The sun contains plasma, gas that has been superheated to the point that electrons are stripped from the nuclei of their atoms. The constant churning of that plasma on the solar surface generates sound waves, which travel toward the interior of the sun before they are bent back toward the surface. It usually takes an hour for sound waves to bounce from one point at the surface to the next, traveling roughly 60,000 to 125,000 miles in the process.
But a pocket of high magnetic activity deep within the sun, destined to rise to the surface and become a sunspot, is more buoyant than the churning plasma around it. So it floats toward the surface faster — and causes sound waves to move faster too. The stronger the sunspot, the more buoyant it is and the faster it rises to the surface.
A team of Stanford University researchers tracked sound-generated activity at different points on the sun's surface and found that sound waves that would normally take an hour to cross from one point to the next traveled 12 to 16 seconds faster when a sun spot was emerging — a surprise, since the researchers expected to see perhaps only one second or so shaved off.
By monitoring sound waves about 37,000 miles below the solar surface, the physicists said, they can predict the emergence of a sunspot one to two days before it appears, depending on how large it is.
"It's very exciting that we can detect them before they become visible," said lead author Stathis Ilonidis, a graduate student studying solar physics at Stanford University. But he added that more data would be needed to show that their results hadn't turned up false positives.