A mighty burp of hot, roiling plasma from the sun slammed into Earth's magnetic field Tuesday afternoon.
For fans of the night sky, the big question is: What kind of show will the collision put on, as charged particles hit the upper atmosphere and turn its nitrogen and oxygen atoms into glowing, sweeping curtains of red, green, and blue northern lights and southern lights?
The southernmost points in the United States where some form of northern lights may appear lie along the US-Canadian border – from northwestern Montana through northern Minnesota, Wisconsin, and Michigan's Upper Peninsula to the northern tip of Maine.
In the parlance of space-weather forecasters at the National Oceanic and Atmospheric Administration's Space Weather Prediction Center in Boulder, Colo., this is likely to be a G-1 (out of 5) solar storm – instead of a gee-whiz event for anyone south of Bemidji, Minn.
While that may be discouraging news for the aurora-starved, for commercial satellite owners and people who have to run electric transmission grids it's relatively good news. Geomagnetic storms can wreak havoc with electrically sensitive segments of today's high-tech infrastructure.
"There may be some temporary communications blackouts in the polar regions. If you're a GPS user needing centimeter accuracy, you'll feel this. If you're measuring currents on the power grid, you'll definitely see some changes in them. But only those intimately familiar with space weather are going to notice any effects," says Douglas Biesecker, a physicist at the Space Weather Prediction Center.
Yet there is little question that the mass of hot, ionized gas ejected by the sun is a big one.
"We haven't seen one of these in a long time," which explains the heightened interest in it, he says.
Indeed, images from NASA's Solar Dynamics Observatory satellite, launched last February, show an eruption known as a coronal-mass ejection (CME) that solar physicist John Raymond calls "truly spectacular."
"This took place over a huge fraction of the area of the sun," says Dr. Raymond, a researcher at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. The eruption began in one region, "but then it apparently disturbed regions fairly far away so that large ribbons of cool gas lifted off" from the star.
It was one of four eruptions that occurred Sunday. Space-weather forecasters also are keeping a close eye on the plasma speeding earthward from one of these other outbursts.
Several factors determine how intense a geomagnetic storm will be once a CME arrives, researchers say. They include the plasma's speed, its density (on average, perhaps one charged particle for every 0.06 cubic inches), whether Earth takes a direct hit from the densest region of the plasma, and the orientation of the plasma's magnetic field.
This ejection certainly had the speed at launch, Dr. Biesecker notes. Scientists clocked it at a hair less than 2.7 million miles an hour. So far, however, the plasma's speed near Earth has slowed to a more sedate 1.3 million miles an hour.
NOAA is testing a new space-weather forecast model that indicated Earth wouldn't take a direct hit from the densest part of the cloud, but instead would find itself enveloped roughly halfway between the densest region and the cloud's edge as the plasma sped past.
That may help account for the low ranking forecasters are giving this encounter.
Magnetic fields also matter.
Earth's field is not uniform around the planet, but somewhat flattened on the sunward half and stretched into a cometlike tail on the night half. As the plasma strikes Earth's magnetic field, it squashes and stretches Earth's field more than usual.
If the plasma's magnetic field is oriented in a way that offers the same average polarity to Earth as the planet's own field, it's like an express train passing a local station: The plasma's field races past Earth's magnetotail with little effect on the locals.
But if the polarities are opposite and attract, the plasma's field couples to the magnetotail and stretches it until it can stretch no farther. It snaps back toward Earth. The energy released in that snap accelerates charged particles already in the magnetotail, hurtling them on their collision course with Earth's upper atmosphere at the poles, where Earth's fields originate.
The more energy a snap releases, the stronger the storm and the lower the latitudes at which people can see auroral displays.
However, with today's orbiting sensors, the orientation of the plasma's field becomes clear only after it's passed and scientists have had a chance to study the data.