5.4 Gnevyshev–Ohl Rule (Even–Odd Effect)

Gnevyshev and Ohl (1948Jump To The Next Citation Point) found that if solar cycles are arranged in pairs with an even numbered cycle and the following odd numbered cycle then the sum of the sunspot numbers in the odd cycle is higher than in the even cycle. This is referred to as the Gnevyshev–Ohl Rule or Even–Odd Effect. This Rule is illustrated in Figure 35View Image. With the exception of the Cycle 4/5 pair, this relationship held until cycle 23 showed that the cycle 22/23 pair was also an exception. If cycle amplitudes are compared then the Cycle 8/9 pair is also an exception. This rule also holds for other indicators of cycle amplitude such as sunspot area. While much has been said about this rule relative to the 22-year Hale cycle, it is difficult to understand why the order (even-odd vs. odd-even) of the pairing should make a difference. The observed effect does however impact flux transport models for the surface fields (see Sheeley Jr, 2005, for a review). Since the odd cycles all have the same magnetic polarity, stronger odd cycles will tend to build up polar fields of one polarity to the extent that the transport during the even cycles cannot reverse the polar fields without associated changes in transport.
View Image

Figure 35: Gnevyshev–Ohl Rule. The ratio of the odd cycle sunspot sum to the preceding even cycle sunspot sum is shown with the filled circles. The ratio of the odd cycle amplitude to the preceding even cycle amplitude is shown with the open circles.

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