The northern lights didn't make a hoped for appearance Thursday morning but could appear Thursday night, a NASA expert says.
The aurora borealis isn't usually visible in the continental United States. However a solar storm on Tuesday may have been big enough to push the shimmering lights southward so that people in northern states can see them--if the local weather is clear.
The solar storm's original predicted arrival time turned out to be too early, said NASA's Joseph Minow, at the Marshall Space Flight Center in Huntsville, Ala.
The wave arrived at Earth at 3:10 p.m. ET Thursday, he said.
"The good news for North American observers is onset of the event is coinciding with sunset so there is a lot of time with night skies ahead of us," Minow said.
The northern lights appear when atoms in the Earth's high-altitude atmosphere collide with energetic charged particles from the sun. They usually appear as shimmering green waves of light in the nighttime sky in polar latitudes. Much more rarely, they can be red and even blue.
Solar storms occur when "the (sun's) magnetic field gets twisted up in a high-energy state and it relaxes, and that releases a tremendous amount of energy," said Jeffrey Newmark, a solar physicist with NASA.
The storms first release a solar flare, a burst of light and high-energy particles. The light reaches the Earth in eight minutes, the high-energy particles about an hour later, Newmark said.
There is also what scientists call a "coronal mass ejection" but which lay people call a solar storm. This cloud of matter contains billions of tons of energetic hydrogen and helium ions as well as protons and electrons ejected from the sun's surface.
Predicting exactly when the energy will arrive is never easy, said Joe Kunches with the National Oceanic and Atmospheric Administration's Space Weather Prediction Center in Boulder, Colo.
Astronomers can see the flare when it leaves the sun but it's invisible until it arrives here on Earth.
The only early warning available comes from NASA's Advanced Composition Explorer satellite, which sits about 1 million miles out. It picks up readings of solar bombardments.
Even at that distance, "as the wave passes the satellite, we have only half an hour's warnings before it gets here," Kunches said..
These kinds of solar storms are a normal part of the sun's activity. They ebb and flow on a roughly 11-year cycle, Newmark said. This is a moderate one.