Auroras filled much of the sky for several nights in mid-May a historic geomagnetic storm headed 100 kilometers above our heads. Seeing auroras so deep in the tropics may have been 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 blinding light is possible in the near future.
This is because we are quickly 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 it is these that are responsible for vibrant auroras.
The big aurora show on May 10, 2024 was the result of three CMEs flying from the Sun's outer atmosphere to 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 an 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 merge before engulfing Earth's atmosphere. The combined energy of three CMEs hitting our planet simultaneously unleashed an aurora show for the ages.
AR3664 on May 10, 2024.Photo: NASA/SOHO
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 Earth itself. You could see AR3664 without magnification simply by looking at the sun through eclipse glasses.
It turns out that the sheer size of AR3664 has made a significant contribution to our generation's aurora display. Such spots on the solar surface often disrupt the region's magnetic field, causing an 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 blackouts.
The Sun's surface rotates about every three and a half weeks, meaning sunspots are only visible to Earth for a week or two, depending on where they form on the Sun's surface.