Some of the biggest geomagnetic storms in decades lit up Earth’s atmosphere throughout 2024 with dazzling auroras. But which was biggest? Eelco Doornbos of KNMI has used different aspects of Swarm data, his Space Weather Timeline Viewer, and reports of visible aurora from the ground, to investigate.

On 10 May 2024, the northern lights dazzled people as far south as the Canary Isles and even Mexico. It was the biggest solar storm to hit Earth in over two decades.

Perhaps even more people witnessed another huge solar storm arrive at Earth on the evening of 10 October 2024, when the evenings were already getting darker earlier on. The northern lights began at dusk for those watching the sky in Europe and continued until dawn for stargazers in America.

Why are there such strong northern lights this year?

The Sun is currently at the peak of its activity. The fiery giant at the core of our solar system ticks along in 11-year cycles, due to a regular shifting of its magnetic field.

At its most intense period, the Sun frequently releases chunks of its magnetic field in events known as coronal mass ejections (CMEs), which fling clouds of extremely hot charged particles into space.

Sometimes, Earth gets in the way. When a CME hits, depending on the direction and scale of the impact, Earth’s magnetic field can be warped by the solar material, which leads to a geomagnetic storm and auroras.

ESA’s Swarm satellites, a constellation of identical triplets designed to measure Earth’s magnetic field, have even had to wrestle with this increased solar activity in recent years. It makes Earth’s atmosphere soupier, meaning satellites must burn more fuel to stay in orbit.

But their position in low Earth orbit also means they are perfectly placed to measure the effects of CMEs on Earth’s space environment and the power of geomagnetic storms, and therefore to answer the question: which was biggest, 10 May or 10 October?

Magnetic perturbation

One way of doing the comparison is to use magnetic perturbation data.

Magnetic perturbations happen when currents of charged particles travel through Earth’s magnetic field and the region where the upper atmosphere transitions into space. We see rapid variations of these perturbations at high latitudes, related to the bursts of currents that generate the aurora.

These perturbations are always around, but outside of the geomagnetic storm they remain small and at high latitudes, above 60 degrees. During the storms of 10 May and 10 October 2024 they reached much larger amplitudes, as well as lower latitudes. 

“One thing that is immediately clear by comparing the May and October events using this data is that the strong currents during the May event lasted longer, while the October storm started with a peak that was nearly as strong but settled down more quickly,” says Eelco. “This makes sense because the May storm was caused by a day-long succession of multiple CMEs, while the October storm was caused by just one.” 

Swarm’s magnetic observations were compared with visual observations of the aurora from the VIIRS instrument on the NASA/NOAA JPSS satellites. Although these are different satellites that were never near the same place at the same time, we see that the intensity and latitude of the visual aurora and the magnetic perturbations varied in a similar manner.

Using the space weather timeline viewer and FAST processing of Swarm data, which recently got its own near-real time Kp-like space weather hazard index, we can now gain insights from the data during such storms within a day. 

Satellite drag

The power of geomagnetic storms can also be measured by comparing the effect of satellite drag on the Swarm satellites.

We mentioned earlier that Swarm had to wrestle against a denser atmosphere. Earlier in the solar cycle, Alpha and Charlie –the lower-orbiting pair of Swarm’s trio of satellites that are prone to the effects of space weather- were moved up over a series of manoeuvres to ensure that they would not deorbit due to the increasing activity of the Sun.

During solar maximum years, the upper atmosphere increases in temperature and expands due to the higher level of solar extreme ultraviolet radiation. This causes continuously higher drag on satellites, causing them to lose altitude faster.

Extreme geomagnetic storms cause considerable further heating and expansion of the upper atmosphere. In just one day they can cause a similar level of altitude loss as would otherwise happen in weeks. For satellites in low Earth orbit, you can imagine it like running against a strong wind. It takes more energy to keep going on your trajectory. This is what we mean by satellite drag.

At TU Delft, Jose van den IJssel processes just the data we need to measure that. By using information on the change in the trajectory of the satellite, which is measured using the Global Positioning System (GPS), she can calculate the variations in the density of the upper atmosphere.

“It is very interesting to compare the upper atmospheric density and drag on Swarm during these two large storms,” says Eelco. “While again the May storm wins in terms of the maximum magnitude of the density and drag increase, we now see that the duration looks very similar.”

In May we see that the density starts to reduce considerably even while the currents causing the geomagnetic perturbations were still going strong. This is an indication that the upper atmosphere sets off its own cooling mechanism to get rid of the extra energy that it gets from the currents of charged particles.

This phenomenon was already known to scientists, but it was not often so easily visible as in these Swarm observations.

We have a winner

So, it looks like overall the geomagnetic storm of 10 May takes the crown of the biggest in 2024.

But the real winner may well be space weather scientists, who can use the events, along with Swarm FAST data and tools such as KNMI’s space weather timeline viewer, to study the interactions between space weather and Earth’s atmosphere in greater detail than ever before.

The analysis performed by Eelco here highlights some of the phenomena that are now more visible to scientists, in a much quicker timeframe.

As we remain in the period of heightened solar activity, it’s likely that 2025 will bring further geomagnetic storms to put Swarm FAST data to the test once again.