The solar storm of May 2024, from the flare to the aurora
In the first full week of May 2024 the Sun threw at least seven clouds of charged gas toward the Earth. Three days after the first flares, they arrived. People who had never seen an aurora saw one from their own street.
Early May 2024
The surface of the Sun is hot gas threaded with magnetic field. In places the field bunches into strong, twisted bundles. From Earth they show as groups of dark spots.
The twist stores energy. When a bundle is stressed too far it snaps into a simpler shape, and the energy has to go somewhere.
7 to 11 May
Part of that energy leaves as light: a solar flare, a burst of X-rays and ultraviolet that lasts from minutes to hours.
Late on 7 May one region of the Sun produced two strong flares. Over the next four days there were eight of the most powerful class. The strongest was rated X5.8.
8 minutes 20 seconds later
A flare travels at the speed of light, so it reaches the Earth 8 minutes and 20 seconds after it happens, at the same moment we see it. Nothing can warn of it.
Its X-rays charge the upper air on the sunlit side of the planet, and some radio signals that bounce off that layer fade out. The sky does not change. The aurora comes from something slower.
The same hours
The same snap can throw a piece of the Sun’s outer atmosphere into space: billions of tonnes of charged gas, carrying its own magnetic field. It is called a coronal mass ejection.
These clouds leave at anything from under 250 to nearly 3,000 kilometres a second. That week the Sun launched at least seven toward the Earth, the fastest at about 4.8 million kilometres an hour.
Three days
The fastest clouds can cross in 15 to 18 hours. Slower ones take several days. They swell as they go, and a large one can be a quarter as wide as the whole gap between the Sun and the Earth by the time it arrives.
This time the later clouds caught up with the earlier ones. They reached the Earth bunched together on 10 May, three days after the first flares.
10 May
The Earth has a magnetic field of its own. The steady wind from the Sun squeezes it on the day side to about ten times the Earth’s radius, and draws it out on the night side into a tail hundreds of radii long, far past the orbit of the Moon.
Most of the time this holds the Sun’s gas off.
10 May, 22:54 UTC
What matters most is which way the cloud’s magnetic field points. If it points south, opposite to the Earth’s, the two fields join, and energy pours into the space around the planet for hours.
At 22:54 UTC on 10 May the disturbance reached G5, the top of the five-step scale. That had not happened since 2003.
The night side
The energy collects in the long tail on the night side. There electrons are sped up and sent back along the magnetic field, which leads them down toward the poles.
They come down in two rings around the magnetic poles. On an ordinary night the northern ring lies between about 60 and 75 degrees of latitude. In a great storm the rings widen and slide toward the equator.
From the ground
The electrons hit the thin air between 80 and 500 kilometres up. They knock energy into atoms of oxygen and nitrogen, and the atoms give it back as light. A neon sign works the same way.
Green, the commonest colour, comes from oxygen between about 100 and 200 kilometres up. Above 200 kilometres oxygen glows red. Nitrogen adds blue at the same height as the green, and pink below 100 kilometres, along the lower edge.
Near midnight
Early in the night the light hangs in arcs from one horizon to the other, made of tall rays like the folds of a curtain. Near midnight the arcs begin to twist and sway.
Then they can spread across the whole sky, moving fast and very bright.
10 and 11 May
That night the aurora was seen from the southern United States and from northern India. Reports came from as low as 26 degrees of magnetic latitude.
It may be among the lowest-latitude auroras of the past five hundred years; scientists were still assessing that when NASA described the storm.
By one measure kept since 1957, the storm matched the great storms of 1958 and 2003.
Everything in the sky that night had left the Sun three days before.
The dates and figures for May 2024 are those published by NASA and by NOAA’s Space Weather Prediction Center. The general figures (speeds, distances, heights, colours) are from the same two agencies. The pictures are drawn, not photographed, and are not to scale: on the scale at which the Sun fills the screen, the Earth would be a dot far outside it, and the magnetic field lines are a diagram of the field’s shape. The aurora in the last scenes is a drawing of the forms and colours described in the sources, not a record of what any one place saw.