Sun has strange 'spin cycle'

Detailed observations reveal gas layers

By BBC News Online science editor Dr David Whitehouse

Scientists have discovered that two parallel layers of gas deep beneath the Sun's surface are speeding up and slowing down in a co-ordinated way.

As our star rotates, one gas layer gradually spins faster while the other reduces speed. Solar scientists are at a loss to explain the phenomenon, which occurs in regular 12 to16-month cycles.

Rivers of gas move towards the solar equator

"It's not what we expected at all," says Stanford research physicist Jesper Schou. "It comes totally out of the blue."

Writing in the 31 March issue of the journal Science, Schou and colleagues say that the unusual but regular changes only occur above and below a section of the Sun known as the interface layer or tachocline.

This lies about 217,000 km (135,000 miles) below the solar surface. The tachocline separates the Sun's two major regions of gas: the radiative zone, which includes the energy-generating core, and the convection zone near the surface.

Mysterious cycle

Solar experts believe that the tachocline may be the source of powerful magnetic fields that produce strong solar flares and solar winds, and create sunspots that mysteriously appear and disappear during an 11-year cycle.

No-one knows how the Sun's enormous magnetic fields are generated, or why they reverse polarity from positive to negative every 11 years. But the discovery that the area surrounding the tachocline varies its rotation in a regular pattern could be a clue to solving the mystery.

The researchers used data from the global network of solar monitoring telescopes (Gong) as well as the Soho satellite positioned in a stable orbit 1.6 million km (a million miles) closer to the Sun than the Earth. The discovery that the inner Sun spins at different rates at different latitudes is consistent with earlier studies showing that the surface of the Sun also rotates at different speeds.

At the equator, it takes about 25 days for the surface of the Sun to rotate on its axis. But at the poles, surface rotation requires roughly 33 days.

Commenting on the discovery, Stanford Professor Philip Scherrer said: "It's pretty neat to look at that kind of detail inside a star. It's really fun!"