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The largest geomagnetic storm in 20 years set off displays of the auroras recently, and another show is expected

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It's been a fruitful season for viewing the northern lights, and there's a good reason for that.

The largest geomagnetic storm in 20 years set off displays of the auroras across Canada and the U.S. on May 10, and a smaller coronal mass ejection (CME) lit up the skies over the northern U.S. and parts of Canada again three weeks later.

Another eruption of solar material that left the sun on June 1 is expected to hit Earth's magnetic field this week, triggering a geomagnetic storm and more auroras.

According to the U.S. government's National Oceanic and Atmospheric Administration (NOAA), the latest one will be the weakest of the spring's geomagnetic storms so far. It's forecast to be a G1 storm — the lowest level of severity out of five — while the storms of May 31 and 10 were categorized as G2 and G4 storms, respectively.

Whether or not you've managed to catch a glimpse of the auroras this spring, you may be wondering what they have to do with the sun, and why they seem to be happening more than usual lately.

Birth of a geomagnetic storm

Geomagnetic storms take place in Earth's magnetosphere — the area surrounding Earth where the dominant magnetic field is the planet's, rather than that of outer space — and filter down into the magnetic field.

However, the phenomena that trigger geomagnetic storms begin more than 151 million kilometres away: on the surface of the sun.

Explosions from more active areas of the solar surface sometimes send magnetically charged material into space in the form of either a CME or a solar flare. The geomagnetic storms of May 10 and 31, as well as this week's storm, were all triggered by CME events.

Northern lights glow in the night sky above the village of Daillens, Switzerland, early Saturday, May 11, 2024. (Laurent Gillieron/Keystone via AP)

However, both CMEs and solar flares can trigger geomagnetic storms, and when particles from these events react with oxygen and nitrogen in the atmosphere, they help create the northern and southern lights.

Although they are similar, there are some key differences between solar flares and CMEs.

Difference between CMEs and solar flares

According to NASA, CMEs are large clouds of magnetized particles — also known as solar plasma — that are released into space from the sun's magnetic fields during solar eruptions. Solar flares are energetic bursts of light and radiation triggered by the release of magnetic energy on the sun's surface.

"One can think of the explosions using the physics of a cannon," NASA explains in a video about the differences between solar flares and CMEs.

"The flare is like the muzzle flash, which can be seen anywhere in the vicinity. The CME is like the cannonball, propelled forward in a single, preferential direction, this mass ejected from the barrel only affecting a targeted area."

Solar flares and CMEs can happen both independently and together, according to NASA, and they both tend to occur near sunspots, which are the locations on the sun's surface where its magnetic field is strongest.

How common are they?

According to CTV News Science and Technology Specialist Dan Riskin, the frequency of CMEs and flares varies with the solar cycle. Right now, we're approaching the peak of that cycle.

"The sun goes through these 11-year cycles where every 11 years it's burping a lot and sending off these big coronal mass ejections," Riskin said in an interview with CTV News on May 10. "And then it goes through a dip where it's not very active for a while, and then it gets active again every 11 years."

NASA typically observes one CME per week at solar minimum, and two or three per day at solar maximum. Solar flares are a little more common, occurring at an average rate of about one per day at solar minimum. There can be as many as 20 per day during solar maximum.

According to the NOAA's Space Weather Prediction Center, the sun should reach solar maximum in July 2025.

Effects on Earth

Because they contain different material and travel differently, flares and CMEs also affect the planet differently. Both CMEs and solar flares also come in a range of intensities, and their effects on Earth will vary depending on how severe they are.

"The energy from a flare can disrupt the area of the atmosphere through which radio waves travel. This can lead to degradation and, at worst, temporary blackouts in navigation and communications signals," NASA explains.

"On the other hand, CMEs can funnel particles into near-Earth space. A CME can jostle Earth’s magnetic fields, creating currents that drive particles down toward Earth’s poles."

Magnetic changes near Earth can affect high-frequency radio waves, causing radios to transmit static. They can also cause GPS coordinates to stray by several metres and create electrical currents in utility grids that can overload electrical systems when power companies are caught off guard.

Fortunately, Riskin says we're usually well prepared for geomagnetic storms on Earth.

"For the most part, we have got pretty good infrastructure to watch for this stuff," he said.

"With that said… if there is anything that is truly substantial we are so much more reliant on those networks of power and telecommunications, so much more than ever before in our history, that the stakes are a lot higher." 

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