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Auroras filled much of the world’s skies for several nights in mid-May as a historic geomagnetic storm coursed 100 kilometers above our heads. Being able to see auroras so deep into the tropics was possibly a once-in-a-lifetime experience, but there will almost certainly be more strong geomagnetic storms later this year, giving hope to aurora watchers around the world that more dazzling lights are possible in the near future.
This is because we’re quickly approaching solar maximum, the peak of our star’s predictable 11-year cycle of activity. Solar flares and coronal mass ejections, or CMEs, are more common during and just after solar maximum, and it’s these that are responsible for vivid auroras.
The great aurora show on May 10, 2024, was the result of three CMEs that surged out of the sun’s outer atmosphere and headed toward Earth. A CME is a collection of magnetized plasma ejected from the sun’s exceptionally hot outer atmospheric layer, the corona, as a result of a disruption in the sun’s magnetic field.
On May 10, each successive CME moved a little faster than the one before it, allowing all three bursts of charged particles to merge before washing over Earth’s atmosphere. The combined energy of three CMEs hitting our planet at once unleashed an aurora show for the ages.
These CMEs were associated with Active Region 3664, a collection of relatively cold and dark sunspots on the sun’s surface that grew more than 15 times larger than the Earth itself. You could see AR3664 without magnification simply by peeking up at the sun through a pair of eclipse glasses.
It turns out that the enormity of AR3664 was a major contributor to our generational aurora display. Such spots on the solar surface often disrupt the region’s magnetic field, creating an instability and realignment that can force the release of a CME or even a powerful solar flare—a burst of electromagnetic radiation that can cause radio blackouts.
The surface of the sun rotates every three and a half weeks or so, meaning that sunspots are only visible to Earth for a week or two, depending on where they form on the solar surface.
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AR3664 rotated out of view not long after the mid-May auroras. As sunspots evolve relatively quickly, it’s unlikely the old spot will return in early June bearing any resemblance to the region we saw in the middle of May.
Additional batches of sunspots spun into view soon after the AR3664 departed. These regions are far smaller than the enormous cluster that triggered our historic aurora display—only two or three times larger than Earth at most—but their presence is a sign that the sun’s activity will be on the upswing over the next 18 months. Sunspots grow more common as we reach the solar maximum. Many spots can blemish the sun during solar maximum, while we can go days without a single sunspot during solar minimum.
Scientists predict that we’ll reach solar maximum in July 2025, with heightened activity continuing for a while thereafter, giving us more than a year and a half with better-than-normal odds of spotting auroras at lower latitudes.
It’s also likely that additional CMEs will produce spectacular auroras over Earth’s higher latitudes as we approach the maximum—though folks in places like Alaska, Canada, and the Nordic countries will have to wait until the sun finally sets again to see auroras. Knowing far in advance when auroras will appear, however, isn’t possible.
Even though the US National Oceanic and Atmospheric Administration’s agency dedicated to tracking solar activity is called the Space Weather Prediction Center (SWPC), “space weather” is a bit of a misnomer compared to the weather we see here on Earth. Terrestrial weather forecasting is a miracle of modern science. Meteorologists can predict the general outline of a large tornado outbreak or a tropical disturbance’s potential growth into a hulking hurricane more than a week in advance.
Space weather, on the other hand, is very much a “watch and wait” sort of business. Only once a CME or solar flare bursts forth from the sun can scientists then begin predicting its trajectory and potential impacts on Earth over the next several days.
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If satellites detect a major CME heading toward Earth, scientists begin issuing predictions on how it’ll affect everything from power grids to GPS signals. One of the most useful forecasts for everyday folks like us is the Kp Index. This is a measure of geomagnetic activity as it relates to auroras. The index runs from Kp 0 to Kp 9, with higher values indicating higher activity that can produce auroras farther away from the poles.
Values of Kp 3 are sufficient for vivid auroras over the far northern latitudes. Major cities like Oslo in Norway and Reykjavik in Iceland, as well as most populated cities in Canada, have the opportunity to see auroras once we hit Kp 5. Any readings higher than Kp 5 begin to send the aurora deeper into Europe, Asia, and North America.
The German Research Centre for Geosciences has kept meticulous track of the Kp Index every three hours since January 1, 1932. Out of more than a quarter of a million data points over the past 92 years, scientists have witnessed the Kp Index smack the top of the scale just 32 times.
Three of those instances occurred during the May 10-11 geomagnetic storm, the longevity of which allowed millions of people around the world to witness incredible sights.
Before this year’s event, the last time the index peaked at a Kp 9 was during a solar storm that struck around Halloween 2003, during which auroras were visible deep into Europe and the United States. If you’re hoping to see auroras at lower latitudes again, cross your fingers for decent sunspots and a high Kp Index as we approach solar maximum next year.
