5.2 Fossil fields and the 22-yr cycle
The presence of a large-scale, quasi-steady magnetic field of fossil origin in the solar interior has long
been recognized as a possible explanation of the Gnevyshev-Ohl rule (see Panel E of Figure 19).
The basic idea is quite simple: The slowly-decaying, deep fossil field being effectively steady
on solar cycle timescales, its superposition with the 11-yr polarity reversal of the overlying
dynamo-generated field will lead to a 22-yr modulation, whereby the cycle is stronger when the fossil
and dynamo field have the same polarity, and weaker when these polarities are opposite (see,
e.g., Boyer and Levy, 1984; Boruta, 1996). The magnitude of the effect is directly related to the
strength of the fossil field, versus that of the dynamo-generated magnetic field. All of this,
however, presumes that flows and dynamical effects within the tachocline still allow “coupling”
between the deep fossil field below, and the cyclic dynamo-generated field above. It is far from
obvious that this coupling could indeed take place in the simple manner implicitly assumed in
dynamo models, that typically incorporate the effect of fossil fields via the lower boundary
condition.
One strong prediction is associated with this explanation of the Gnevyshev-Ohl rule: While the pattern
may become occasionally lost due to large amplitude fluctuations of other origin, whenever it is present
even-numbered cycles should always be of lower amplitudes and odd-numbered cycles of higher amplitude
(under Wolf’s cycle numbering convention). Evidently, this prediction can be tested observationally,
provided one can establish a measure of sunspot cycle amplitude that is truly characteristic of the
strength of the underlying dynamo magnetic field. Taken at face value, the analysis of Mursula
et al. (2001), based on cycle-integrated group sunspot numbers, indicates that the odd/even pattern
has reversed between the time periods 1700-1800 and 1850-1990 (see their Figure 1). This
would then rule out the fossil field hypothesis unless, as argued by some authors (see Usoskin
et al., 2001, and references therein), a sunspot cycle has been “lost” around 1790, at the onset
of the so-called Dalton minimum. For more on these matters, see Section 6 in Usoskin and
Mursula (2003).
).
The basic idea is quite simple: The slowly-decaying, deep fossil field being effectively steady
on solar cycle timescales, its superposition with the 11-yr polarity reversal of the overlying
dynamo-generated field will lead to a 22-yr modulation, whereby the cycle is stronger when the fossil
and dynamo field have the same polarity, and weaker when these polarities are opposite (see,
e.g., Boyer and Levy, 1984; Boruta, 1996). The magnitude of the effect is directly related to the
strength of the fossil field, versus that of the dynamo-generated magnetic field. All of this,
however, presumes that flows and dynamical effects within the tachocline still allow “coupling”
between the deep fossil field below, and the cyclic dynamo-generated field above. It is far from
obvious that this coupling could indeed take place in the simple manner implicitly assumed in
dynamo models, that typically incorporate the effect of fossil fields via the lower boundary
condition.
