Night skies across the U.S. danced with streaks of green and magenta this past week. People as far south as Asheville, N.C., and Phoenix, Ariz., had the unexpected pleasure of witnessing the Northern Lights.
“My Twitter feed was inundated. Every femtosecond, a new picture appeared,” says Scott McIntosh, a solar physicist and deputy director of the National Center for Atmospheric Research. “It was pretty special.”
The driving force behind the unusual light show was a severe geomagnetic storm that caught space weather forecasters by surprise. Rated at level 4 on a five-point scale, it was “the biggest storm in many years,” says Elizabeth MacDonald, a space physicist at NASA’s Goddard Space Flight Center. In 2017 the most recent storm of a similar magnitude caused auroras as far south as Arkansas.
“I think that the magnitude of what happened last week was a bit of a surprise—which tells you a lot about our actual understanding of what’s going on,” McIntosh says.
Auroras appear in the sky when charged particles ejected from the sun interact with Earth’s magnetic field. This field envelopes our planet in a protective bubble, deflecting the slow-moving particles that normally stream off the sun’s surface. But eruptions, flares and “holes” on the sun’s surface can all unleash fast-moving particles that rattle our magnetic shield. When this happens, electrons can surf the field straight down to Earth’s magnetic poles, says Jim Schroeder, a plasma physicist who studies auroras at Wheaton College in Illinois.
These charged particles then “bounce around like a pinball, plunging deeper and deeper into the atmosphere,” Schroeder explains. Inside the atmosphere, they collide with and excite atoms of nitrogen and oxygen. When these excited atoms eventually return to their normal ground state, the excess energy from the collision shoots off in the form of vibrant light. The color of the light ranges from vivid green to deep magenta, depending on the elements involved and the speed of the initial collision.
Normally, the lights occur within the “auroral ovals” that encircle the Arctic and Antarctic. But during severe storms, these particles can travel farther from the poles, expanding the auroral oval to encompass more of the globe. On Halloween in 2003 the ghostly lights appeared as far south as Florida. In September 1859, during the largest geomagnetic storm ever recorded, auroras lit up the Caribbean.
This past week, just as the Northern Lights stretched southward, the Southern Lights also stretched farther north. People in Tasmania—at a Southern Hemisphere latitude comparable to northern California in the Northern Hemisphere—spotted the auroras as well.
Farther away from the poles, the auroras might appear more magenta than green, MacDonald explains. This may be because auroras tend to appear red if they’re higher in the atmosphere and green if they’re closer to the ground. People in southern U.S. states such as Arizona would have viewed the lights by looking north, with the low-altitude green hidden behind the horizon thanks to the curvature of Earth. This would leave only an eerie haze of magenta.
MacDonald strongly encourages anyone who witnessed one of the auroras to report that observation to Aurorasaurus, a citizen science project she helps run. The goal is to use sightings gathered from the public to get better at predicting when and where the elusive phenomena will appear.
Scientists track auroras and geomagnetic storms carefully—and not just for the benefit of aurora chasers. Strong storms can cause serious problems on the ground such as interference with power grids and pipelines. “We have some satellites that watch the sun. But they can’t always see everything that’s coming at us,” MacDonald says. Usually, officials can tell a storm is coming a few days in advance. “In this case, it was not even as good as that. The forecast was trickier,” she says.
With these “stealthy” solar events, “there’s a kind of meager-looking eruption and then, all of a sudden, wham!” McIntosh says. His team is working to develop a technique that is based on so-called Doppler telescopes and that can detect when charged particles are headed toward Earth. In the future, this could give scientists an earlier heads-up on stealthy storms.
The main cause of the storm was a “fairly impressive” eruption on the surface of the sun called a coronal mass ejection, says Bill Murtagh, program coordinator at the National Oceanic and Atmospheric Administration’s Space Weather Prediction Center. These eruptions belch out giant loops of plasma. The recent coronal mass ejection’s effect may also have been amplified by another solar phenomenon observed last week, called a coronal hole, says W. Dean Pesnell, an astrophysicist on NASA’s Solar Dynamics Observatory mission. These holes beam high-speed solar winds out into space.
When all of the charged particles from these events collided with Earth’s magnetic field, they had “more of an impact than we expected,” Murtagh says. It was a “perfect coupling” of the magnetic field of the particles and the magnetic field of Earth—and unfortunately, he says, such chance couplings are “impossible to predict.”
The time of year may have played a role. Around the spring and fall equinoxes, the tilt of Earth’s magnetic field couples particularly well with these charged particles from the sun, Pesnell says. This recent storm happened only three days after the spring equinox.
“It really was a confluence of everything falling into place at the right time,” says Bob Leamon, a solar physicist at NASA’s Goddard Space Flight Center.
Currently the sun is a hotbed of activity—and it will only get more turbulent as it approaches the peak of its 11-year cycle. That point, called solar maximum, will likely occur in 2025, when the sun’s magnetic field flips. The most intense activity will occur during and after this peak in the sun’s cycle. “However impressive this was last week,” Leamon says, “we’re still on the way up to solar maximum.”