Peeking Around the Sun's Edge express
The Sun is not a calm sphere. Waves constantly ripple through its interior, like a bell after being struck. These oscillations reach the surface and reveal what's happening on the far, invisible side. Astronomers have long been able to 'listen' to this ringing and find large sunspot groups there. But until now, they couldn't tell the key detail: which pole of a sunspot is north and which is south.
A new study proposes a way to fix that. Take a far-side map obtained from seismic waves (a helioseismic map). On it, an active region appears as a pair of spots — one bright, one dark. It turns out that by examining how the signal varies along east and west, you can separate these two components. Then Hale's law comes into play: in each solar activity cycle, the leading spot of a group has one polarity, and the trailing spot has the opposite. Knowing the current cycle, we assign the poles correctly.
This yields a magnetic map of the invisible side — almost like a regular one, but built from the trembling of the solar surface. It can be stitched together with a map of the visible side to get a complete portrait of the Sun's magnetic field. This is needed for solar wind models and space weather forecasts: if a storm is brewing on the far side, we'll know in advance, not when it emerges from behind the limb.
The solar bell continues to ring. Now its chime tells us not only where sunspots are, but also how they are magnetized.
Original abstract
as written by the authors arXiv:2603.02614
Understanding and monitoring solar active regions is essential for operational space-weather forecasting and improved solar dynamo modeling. This requires comprehensive 360-degree observations of the Sun. While space-weather forecasting has long relied successfully on high-quality observations of the Earth-facing hemisphere, a critical gap remains due to the lack of direct, continuous magnetic field measurements of far-side active regions, particularly magnetic field strength, polarity configurations, and related parameters. We present a methodology for inferring magnetic field distributions of active regions in helioseismic maps of the far hemisphere. The analysis focuses on identifying the magnetic polarities of opposing components of a helioseismic signature and applying stable, continuous polarity assignment to large-scale magnetic structures derived from such maps. These helioseismic signatures reliably resolve strong active regions, especially those that later appear as major rotation regions when they rotate into Earth view. Polarity boundaries are identified by analyzing the bimodal longitudinal variance profile of the seismic signal within each region, after which Hales law is applied to establish east-west ordering consistent with the solar cycle. The method produces polarity-resolved far-side magnetograms suitable for integration with near-side observations, enabling construction of full-Sun magnetic boundary conditions for coronal and solar wind modeling and providing a critical step toward improved heliospheric simulations and operational forecasting.
Hale's law has held for over a hundred years: in each 11-year cycle, the leading spots in groups in one hemisphere have the same polarity, and in the next cycle it flips to the opposite.