SOLARNET Conference: The Many Scales of the Magnetic Sun

Solar dynamo models can explain the production and maintenence of the solar magnetic fields on two different spatial scales: the large-scale solar dynamo, manifested in the cycle-dependent appearance of solar active regions, and the small-scale solar dynamo, driven by the turbulent convective motions close to the solar surface. Disentangling these two mechanisms observationally is extremely challenging, but provides important input for the dynamo models. Highly stable long-term observations with sufficient magnetic sensitivity are required.

The Helioseismic Imager on board the Solar Dynamics Observatory (SDO/HMI) has measured the solar magnetic field for more than one solar cycle. In ths work, we present the results of these 13-year long magnetic field measurements in the most quiet patches of the solar surface after applying a combination of a temporal and spatial averaging algorithm and a careful selection process. The results provide strong evidence for a solar-cycle modulation in even the most quiet patches when they include network fields. In contrast, the modulation is absent in the internetwork fields, where only the interior of the supergranular cells is included in the analysis.

We interpret the absence of a solar-cycle modulation in the internetork fields as a consequence of them being generated by the small-scale (or fluctuating) dynamo acting on scales smaller than a supergranular cell. The solar cycle modulation when including the network fields can be explained by tangling of the solar-cycle dependent large-scale sub-surface field.