Theories of the Solar Cycle and Its Effect on Climate

Abstract

In the first part, some views of the solar magnetic cycle are discussed and confronted with observations, with focus on two aspects at the core of most models: the role of convective turbulence, and the role of the ‘tachocline’ at the base of the convection zone. The shorter second part discusses the possibility that the magnetic field of the Sun might influence its brightness enough to have an effect on the Earth’s climate. The standard view, which treats the solar cycle as a result of the interaction between turbulent convection and magnetic fields is shown to be misplaced. The main ingredient of the solar cycle, apart from differential rotation, is instead buoyant instability of the magnetic field itself. The source of the magnetic field of the solar cycle is usually assumed to be located in the ‘tachocline’: the shear zone at the base of the convection zone. The cycle cannot be powered by the radial shear of the tachocline as assumed in these models, however, since the radiative interior does not support significant shear stresses. Instead, it must be powered by the latitudinal gradient of the rotation rate in the convection zone, as in early models of the solar cycle. The Sun’s brightness is known to vary in sync with the sunspot cycle, but our understanding of the mechanisms involved make it unlikely that it has a significant effect on climate, whether on short (decades) or longer time scales.