Guest post by David Archibald
Long suspected, it seems that this has now been confirmed by a paper in Astronomy and Astrophysics with the title “Is there a planetary influence on solar activity?” by Abreu et al that was published on 22nd October, 2012.
From the Discussion and Conclusions section:
The excellent spectral agreement between the planetary tidal effects acting on the tachocline and the solar magnetic activity is
surprising, because until now the tidal coupling has been considered to be negligible. In Appendix A we show that the possibility of an accidental coincidence can be ruled out. We therefore suggest that a planetary modulation of the solar activity does take place on multidecadal to centennial time scales.
The authors note that current solar dynamo models are unable to explain the periodicities in solar activity such as the 88 year (Gleissberg), 104 year, 150 year, 208 year (de Vries), 506 year, 1000 year (Eddy) and 2200 year (Halstatt) cycles. They adopted a different view by regarding the planets and the solar dynamo as two weakly coupled non-linear systems.
The idea that planetary motions may influence solar activity seems to have been initiated by Rudolf Wolf in the 1850s. While energy considerations clearly show that the planets cannot be the direct cause of solar activity, they may perturb the solar dynamo.
Specifically, the authors calculated planetary torque at the tachocline. The tachocline of the sun is a shear layer which represents a sharp transition between two distinct rotational regimes: the differentially rotating convection zone and the almost rigidly rotating radiative interior. The tachocline plays a fundamental role in the generation and storage of the toroidal magnetic flux that eventually gives rise to solar active regions. A net tidal torque is exerted in a small region close to the tachocline due to the buoyancy frequency originating from the convection zone matching the tidal period. The tachocline is thought to be non-spherical – either prolate (watermelon-shaped) or oblate (pumpkin-shaped). The authors’ model describes planetary torques acting on a non-spherical solar tachocline.
Figure 5 from the paper shows the 10Be record, shown as modulation potential, and planetary torque in the frequency domain:
Panel a is the Fourier spectrum of the solar activity quantified by the solar modulation potential. Panel b is the Fourier spectrum of the annually averaged torque modulus. The spectra display significant peaks with very similar periodicities: The 88 year Gleissberg and the 208 year de Vries cycles are the most prominent, but periodicities around 104 years, 150 years, and 506 years are also seen.
The match between theory and the physical evidence is very, very good. As the authors put it,”there is highly statistically significant evidence for a causal relationship between the power spectra of the planetary torque on the Sun and the observed magnetic activity at the solar surface as derived from cosmogenic radionuclides.”
They also advance a plausible mechanism which is that the tachocline, playing a key role in the solar dynamo process, is a layer of strong shear which coincides more or less with the layer of overshooting convection at the bottom of the convection zone. The overshoot layer is thought to be crucial for the storage and amplification of the magnetic flux tubes that eventually erupt at the solar photosphere to form active regions. Small variations in the stratification of the overshoot zone “of about -10-4 may decide whether a flux tube becomes unstable at 2·10-4 G or at 10-5 G. This makes a great difference, because flux tubes that do not reach a strength close to 10-5 G before entering the convection zone cannot reach the solar surface as a coherent structure and therefore cannot form sunspots.” This sounds like an explanation for the Livingstone and Penn effect of fading sunspots.
Figure A.1 from the paper also shows the very good correlation between cosmogenic radionuclides from the period 300-9400 years BP and the model output:
If planetary torque modulates solar activity, does solar activity in turn modulate the earth’s climate? Let’s have a look at what the 10Be record is telling us. This is the Dye 3 record from Greenland:
All the cold periods of the last six hundred years are associated with spikes in 10Be and thus low solar activity. What is also telling is that the break-over to the Modern Warm Period is associated with much lower radionuclide levels. There is a solar mechanism that explains the warming of the 20th Century. It is also seen in the Central England Temperature record as shown in the following figure:
This paper is a major advance in our understanding of how solar activity is modulated and in turn its effect on the earth’s climate. It can be expected that planetary torque will progress to being useful as a tool for climate prediction – for several hundred years ahead.
J.A. Abreu, J. Beer, A. Ferriz-Mas, K.G. McCracken, and F. Steinhilber, Is there a planetary influence on solar activity?” Astronomy and Astrophysics, October 22, 2012
Thanks to Geoff Sharp, the full paper can be downloaded from here.