Keith Johnson, associate director of the Fleischmann Planetarium at the University of Nevada, Reno, said he had never seen such a luminous display of the lights so far south.
Residents in Nevada, California, and Arizona witnessed a shimmering display of the red and green Northern Lights on Friday night following the eruption of solar flares.
Satellite readings show that the biggest sunspot cluster for at least 10 years has developed on the upper right quarter of the side of the Sun visible from Earth.
The colourful shimmering glow of the Northern Lights occurs when energetic particles resulting from the eruptions strike the Earth's upper atmosphere.
Colour and shape
Thousands of people in Nevada enjoyed what astronomers were describing as the best display of the Northern Lights in at least 20 years.
Keith Johnson, associate director of the Fleischmann Planetarium at the University of Nevada, Reno, said he had never seen such a luminous display of the lights so far south.
He said the skies had begun to glow red, and then light-green coloured rays had begun to appear as darkness fell on Friday.
He said: "It was sensational. You could see some actual colour, shape and structure to the displays.
"I saw large lumps of light, rays of light and sheets of light. I even saw some slow motion in them. The colours were obvious but not very vivid."
Mistaken for UFOs
Light from the solar flares was also reported near cities including Palm Springs and Sacramento in California, Flagstaff, Arizona, and Albuquerque and Carlsbad in New Mexico.
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.