The biggest sunspot cluster seen in at least 10 years has developed on the upper right quarter of the side of the sun visible from Earth, according to satellite readings.
Friday night, the light from solar flares was reported near cities including Palm Springs and Sacramento, Calif.; Flagstaff, Ariz.; and Albuquerque and Carlsbad, N.M.
"It has totally lit up the sky. We've had dozens and dozens of calls. People want to know what it is," said Bill Seigel, a producer at radio station KESQ in Palm Desert, 115 miles east of Los Angeles. "Some people thought it was UFOs."
Just north of Albuquerque, David MacKel was making the rounds at his security job when he saw the lights. He noted it on his report at 11:23 p.m.
"It was blood red. That's all I can say. It was kind of opaque and you could see the stars through it," MacKel said. He said he had seen the Northern Lights while in Alaska, but "the Northern Light move, this was more gaseous. It kind of got me freaked out."
Eddy County, N.M., Deputy Danny Gonzales described it as a purple haze. "It was very distinct in color," he said. "I have never seen anything like it."
Anthony Watts, a meteorologist in Chico, Calif., about 170 miles north of San Francisco, said the glow from the coronal mass ejection was interesting, but posed no threat.
"There's no danger, however there is the likelihood that we'll have radio or television interruptions," Watts said.
The sunspot, which is a cooler, darker region on the sun's surface, is caused by a concentration of temporarily distorted magnetic fields. It spawns tremendous eruptions, or flares, into the sun's atmosphere, hurling clouds of electrified gas toward Earth.
The solar activity can produce an aurora in the night sky, typically over northern latitudes. The colorful, shimmering glow occurs when the energetic particles strike the Earth's upper atmosphere.
NASA scientists said a powerful flare that erupted Thursday rated a class X, the most potent category.
The eruptions triggered a powerful, but brief, blackout Friday on some high-frequency radio channels and low-frequency navigational signals, scientists said. They forecast at least a 30 percent chance of continuing disruptions through Sunday.
In addition to radio disruptions, the charged particles can bombard satellites and orbiting spacecraft and, in rare cases, damage industrial equipment on the ground, including power generators and pipelines.