The aurora borealis filled much of the world’s sky for several nights in mid-May as a historic geomagnetic storm passed 60 miles above our heads. The opportunity to see the aurora borealis so deep in the tropics was likely a once-in-a-lifetime experience, but stronger geomagnetic storms will almost certainly occur later this year, giving aurora watchers around the world hope that more dazzling lights may be possible in the near future.
This is because we are rapidly approaching solar maximum, the peak of our star’s predictable 11-year activity cycle. Solar flares and coronal mass ejections, or CMEs, are more common during and just after solar maximum and are responsible for the dynamic auroras.
The great aurora display that occurred on May 10, 2024, was the result of three CMEs that emerged from the Sun’s outer atmosphere and headed toward Earth. A CME is a collection of magnetized plasma ejected from the extremely balmy outer layer of the Sun’s atmosphere, the corona, by a disruption of the Sun’s magnetic field.
On May 10, each successive CME moved slightly faster than the previous one, allowing all three bursts of charged particles to combine before washing away Earth’s atmosphere. The combined energy of three CMEs hitting our planet simultaneously unleashed an aurora display for the ages.
AR3664 May 10, 2024Photo: NASA/SOHO
These CMEs were associated with Active Region 3664, a collection of relatively chilly and obscure sunspots on the Sun’s surface that was more than 15 times larger than Earth itself. You could see AR3664 without magnification by simply looking at the sun through eclipse glasses.
It turns out that the enormity of AR3664 was a major contributing factor to our generational aurora display. Such spots on the Sun’s surface often disrupt a region’s magnetic field, causing instability and realignment that could force the release of a CME or even a powerful solar flare, a burst of electromagnetic radiation that can cause radio signal blackouts.
The Sun’s surface rotates approximately every three and a half weeks, which means sunspots are only perceptible to Earth for a week or two, depending on where they form on the Sun’s surface.