What Causes the Northern Lights? The Science Behind Auroras

On the night of 10 May 2024, people in Florida, northern India and the Canary Islands photographed the northern lights. A cluster of solar eruptions had reached Earth together and set off the strongest geomagnetic storm since 2003, rated G5, the top of the scale used by the US National Oceanic and Atmospheric Administration (NOAA). For most of those people it was the first aurora they had ever seen, and the explanation for it starts about 150 million kilometres away.

Illustration: the aurora borealis.

From the sun to the magnetic field

The sun constantly blows out a thin stream of charged particles, mostly protons and electrons, called the solar wind. It reaches Earth after two to four days. Earth's magnetic field, the magnetosphere, deflects most of it. The field is squashed on the side facing the sun and stretched out on the night side into a long tail, like a windsock.

A common simplification says that solar wind particles leak in at the poles and light up the air. The real chain is a little more indirect. The solar wind drags on the magnetosphere and loads energy into the tail. When the stretched field there snaps back into shape, it accelerates electrons that were already trapped in the magnetosphere and drives them down along the field lines into the upper atmosphere. Those field lines meet the atmosphere in two rings around the magnetic poles, called the auroral ovals. That is why the aurora is usually seen in places like northern Norway, Iceland, Alaska and northern Canada, which sit under the northern oval, and why its southern twin, the aurora australis, is mostly seen from Antarctica, Tasmania and the far south of New Zealand.

Why green, and why sometimes red

The electrons hit oxygen atoms and nitrogen molecules between about 80 and 500 kilometres up and knock them into excited states. When the atoms and molecules settle back down, they give off light, much as the gas in a neon sign does.

The colour depends on which gas is hit and how high up. The common green, at a wavelength of 557.7 nanometres, comes from atomic oxygen, mostly between about 100 and 250 kilometres. Higher up, oxygen produces a deep red at 630 nanometres. The reason has to do with timing. An oxygen atom excited into the state that gives red light takes nearly two minutes, on average, to release it. Lower down, the air is dense enough that the atom usually bumps into something and loses the energy first. Only in the very thin air above about 200 kilometres do enough atoms stay undisturbed long enough to glow red. Nitrogen gives blue and purple light, often seen as a fringe along the lower edge of bright curtains.

The eye is also part of the story. Human night vision is much more sensitive to green than to red, so faint red aurora often shows up on a phone camera when it is barely visible to the eye. Many people in southern England and the southern United States in 2024 only saw the colours properly when they looked at their screens.

Green light comes from oxygen at around 100–300 km, red from oxygen higher up. Photo: Jason Pittman / Pexels

Storms, cycles and forecasts

The aurora moves towards the equator when the sun sends something stronger than the usual solar wind. The biggest driver is a coronal mass ejection, a cloud of magnetised gas thrown off the sun that can carry billions of tonnes of material. When one hits Earth with its magnetic field pointing southwards, opposite to Earth's own field at the point of contact, it couples strongly to the magnetosphere and causes a geomagnetic storm. The auroral ovals widen and slide towards the equator. In the Carrington event of September 1859, the strongest storm on record, aurora was reported from Cuba and Hawaii.

These storms follow the sun's roughly 11-year activity cycle. The current cycle, number 25, peaked in late 2024, which is why the past two years have brought unusually many aurora sightings at middle latitudes. The declining years after a maximum still bring big storms, so sightings at those latitudes will not stop suddenly.

Forecasting has improved, but it is still short range. NOAA's Space Weather Prediction Center can see a coronal mass ejection leave the sun and estimate its arrival within a window of several hours. Whether it will produce a good display depends on the direction of its magnetic field, which is only measured when the cloud passes a spacecraft about 1.5 million kilometres upstream of Earth, roughly 15 to 60 minutes before it arrives. That is why aurora alerts often come with so little warning.

Earth is not the only planet with auroras. Any world with an atmosphere and a magnetic field can have them. Jupiter's are hundreds of times more energetic than Earth's and are partly powered by its volcanic moon Io, which pumps charged particles into the planet's magnetic field. The Hubble and James Webb space telescopes have photographed them in ultraviolet and infrared light.

Sources

1. NOAA Space Weather Prediction Center, "Aurora". https://www.swpc.noaa.gov/phenomena/aurora
2. NOAA Space Weather Prediction Center, "Coronal mass ejections". https://www.swpc.noaa.gov/phenomena/coronal-mass-ejections
3. NASA Science, "How NASA tracked the most intense solar storm in decades" (on the May 2024 storm). https://science.nasa.gov/science-research/heliophysics/how-nasa-tracked-the-most-intense-solar-storm-in-decades/
4. NASA Space Place, "What is an aurora?". https://spaceplace.nasa.gov/aurora/en/