 1 and 2 years and are most prevalent near the time of cycle maxima (Bazilevskaya et al., 2000). A signal with a similar period is seen in the Tachocline Oscillations – periodic variations in the shear at the base of the convection zone (Howe et al., 2000). These tachocline oscillations have also been found to be intermittent (Howe et al., 2007).

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