Bill Seigel, a producer at radio station KESQ in Palm Desert, east of Los Angeles, said some callers to the station had thought the lights were UFOs.

"It has totally lit up the sky," he said. "We've had dozens and dozens of calls. People want to know what it is."

In Eddy County, New Mexico, Deputy Danny Gonzales described the night sky as a purple haze.

He said: "It was very distinct in colour. I have never seen anything like it."

Meteorologist Anthony Watts, based in Chico, California, said the glow from the solar eruptions was interesting, but did not pose any danger.

However, some disruption was expected to telecommunications and radio and TV signals. On Friday, the eruptions triggered a brief blackout on some high-frequency radio channels and low-frequency navigational channels.

Clouds of gas

The sunspot group, which is a cooler, darker region on the Sun's surface, is caused by a concentration of temporarily distorted magnetic fields. This triggers tremendous eruptions into the Sun's atmosphere, hurling clouds of electrified gas toward Earth.

This particular sunspot group, designated Noaa 9393, is about 140,000 km (86,800 miles) in diameter, according to the National Science Foundation (NSF).

It is so big that it can be seen by unaided, providing filters are used to protect the eyes from damage, the NSF says.

Although very large by normal standards, Noaa 9393 falls some way short of the largest ever recorded. That title is held by a spot group which appeared in 1947. It was three times larger than Noaa 9393.