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Tuesday, August 3, 1999 Published at 17:16 GMT 18:16 UK


Solar flare alert

X-ray image of the Sun

By BBC News Online Science Editor Dr David Whitehouse

Spacecraft and ground-based telescopes monitoring the Sun detected a major "X-class" solar flare on Monday.

Astronomers say that while not spectacularly energetic, it is the most powerful event seen on the Sun's surface for over six months. They add that it has interrupted radio communications on Earth.

[ image: The flare came from this region]
The flare came from this region
The Sun is approaching the maximum of its 11-year cycle of behaviour and is expected to reach a peak next year.

Solar flares are gigantic explosions that take place above the Sun's surface due to the explosive release of magnetic energy. Magnetic fields suddenly collapse and flash-heat gas to millions of degrees. This then explodes in all directions with the release of high-energy radiation.

Astronomers rank solar flares into five categories according to the extent to which they emit X-rays. Class X is the most powerful, being tens times the intensity of M-class flares.

X-class solar flares are the rarest and the most powerful of all types of solar flares. They can occur at any time during the solar cycle, but are more common near the solar maximum, when sunspot regions are complex enough to generate the conditions required to produce such powerful solar explosions.

The last major X-class flare was observed on 28 November 1998.

On impulse

Scientists describe the event as impulsive, which means that the rise of the X-rays to the eventual maximum occurred very rapidly, within a few minutes. Despite being the strongest flare for many months' researchers say that only a modest amount of energy was released in terms of typical solar flare energy.

In human terms, however, the energy involved is staggering. It would take all of the power generating plants in the United States more than 3,000 years to produce the amount of energy that this major X-class solar flare released in just 20 minutes.

[ image: The X-ray spike was sharp]
The X-ray spike was sharp
The flare was associated with what is known as a Type II spectral radio burst or a sweep frequency event. These radio emissions are produced when a shock wave from the solar flare excites the Sun's hot but thin lower atmosphere, causing clouds of electrons to be ejected.

Such type II sweeps are often an indication of a mass being ejected from the Sun but data available about this current burst suggests that this solar flare probably was not associated with a mass ejection.

However the X-rays from this flare were intense enough to have a considerable impact on radio communications.

Reports suggest that in some cases the effect was strong enough to completely blackout all radio communications between points more than 3,000 to 4,000 km apart and up to frequencies as high as 10 MHz for a period of 15 to 30 minutes.

Trouble ahead?

Researchers say that the active region that produced this major flare may be capable of producing another X-class flare before the sunspot region rotates out of view behind the southwest limb of the Sun over the next four or five days.

They are also keeping a close eye on another active region on the Sun which is just crossing the Sun's central meridian.

That region is larger than the one that spawned the recent major flare and may also be capable of producing a major M or X-class flare before it rotates out of view behind the northeast limb over the next seven days.

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