By Dr David Whitehouse
BBC News Online science editor
Imminent disruption is predicted for satellites, power systems and even mobile phones because of a solar storm.
One big sunspot and another on the way
It comes from one of the largest groups of sunspots seen for years. Several times in recent days superhot gas has erupted above them.
The events, called Coronal Mass Ejections, have sent 10 billion tonnes of superhot gas speeding towards Earth.
As well as communication blackouts, aurorae - polar lights - may be seen from mid-latitudes as the gas arrives.
A coronal mass ejection (CME) is an explosion of gas and charged particles into space from a solar flare in the corona, the outermost layer of the Sun's atmosphere. They are associated with sunspots.
The current large sunspot group is one of the largest for years. In area, it is 10 times larger than the surface of the Earth.
It has been captivating astronomers for days and has already produced several powerful solar flares - huge explosions on the Sun's surface.
The Sun has been active
One of its flares was designated as X-class - the most powerful category.
Astronomers say that the latest CME sent nearly 10 billion tonnes of matter toward Earth.
It is expected to reach Earth on Friday, and when it interacts with the planet's magnetic field it could create a significant geomagnetic storm.
Geomagnetic activity associated with CMEs can dramatically disrupt electrical and communications systems.
CMEs can create voltage surges in electric power grids, disrupt radio communications and navigation systems, and prevent normal satellite operations.
In 1997, such a storm shut down an AT&T Telstar 401 satellite that provided
television broadcasts. The following year another storm disrupted a
Galaxy IV satellite that supported automated cash machines and airline tracking systems.
Such storms are also known to affect mobile phone operations and may disrupt wireless internet services.
And there is more to come. Another sunspot group is rotating into view onto the solar disk, showing even more signs of activity.
That particular region caught the attention of solar physicists while it was still on the far side of the
Using a technique based on the velocity of sound waves through the Sun's outer layers astronomers have realised that a second sunspot cluster was on the Sun's far side. It could produce more geomagnetic storms in the next two weeks.