Is the small-scale magnetic field correlated with the dynamo cycle?

TL;DR

This study uses 3D dynamo simulations to investigate how small-scale magnetic fields on the Sun relate to the solar cycle, revealing that their correlation depends on the presence of large-scale dynamos and the strength of the magnetic field.

Contribution

It demonstrates through simulations that small-scale magnetic fields can be positively correlated, uncorrelated, or anticorrelated with the solar cycle depending on dynamo conditions and field strength.

Findings

Small-scale fields are positively correlated with the cycle when only large-scale dynamo operates.

When both dynamos operate, small-scale fields are uncorrelated or anticorrelated depending on field strength.

Anticorrelation occurs when the large-scale field exceeds the turbulence's equipartition value.

Abstract

The small-scale magnetic field is ubiquitous at the solar surface---even at high latitudes. From observations we know that this field is uncorrelated (or perhaps even weakly anticorrelated) with the global sunspot cycle. Our aim is to explore the origin, and particularly the cycle dependence of such a phenomenon using three-dimensional dynamo simulations. We adopt a simple model of a turbulent dynamo in a shearing box driven by helically forced turbulence. Depending on the dynamo parameters, large-scale (global) and small-scale (local) dynamos can be excited independently in this model. Based on simulations in different parameter regimes, we find that, when only the large-scale dynamo is operating in the system, the small-scale magnetic field generated through shredding and tangling of the large-scale magnetic field is positively correlated with the global magnetic cycle. However, when both dynamos are operating, the small-scale field is produced from both the small-scale dynamo and the tangling of the large-scale field. In this situation, when the large-scale field is weaker than the equipartition value of the turbulence, the small-scale field is almost uncorrelated with the large-scale magnetic cycle. On the other hand, when the large-scale field is stronger than the equipartition value, we observe an anticorrelation between the small-scale field and the large-scale magnetic cycle. This anticorrelation can be interpreted as a suppression of the small-scale dynamo. Based on our studies we conclude that the observed small-scale magnetic field in the Sun is generated by the combined mechanisms of small-scale dynamo and tangling of the large-scale field.