Aurora alert: Powerful geomagnetic storm could spark northern lights across US and Europe tonight

Editor's note: The incoming CME hit 6 hours later than predicted and sparked a severe G4 geomagnetic storm. You can read our aurora photo story here. 

A combination of powerful solar events has prompted the National Oceanic and Atmospheric (NOAA) Space Weather Prediction Center (SWPC) to issue a geomagnetic storm warning for today (Sept. 16). 

This is great news for those wishing to see the northern lights the predicted geomagnetic storm could spark auroras deep into mid-latitudes (around 50°) and as far south as California, Missouri and Oregon.

This culprit? In this case, there are actually two.

The first is a large plume of plasma and magnetic field known as a coronal mass ejection (CME) that was released from the sun during a colossal X-class solar flare — the most powerful class of solar flare — on Sept. 14. The X4.5 solar flare peaked at 11:29 a.m. EDT (1529 GMT) and was the fifth largest solar flare of the current solar cycle. 

The CME released during the X-flare is currently barreling toward Earth and is predicted to arrive today (Sept. 16). CMEs carry electrically charged atoms known as ions. When they collide with Earth's magnetosphere, they can trigger geomagnetic storms. During these storms, the ions interact with gases in Earth's atmosphere, emitting energy as light. It is this light that we view as the northern lights, or the aurora borealis, in the Northern Hemisphere, and the southern lights, or the aurora australis, in the Southern Hemisphere.

The second culprit is a coronal hole on the Earth-facing side of the sun. Coronal holes appear as dark regions in the sun's corona (outer atmosphere). They have an open, unipolar magnetic field line structure which essentially allows the solar wind to escape more readily into space. This stream of relatively fast solar wind and the influence of the incoming CME have resulted in the G3 geomagnetic storm watch being issued. 

NOAA classifies geomagnetic storms using a G-scale that measures their intensity, ranging from G1 for minor storms to G5 for the most extreme ones. Powerful G3 conditions are predicted for today (Sept. 16) which means we could be in for quite the aurora show as previous "minor" G1 conditions on July 29/30 still managed to spark dazzling northern lights across the U.S. and Canada.

When will the geomagnetic storm occur?

Current forecasts predict G3 geomagnetic storm conditions will arrive on Sept. 16, though the exact timings are uncertain.

Similar to Earth's weather, space weather is unpredictable and challenging to forecast. While geomagnetic storm warnings of this level are uncommon, they can still sometimes fizzle to nothing. 

"NOAA & NASA predictions indicate arrival anytime from 0600 UTC to 1800 UTC September 16. Likely the storm will arrive on the late side, possibly deflected further east by a fast solar wind stream," space weather physicist Tamitha Skov wrote in a post on X. 

If you use X, I recommend checking out space weather forecaster Sara Housseal's account for regular "CME arrival" updates and general space weather information. 

"CME Update: We've reached the point where we could expect the CME to [arrive] at any time, based on model outputs, but there is still no sign that an arrival is imminent," Houseeal wrote in a post on X this morning (Sept. 16). 

"Now the fun begins of will it arrive later than expected or not at all? Stay tuned!" Housseal continued. 

If you want to keep a watchful eye on the space weather conditions and know when (and where) to look out for auroras, I recommend downloading a space weather app that will let you know the predicted conditions based on your location. I use an app called "My Aurora Forecast & Alerts" available on iOS and Android. But any similar app should do the job. I then couple this with the Space Weather Live app available on iOS and Android. This app is great as it gives you more of an insight into whether the space weather conditions are looking promising for aurora activity.