Over the past week, the night sky across the United States danced in green and magenta stripes. People in the far south, such as Asheville, North Carolina, and Phoenix, Arizona, have experienced the unexpected pleasure of witnessing the Northern Lights.
“My Twitter feed was inundated. Every femtosecond, there’s a new image,” 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 spooked space weather forecasters. Rated a level 4 out of 5, Elizabeth McDonald, an astrophysicist at NASA’s Goddard Space Flight Center, called it “the biggest storm in years.” In 2017, the latest storm of similar magnitude triggered auroras as far south as Arkansas.
“I was a little surprised by the scale of what happened last week. It says a lot about our actual understanding of what’s going on,” McIntosh says.
Auroras appear in the sky when charged particles emitted by the sun interact with the earth’s magnetic field. This field envelops our planet in a protective bubble that deflects slow-moving particles that normally flow from the surface of the Sun. But eruptions, flares, and “holes” in the Sun’s surface can all release fast-moving particles that rattle magnetic shields. When this happens, electrons can pass through the magnetic field and fall straight into the Earth’s magnetic poles, says Jim Schroeder, a plasma physicist who studies auroras at the University of Wheaton.
These charged particles then “bounce around like a pinball and go deep into the atmosphere,” explains Schroeder. It is excited by colliding with nitrogen and oxygen atoms in the atmosphere. When these excited atoms eventually return to their normal ground state, the extra energy from the collisions is released in the form of bright light. The color of the light varies from bright green to deep magenta, depending on the factors involved and the speed of the initial impact.
Light usually occurs within the “Aurora Oval”, which surrounds the North and South Poles. But during violent storms, these particles can travel far from the poles, expanding the auroral ellipse and enveloping more of the Earth. On Halloween 2003, ghostly lights appeared as far as South Florida. In September 1859, the Northern Lights illuminated the Caribbean Sea during the greatest geomagnetic storm ever recorded.
Just as the Northern Lights stretched south last week, so do the Southern Lights. People of Tasmania at latitudes in the southern hemisphere comparable to northern California in the northern hemispherefound the aurora likewise.
Farther away from the poles, the aurora may appear more magenta than green, McDonald explains. This is likely because auroras tend to appear red when they are higher in the atmosphere and green when they are closer to the ground. People in southern US states such as Arizona would have looked north to see the light because the curvature of the earth hides the low-altitude greenery behind the horizon. This leaves only an eerie haze of magenta.
MacDonald urges anyone who has witnessed one of the Northern Lights to report their observations to Aurorasaurus. This is a citizen science project she helps run. The goal is to more accurately predict when and where elusive phenomena will manifest themselves using sightings collected from members of the public.
Scientists closely track auroras and geomagnetic storms, but not just to track them. Strong storms can cause serious problems on the ground, including interference with power grids and pipelines. “There are satellites that observe the sun. But they don’t always see everything that comes to us,” McDonald says. Officials can usually tell when a storm is coming days in advance. “In this case, it wasn’t so good. The prediction was trickier,” she says.
These “stealth” solar events “have a kind of poor-looking eruption and then, all of a sudden, wham!” says McIntosh. His team is working on a technique based on so-called Doppler telescopes that can detect when charged particles are on their way to Earth. In the future, this could help scientists turn their attention to stealth storms sooner.
The main cause of the storm was a “pretty impressive” eruption on the sun’s surface called a coronal mass ejection, said Bill Murtagh, program coordinator for the National Oceanic and Atmospheric Administration’s Space Weather Prediction Center. These eruptions spew huge loops of plasma. The effects of the recent coronal mass ejection could also have been amplified by another solar phenomenon observed last week called coronal holes, said W. Deanpe, an astrophysicist on NASA’s Solar Dynamics Observatory mission. Snell said. These holes release high-velocity solar wind into space.
When all the charged particles from these events collided with the Earth’s magnetic field, they “had a bigger impact than we expected,” Murtagh says. A “perfect union,” he says, and, unfortunately, such fortuitous unions are “impossible to predict.”
Timing may have an effect. Around the vernal and autumnal equinoxes, the tilt of the Earth’s magnetic field couples particularly well with these charged particles from the Sun, Pesnell says. This recent storm occurred just three days after the vernal equinox.
Bob Leamon, a solar physicist at NASA’s Goddard Space Flight Center, said:
The Sun is currently a hotbed of activity that will become increasingly turbulent as it approaches the peak of its 11-year cycle. That point, called the Solar Maximum, will likely occur in 2025 when the Sun’s magnetic field reverses. The most intense activity occurs during and after this peak of the solar cycle. “Last week’s results were impressive, but we’re still nearing the solar maximum,” he says Leamon.
