The science of the aurora borealis: why did we see such intense Northern Lights?

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On the night of 10th May 2024, the skies of Durham were filled with the pink and green hues of the aurora borealis, also known as the Northern Lights. Caused by the most intense geomagnetic storm since October 2003, the aurora’s presence that night had been suggested and somewhat predicted by observing and following the ‘weather’ at the Sun’s surface.

Although the Sun seems an unchanging source of energy in the sky, it is far from being a passive object. As a common star, it is a large sphere made almost entirely of hydrogen gas and plasma, a state of matter characterised by electrically charged particles at extremely high temperatures (from thousands to millions of degrees Celsius) meaning they move very fast, which creates a strong and active magnetic field.

The Sun’s activity goes through cycles of around 11 years which are dictated by the turbulence of this field – 2024 and 2025 mark the current maximum of cycle 25, when the magnetic field entirely flips around. This intense phase is distinguishable by an increased frequency of impressive phenomena at its surface: as the magnetic field contorts, ‘snaps’, and realigns, sudden flashes of light can occur (solar flares), or more impressively, an eruption of matter can be released into space in events called coronal mass ejections (CMEs), with the corona being the solar atmosphere’s outermost layer. While particles are constantly making their way from the Sun to the Earth, both in the form of light/photons and material from banal solar wind, CMEs are impressively large bundles of much more energetic plasma which more noticeably interact with the Earth’s environment by creating geomagnetic storms. They can take a dozen hours to a few days to reach Earth and can be nearly as large as a quarter of an astronomical unit (AU), with 1 AU being the average distance between the Sun and Earth.

Coronal mass ejections (CMEs) are bundles of energetic plasma from the Sun that create geomagnetic storms on Earth, causing effects like the Northern Lights

The Earth has layers of protection that withstand these particle emissions from solar activities. One of these is the magnetosphere, which is a region dominated by the strong magnetic field produced by the molten metals in the Earth’s core. It continuously deflects charged particles, such as the unwavering solar wind, hence preserving the atmosphere which would otherwise be eventually stripped off. These energetic particles are also sometimes captured by the field and channelled to the North and South poles, which is why aurorae are most common in those areas, but occasionally, when they are even stronger, they can directly funnel through. However, when an unusually strong solar event such as a CME occurs (such as the one we witnessed recently), the higher flux of more energetic particles can overcome the magnetic barrier and channelling, and thus are observed at much lower latitudes.

Once they have made it through the gaps in the Earth’s magnetic armour, protons and electrons from the Sun collide with and ionise the particles in the Earth’s higher atmosphere, exciting them to higher states of energy. Different gases emit different colours when returning to their ground state, which is the same principle that old neon lights are based on. We then see a colourful sky: the greens and reds are characteristic of excited oxygen and nitrogen molecules at different altitudes, with green being more common and red (often higher up) being associated to stronger activity. These lights are visible by eye, but even more intense through a camera lens.

Red or pink light in the aurora borealis is associated with stronger activity

Beyond a dazzling spectacle, the induced geomagnetic storm is as suggested in the name, a large-scale disruption of the Earth’s magnetic field. This temporarily deforms and sways it. The change in the magnetic field induces electric fields and therefore currents on Earth, which could be a hazard to electrical and electronic components across the world. Consequently, radio interference, satellite service malfunctions, power outages and grid emergencies can occur, which was the case most significantly in recent memory in 1989 and in recorded history at the Carrington event in 1859, which was the most intense geomagnetic storm ever recorded.

Understanding space weather and more specifically solar activity is therefore key to ensuring safety on Earth. The Quantum Light and Matter group (QLM) present in Durham is partly devoted to this topic. By studying multitudes of phenomena at the Sun’s surface, whether it be solar flares, CMEs or more, they are imaging a simulation map of the Sun’s magnetic field to hence be able to predict solar weather. Mathematical models are used for pattern recognition, as signature telltale signs seem to indicate imminent events – though these are statistical methods, their predictions could allow for a supplementary 20- to 26-hour time frame in which to prepare for any incoming solar storm. This time, the National Oceanic and Atmospheric Administration of the United States had observed multiple CMEs on 8th May and consequently issued a severe geomagnetic storm warning for Friday 10th, correctly so.

Even though solar weather is an important point of concern, the CMEs which the Earth happened to intersect this May had no grave effects. Lower latitudes including Durham were able to instead enjoy a pleasant show of lights. And to those who were asleep on that fateful Friday night: stay alert in the coming months, as the solar maximum, and therefore the intense activity, may not yet be over!

Image: A photo of the Northern Lights seen from Durham. Image credit:

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