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Monday, November 15, 1999 Published at 11:00 GMT


An active Sun

The Sun as seen on Thursday by the Big Bear Solar Observatory, US

By BBC News Online Science Editor Dr David Whitehouse

As it approaches the peak of its 11-year cycle of activity, the Sun is putting on a magnificent display of sunspots on its surface. The maximum activity is expected in the middle of next year.

Observers say that the current series of sunspots is particularly fine.

[ image: Close-up on a sunspot group]
Close-up on a sunspot group
Each spot is about the size of the Earth. They appear dark only by contrast. The normal temperature on the Sun's surface is 6,000 degrees Celsius. A sunspot is at a temperature of about 3,000 degC. If it were on its own it would be brighter than an arc lamp.

Sunspots are caused when magnetic fields, coiled in the shape of a tube, which emerge from below the Sun's visible surface.

As the tube grows and moves in the solar atmosphere it becomes filled with super hot gas that radiates X-rays.

The result is that images taken from the Yokoh X-ray telescope in Earth orbit show loops of glowing gas.

A similar thing is seen from the SOHO satellite which is positioned a million miles nearer the Sun than the Earth.

[ image: Yokoh sees gas-filled magnetic loops]
Yokoh sees gas-filled magnetic loops
Solar astronomers are keeping a careful watch on these active regions as, at any time, the magnetic field tubes may collapse liberating immense energy.

When that happens vast amounts of super hot material called plasma will be explosively heated and be expelled into space along with a burst of X-rays. Astronomers call these events solar flares.

Last week a cloud of hot gas, intertwined with a magnetic field, was ejected from the Sun and took three days to reach us.

As it interacted with the Earth's magnetic field it caused moderate geomagnetic disturbances and aurorae at the poles.

[ image: The SOHO satellite's view]
The SOHO satellite's view
Scientists expect that such disturbances and displays will increase in frequency and intensity in the near future as the Sun approaches maximum activity.

Solar storms may affect electricity supplies for countries in high latitudes but they may also affect communication satellites causing mobile phone and pager problems.

Some believe that solar interference may be more of a problem than the Y2K computer problem.

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