Early-warning system for sunspots

Sunspots can trigger major eruptive events on the solar surface, which can lead to widespread power outages on Earth as well as wreaking havoc on telecommunication and navigation systems. But researchers in the US say that they have developed a technique that could detect emerging sunspots a full day or two before they appear.

Although scientists have been observing and documenting sunspots for more than 400 years, the origin of these cool, dark regions of strong magnetism on the solar surface have remained largely mysterious. Theories suggest that they are formed from complex motions of hot plasma inside the Sun. The idea is that acoustic waves that form near the surface propagate deep inside the Sun before returning to the surface at a different location because of refraction.

Emerging storms

Now, Stathis Llonidis and colleagues at Stanford University have taken this theory as the basis for a technique to locate the emergence of sunspots within the interior of the Sun. Llonidis' team uses a specific helioseismology technique – called time–distance helioseismology – to analyse the time taken for these acoustic waves to propagate through the solar interior.

The technique involves selecting a pair of points on the solar surface separated by a specific distance between 100,000–200,000 km. Some of the acoustic waves excited near the location of one of these points will propagate 60,000 km into the Sun before returning to the surface near the location of the corresponding point. It usually takes about one hour for the acoustic waves to make this journey. However, if the waves pass through an emerging sunspot, then they speed up and the journey time is reduced slightly – for a large sunspot region this effect is about 12–16 s.