Magnetic fields on the surface of the Sun appear as dark spots and bright faculae. But they are born deep inside, where flows of hot gas act like a giant dynamo. The question is whether sunspots on the surface can predict what will happen inside the star years from now.

The authors applied a mathematical theorem similar to the rule linking the area of a shape to its boundary. This theorem allowed them to connect the magnetic field on the surface to processes inside the Sun. It turns out that the surface becomes a predictor when its field is effectively dragged back inward by plasma flows. It's like foam on the ocean surface revealing deep currents.

The researchers tested five different models of the solar dynamo. In two types of models, the prediction works, but for different reasons. In one model, the surface field is carried back inside; in another, the surface field directly reflects the internal field. This finding helps choose the right model for forecasting solar activity.

Now scientists know which features to look for in models to predict the behavior of the Sun. It's like finding the right compass for a journey into the star's interior.