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Last Updated: Tuesday, 28 October, 2003, 14:27 GMT
Our Sun's regular mood swings
By Dr David Whitehouse
BBC News Online science editor

Huge sunspots erupting on the face of the Sun, millions of tonnes of superheated gas streaming across space, turbulence in the Earth's magnetic field, threats to power systems and satellites.

Our Sun's predictable instability
Our Sun's predictable instability
Sounds dramatic - but in reality it is one of the Sun's regular reminders that it is not an unwavering ball of light but has moods of its own.

Despite some uninformed commentaries, current events on the Sun's surface are neither unexpected nor unusual.

In reality, there is nothing particularly peculiar about the Sun's present spate of activity. We have seen it all before.

Long history

Sunspots have been seen since antiquity and have been studied in detail since the invention of the telescope in the early 17th Century. (Never look at the Sun through a telescope or with the naked eye)

The 11-year coming and going of sunspots was recognised in the 19th Century. Shortly afterwards, a similar pattern was seen in geomagnetic activity - magnetic storms and aurorae.

Magnetic gas clouds from the Sun strike the Earth's environment and, if the conditions are right, cause a flutter in the Earth's magnetic sheath.

We now know that the solar cycle is caused by the interplay of rotation and convection.

The outer layer of the Sun is bubbling; the inner layers spin faster than the surface. The result is a magnetic dynamo and sunspots are where huge magnetic coils of energy break surface.

Flares are caused by instabilities in the magnetic field. Some times the field can collapse abruptly causing a sudden release of pent-up energy.

Little Ice Age

Looking back in history there seems to have been a prolonged period between 1645 and 1715 when there were very few sunspots to be seen.

It appears to be a real effect - not just because astronomers were not looking - something did happen to the Sun in the 17th Century.

Soho image
Crossing the void to the Earth
Curiously, this lack of spots coincided with a lengthy cold spell on Earth - the so-called Little Ice Age. This shows the link between the Sun and climate.

But large flares and the concomitant geomagnetic activity do not always follow the 11-year cycle of activity.

Intense auroral and geomagnetic storms can occur at anytime during the solar cycle, although there is a slight preference for the years of declining activity.

Anywhere, anytime

For example, the most severe geomagnetic storm ever recorded was on 17 September 1941, about 53 months after the Sun's peak in its activity cycle - closer to solar minimum than maximum.

Another dramatic one took place in 1989. The magnetic field fluctuations induced currents in Canada's power grid causing a shutdown. This was quite early in the Sun's cycle.

A particularly dramatic and intense flare took place in August 1972 in between the Apollo 16 and 17 manned missions to the Moon - late in the solar activity cycle.

If it had it occurred when astronauts were in space, beyond the protective cocoon of the Earth's radiation belts, they could have been severely injured or possibly killed.

It is estimated that such extreme radiation conditions occur for just two days every 20 years. Even so, if astronauts are to ever venture into deep space, they must be prepared for them.

According to the latest estimates of that uncertain science - predicting future solar activity - the next solar minimum should be in 2006 rising to a maximum in 2010.

As for the flares, they could happen anytime.




SEE ALSO:
Angry sunspots continue to rumble
27 Oct 03  |  Science/Nature
Earth put on solar storm alert
24 Oct 03  |  Science/Nature
Sun unleashes the big one
04 Apr 01  |  Science/Nature


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