This "coronal mass ejection" reached Earth almost 12 hours earlier than top space forecasters predicted.
Forecasting solar storms is especially difficult right now, even as the sun is getting more active.
A giant explosion lashed out from the sun this week, sending a flood of solar material careening toward Earth faster than forecasters realized.
It started with a giant loop of plasma arcing from the sun on Saturday. The snakey filament grew, oozing further and further from the sun's surface, until it accelerated away and burst into space.
Keith Strong, a solar physicist who has worked for Lockheed Martin and NASA, shared footage of the eruption on X, the platform formerly known as Twitter.
"THE BIGGEST ERUPTION I HAVE EVER SEEN!" Strong wrote. "Note it covers over half the sun."
—Keith Strong (@drkstrong) September 17, 2023
NASA's Solar Dynamics Observatory also captured the event. In the video clip below, which is a time-lapse covering several hours, you can see the plasma arc gathering, then erupting, in the center-right of the sun's disc.
Because it occurs in the sun's outer atmosphere, the corona, this type of eruption is called a coronal mass ejection (CME). CMEs fling charged, super-hot plasma into space, and sometimes — like in the case of this CME — that plasma strikes Earth.
Forecasters at the Space Weather Prediction Center (SWPC), a branch of the National Oceanic and Atmospheric Administration, announced that the CME would trigger a geomagnetic storm — a powerful disturbance in the planet's magnetic field, which can disrupt radio communications, drag satellites out of orbit, and even occasionally disable power grids.
On the bright side, these solar storms also make stunning displays of the Northern Lights, or aurora borealis, visible in the middle of the US.
SWPC forecasters calculated that the CME from the giant filament eruption would reach Earth on Monday evening.
But around 9 a.m. Eastern Time, NOAA sensors in space captured a sudden, dramatic shift in Earth's geomagnetic field. The CME had already arrived, nearly 12 hours earlier than forecasted.
Why was the space-weather forecast so wrong?
To forecast the arrival time and intensity of solar storms, NOAA relies on spacecraft to act as "buoys" to gather data on the stream of particles flowing from the sun toward Earth. Right now, though, only one of those buoys is in a helpful position.
"We only have a single viewpoint of the sun for the next year or so. It's like playing tennis with one eye closed — we have poor depth perception," Matt Owens, a professor of space physics at the University of Reading, told Insider in an email.
"Making precise timing predictions is difficult due to the complex nature of space weather, along with a limited number of space-based sensors," Bryan Brasher, a spokesperson for SWPC, told Insider via email.
The other piece of the solar-storm forecast is the strength of the storm once it arrives. That's "very difficult" to predict, according to Daniel Verscharen, an associate professor of space and climate physics at University College London.
"This always depends on the direction of the magnetic field in the plasma cloud. If it is pointing in the opposite direction to the Earth's magnetic field, then the geomagnetic storm becomes particularly strong," he told Insider via email.
The SWPC got its intensity forecast right for this CME, though. It predicted a moderate geomagnetic storm, which is classified as G2, with a change for a strong G3 storm. The storm that ultimately occurred was mostly G2, peaking at G3 overnight.
More solar storms are surely coming
This was just the latest in a series of 22 CMEs that occurred in one week, with three of them pointing at Earth, according to NASA.
"The sun's been very active this last week," Owens said.
In fact, the sun has been getting more active for years, as it builds toward a decadal peak of activity. Experts initially predicted the sun would hit its peak in 2025, but now it looks closer to mid-2024.
As a result, more solar storms have been coming our way.
We got lucky this time
Another structure on the sun — called a coronal hole — is also forming this week, bringing with it fast solar winds. On a different day, all of these effects combined could have created a very violent solar storm.
The best-case scenario is that incoming storms trigger beautiful aurorae, much further south than usual. That's what happened this time: Spectators reported seeing the aurora in Montana, Missouri, Virginia, and the UK.
In the worst-case scenario, which is very rare, all the conditions align to send a very fast and very powerful solar storm to Earth. This once-a-century event would be so powerful it could disrupt Earth's geomagnetic fields and damage infrastructure, like power grids and satellites.
But this week, the eruptions and coronal holes are "a bit too slow and a bit too spread out," according to Owens.
"These are all the right ingredients for some serious space weather — multiple Earth-directed CMEs and some fast coronal hole wind — but it's not quite been combined in the way to cause real concern in this instance," Owens said, adding:
"There's a good chance for some decent aurora the next couple of nights. But probably not any major concerns for power grids, etc."
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