By BBC News Online Science Editor Dr David Whitehouse

A flare occurred on the surface of the Sun on Thursday, 17 February, that threw into space a vast cloud of superheated gas called a coronal mass ejection (CME) - and it appears to be headed for Earth!

When it reaches us over the weekend, it is likely to interact with the Earth's magnetic field, starting what scientists call a geomagnetic storm. One manifestation of this storm may be a display of Aurorae, or Northern or Southern lights.

On Thursday, what was described as a medium-sized solar flare erupted from a sunspot group near the middle of the solar disk. It was accompanied by a coronal mass ejection event that appears to be headed directly for us.

The two M-class solar flares that erupted in quick succession were unexpected as the sunspot groups they came from displayed very little activity prior to flaring. The second flare occurred near a small sunspot identified by astronomers as active region number 8872.

This eruption was accompanied by a coronal "halo event" when a CME appears to get larger and larger forming a halo around our Sun.

Satellite failures

CME's can carry up to 10 billion tonnes of plasma travelling at speeds as high as 2000 km/s. If they reach the Earth, they can cause geomagnetic storms, which have been linked to satellite failures.

In some extreme cases, geomagnetic storms can induce electric currents in the Earth and oceans that can damage electric power transmission equipment.

Radiation and particles from solar flares take eight minutes to arrive on Earth, but the slower-moving gas from a coronal mass ejection usually takes days to reach us.

The United States National Oceanic and Atmospheric Administration (NOAA) agency says that both flares were associated with disappearing solar filaments. These are loops of magnetic fields that contain hot gas. As the magnetic fields become unstable the hot gas is ejected into space.

As the Sun reaches it maximum period of activity later this year, CME's will become more common.