The polar vortex may be gone, but another extreme event is coming. Space weather experts say northern states may be able to see the aurora borealis, or northern lights, Thursday and Friday morning.
"We're hoping," said Joe Kunches with the National Oceanic and Atmospheric Administration's Space Weather Prediction Center in Boulder, Colo. "The sun has done its part, it produced a fast, energetic eruption about the middle of the day Tuesday."
That eruption blew off a piece of the solar atmosphere that is winging its way toward Earth, said Jeffrey Newmark, a solar physicist with NASA. It will take about two days to reach us.
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.
A solar storm is 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," Newmark said.
First there is a solar flare, a release of light and high-energy particles. The light reaches the Earth in eight minutes, the high-energy particles about an hour later, Newmark said.
At the same time, there is a "coronal mass ejection," sometimes called a solar storm. It 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 difficult.
NASA's Advanced Composition Explorer satellite sits about 1 million miles away from the Earth, picking 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.
It takes the Earth up to 36 hours to pass through the cloud, so it is possible there may be lights Wednesday night going into Thursday morning, said Mark Paquette with AccuWeather in State College, Pa. The clearest skies and best viewing will be in the Northeast and the Great Lakes region, he said.
It's more likely the lights, should they appear, will be visible Thursday night going into Friday morning, he said.
The energetic particles can affect spacecraft, communication and power grids, GPS satellites and even planes flying near the poles.
Because aircraft can't be guaranteed undisturbed communication as they fly at high altitudes, air-traffic control might push them down a little more toward the equator, Kunches said.
Astronauts at the International Space Station "will be told to stay indoors," Newmark 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, and the flare came from a region of the sun's surface directly facing the Earth.
"In July of 2012, there was a much larger event, but it was on the back side of the sun facing away from us, so we barely felt it," he said.