A strong solar storm is expected to hit Earth shortly, and experts warn it could disrupt power grids, satellite navigation and plane routes.
The storm - the largest in five years - will unleash a torrent of charged particles between 06:00 GMT and 10:00 GMT, US weather specialists say.
They say it was triggered by a pair of massive solar flares earlier this week.
It means there is a good chance of seeing the northern lights at lower latitudes, if the skies are clear.
The effects will be most intense in polar regions, and aircraft may be advised to change their routings to avoid these areas.
In the UK, the best chance to see them will be on Thursday night, the British Geological Survey says.
"It's hitting us right in the nose," said Joseph Kunches, an expert at the US National Oceanic and Atmospheric Administration (Noaa).
He described the storm as the Sun's version of Super Tuesday - in a reference to the US Republican primaries and caucuses in 10 states.
"Space weather has gotten very interesting over the past 24 hours," Mr Kunches added.
The charged particles are expected to hit Earth at 4,000,000 mph (6,400,000 km/h), and Noaa predicts the storm will last until Friday morning.
Images of the Sun's region where the flares happened show a complex network of sunspots indicating a large amount of stored magnetic energy.
Other solar magnetic storms have been observed in recent decades.
One huge solar flare in 1972 cut off long-distance telephone communication in the US state of Illinois.