By Dr David Whitehouse
BBC News Online science editor
Astronomers have discovered a key fact required to understand the Sun's 11-year cycle of activity.
A large flare glares from the Sun's limb...
Sunspots and flares on the Sun's surface follow the cycle, but expelled gas clouds do not.
It seems that these ejections trail the sunspot peak - they peaked in 2002, two years after sunspots.
The expelled gas takes away the Sun's old magnetic skin allowing a new one to emerge to start a new cycle.
Rotation and convection
The Sun's 11-year cycle of activity - as recognised by the coming and going of sunspots - has been known since 1843, when Heinrich Schwabe, a German astronomer, noticed the pattern.
Years later the activity was recognised as being of magnetic origin by George Ellery Hale, the US astronomer, who, in 1908, saw that sunspots were intensely magnetic.
Since then many theories have been put forward to explain the solar rhythm.
...and a gas cloud starts on its way
The accepted theory is that the sunspot cycle is a consequence of rotation and convection inside the Sun.
The fact that the Sun's outer layers are bubbling, and that the Sun rotates faster at the equator than the poles, and faster on the inside than on the surface, results in a solar dynamo that, over 11 years, becomes increasingly wound up.
So at some stage during the magnetic cycle the Sun has to somehow shed its old, contorted magnetic skin, and allow a newer, less troubled one, to emerge.
'Like a snake'
The Soho (Solar Heliospheric Observatory) satellite may have obtained evidence about how the Sun does it.
Eight years of observing gas eruptions - coronal mass ejections (CMEs) - show that they are removing the Sun's old magnetic field bit by bit, first from one pole and the equator, and then the other pole.
"The Sun is like a snake that sheds its skin," says Nat Gopalswamy of Nasa's Goddard Space Flight Center, author of a report in the Astrophysical Journal.
"In this case, it's a magnetic skin. The process is long, drawn-out and it's pretty violent.
"More than a thousand coronal mass ejections, each carrying billions of tonnes of gas from the polar regions, are needed to clear the old magnetism away. But when it's all over the Sun's magnetic stripes are running in the opposite direction."
"This analysis of nearly eight years of CME data is a big step forward in making sense of space weather," says Joseph Gurman, Nasa project scientist for Soho.
"By identifying the solar origin of these events with CMEs of different speeds and appearances, and at different latitudes, it improves our capability to predict space weather that can affect the Earth, at different phases of the solar activity cycle."