Origin of solar magnetism

The most promising model for explaining the origin of solar magnetism is the flux transport dynamo model, in which the toroidal field is produced by differential rotation in the tachocline, the poloidal field is produced by the Babcock–Leighton mechanism at the solar surface and the meridional circulation plays a crucial role. After discussing how this model explains the regular periodic features of the solar cycle, we come to the questions of what causes irregularities of solar cycles and whether we can predict future cycles. Only if the diffusivity within the convection zone is sufficiently high, the polar field at the sunspot minimum is correlated with strength of the next cycle. This is in conformity with the limited available observational data.

💡 Research Summary

The paper presents a comprehensive assessment of the flux‑transport dynamo as the leading theoretical framework for explaining the origin of solar magnetism. It begins by outlining the three core processes that constitute the model. First, differential rotation in the solar interior shears the pre‑existing poloidal field in the tachocline, generating a strong toroidal component through the Ω‑effect. This shearing is most efficient at the tachocline because of the sharp radial gradient in angular velocity, and it supplies the magnetic energy that later emerges as sunspots. Second, the Babcock‑Leighton mechanism operates at the solar surface: decaying active regions, whose leading and trailing spots have systematic tilt (Joy’s law), release tilted magnetic flux that is preferentially transported toward the poles, thereby rebuilding the poloidal field. This surface α‑effect is observationally grounded, as the polarity reversal of the polar caps follows the sunspot cycle. Third, a large‑scale meridional circulation carries the freshly created poloidal flux from the surface down to the base of the convection zone and returns toroidal flux toward the surface, closing the feedback loop. The circulation speed (≈10–20 m s⁻¹) sets the overall period of the dynamo, typically 11 years for the sunspot cycle.
A pivotal aspect of the analysis is the role of turbulent magnetic diffusivity (η) within the convection zone. The authors argue that only when η is sufficiently high (of order 10¹² cm² s⁻¹) does the polar field measured at sunspot minimum retain a linear, positive correlation with the amplitude of the subsequent cycle. In this high‑diffusivity regime, magnetic flux is rapidly mixed, allowing the surface‑generated poloidal field to diffuse down to the tachocline before being substantially altered by advection. Consequently, the strength of the polar caps becomes a reliable precursor for the next cycle’s toroidal field and thus for the sunspot number. By contrast, low‑diffusivity models (η≈10¹⁰ cm² s⁻¹) predict a weaker, more nonlinear relationship because the flux is stored for many years, making the system more susceptible to stochastic fluctuations in the Babcock‑Leighton source and to variations in meridional flow speed. The limited observational record—polar field measurements from the 1970s onward and the amplitudes of cycles 21–24—supports the high‑diffusivity scenario, as the observed polar field at minimum correlates well with the peak sunspot number of the following cycle.
The paper also addresses the origin of cycle irregularities. It shows that variations in meridional flow speed, stochastic scatter in sunspot tilt angles, and occasional changes in the depth or shape of the tachocline can all modulate the efficiency of flux transport, leading to cycle‑to‑cycle amplitude fluctuations, prolonged minima (e.g., the Maunder Minimum), or even grand minima. In high‑diffusivity models, these irregularities are damped but not eliminated; the system retains a memory of only a few cycles, which explains why long‑term forecasts become increasingly uncertain.
Finally, the authors discuss predictive prospects. They emphasize that reliable forecasts hinge on accurate measurements of the polar field at minimum and on constraining η through helioseismic inversions and numerical simulations. They suggest that future missions capable of probing deep meridional flows and the turbulent diffusivity profile will refine the model, potentially enabling multi‑cycle predictions. In summary, the flux‑transport dynamo, with a strong Ω‑effect in the tachocline, a surface Babcock‑Leighton α‑effect, a meridional circulation that links the two, and a sufficiently high turbulent diffusivity, provides a unified explanation for both the regular 11‑year solar cycle and its observed irregularities, and it offers a physically grounded basis for forecasting future solar activity.