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.
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.