How solar Rossby hills would appear if their relative height is magnified by a factor of about one million.

Close-up videos of the Sun reveal a panorama of constantly bubbling, turbulent gas. Yet this scene of perpetual motion is not the whole story. Scientists using the Michelson Doppler Imager (MDI) experiment on the ESA-NASA SOHO spacecraft have discovered that the surface of the Sun is covered with long-lasting depressions and humps that are very similar to features on the Earth's oceans.

According to a report in the journal Nature, analysis of MDI data has produced the most sensitive measurements ever made of our nearest star's shape. The authors conclude that the Sun is covered with "hills", each about 100 metres (330 ft) high and 90,000 km (56,250 mls) apart. If the Sun was scaled down to the size of the Earth, these features would appear as small bumps only a few centimetres in height.

The undulations are caused by a phenomenon called Rossby waves, which produce a 'grid' pattern of weak cyclones that generate the hills and valleys on the Sun's visible surface. To detect these hills the MDI experiment measured the changing shape of the solar limb over almost a three year period as the Sun's rotation carried the Rossby waves around it.

"This new understanding of the solar "mountains" which cover its surface will help us solve some longstanding mysteries, like why the Sun rotates more slowly at the poles than at its equator," said Jeffrey Kuhn from the University of Hawaii Institute for Astronomy. "Rossby waves are a new and sensitive probe of the Sun's peculiar interior rotation. We can use their measured properties much like how we have learned about the Earth's interior when we study the vibrations caused by earthquakes."