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The Sun has unleashed its strongest flare in four years, observers say.

The eruption is a so-called X-flare, the strongest type; such flares can affect communications on Earth.

Nasa's Solar Dynamics Observatory (SDO) spacecraft of extreme ultraviolet radiation emanating from a sunspot.

The British Geological Survey (BGS) has issued a geomagnetic storm warning, and says from the northern UK.

The eruptions are expected to hit the Earth's magnetic field over the next couple of days, causing an increase in geomagnetic activity.

The monster flare was recorded at 0156 GMT on 15 February and directed at the Earth. According to the US space agency, the source of this activity - sunspot 1158 - is growing rapidly.

Solar flares are caused by the sudden release of magnetic energy stored in the Sun's atmosphere.

Preliminary data from the Stereo-B and Soho spacecraft suggest that the explosion produced a fast but not particularly bright coronal mass ejection (CME) - a burst of charged particles released into space.

The unpredictable activity on the Sun can interfere with modern technology on Earth, such as electrical power grids, communications systems and satellites - including the satellite navigation (or sat-nav) signals used on Earth.

On Wednesday, the BGS released a rarely seen that provide an insight into "space weather" stretching back to the Victorian era.

BGS scientists say that studying past solar activity could inform the prediction of future space weather and help mitigate threats to national infrastructure.

In 1972, a geomagnetic storm provoked by a solar flare knocked out long-distance telephone communication across the US state of Illinois.

And in 1989, another storm plunged six million people into darkness across the Canadian province of Quebec.

Displays of the Northern Lights (Aurora Borealis) have already been seen further south than usual in Northern Ireland and elsewhere in the UK. And further activity is expected over the next few days.

Researchers say the Sun has been awakening after a period of several years of low activity.

Paul.Rincon-INTERNET@bbc.co.uk