People from Montana to Missouri reported sightings of the aurora borealis overnight, and forecasters said the phenomenon also known as the northern lights would be visible over parts of the West and Midwest until about dawn on Tuesday.
The northern lights get their name for lighting up the sky at higher latitudes. On Monday night, the phenomenon stained the night sky with green and purple blotches in some parts of the United States, and red or purple pillars or curtains in others.
“Most of the time when the northern lights occur, it is just kind of a green glow on the northern horizon,” said Grant Hicks, a meteorologist at the National Weather Service office in Glasgow, Mont., where the staff was posting images of the northern lights overnight.
“Once you get to see the pillars and the curtains, and they start dancing and moving around, that can be more fun to watch,” Mr. Hicks said, speaking by phone early Tuesday morning.
Sometimes the northern lights are only visible with the help of a camera. But Tyler Schlitt, a part-time photographer who lives near St. Louis, said by phone that they were already “highly visible” to the naked eye in part of eastern Missouri on Monday night.
“Some people will mistake it as clouds or something,” Mr. Schlitt, 32, said from a field near the city of Elsberry, Mo. “But if you have a good aurora show, you’ll likely see the aurora.”
There were many other reports overnight of aurora sightings by photographers and social media users and across the American West and Midwest, as well as in parts of Canada and Britain.
The Space Weather Prediction Center, which is part of the National Oceanic and Atmospheric Administration, had said earlier that a “moderate” geomagnetic storm was expected in the northern United States late Monday into Tuesday. It said in a fresh warning early Tuesday morning that “strong” geomagnetic activity was occurring, meaning that the northern lights “may be seen as low as Pennsylvania to Iowa to Oregon.”
Space weather experts believe the geomagnetic storm was probably caused by an eruption on the sun late last week that produced a “coronal mass ejection,” the technical term for a large expulsion of plasma and magnetic field.
Space weather experts measure geomagnetic disturbances on a five-tier scale that forecasts minor, moderate, strong, severe and extreme effects on power grids, satellites and other things. The higher the level, the farther south the northern lights tend to be visible. In a severe or extreme disturbance, they can be seen as far south as Florida, said William Murtagh, the program coordinator at the Space Weather Prediction Center.
Mr. Murtagh said in an email after 2 a.m. Eastern time on Tuesday that while the worst of the latest disturbance was probably over, it could possibly continue producing aurora visible to the naked eye over some northern states.
A project manager at the center, Lt. Bryan R. Brasher, said that “unsettled to active levels of geomagnetic activity” would probably linger into Wednesday. But he said it was unlikely that the aurora would be visible as far south on Tuesday night as it had been late Monday.
Mr. Schlitt, the photographer in Missouri, said by phone around 11 p.m. on Monday that the red pillars he saw earlier in the evening had since faded. He had a 90-minute drive home and an early workday ahead, he said, so he planned to pack up soon — unless the colors on his camera screen were to suddenly intensify.
“If I see a whole bunch, I will stay out a little bit later,” he said.