On the 26th and 27th of February 2023, the UK experienced a rare treat - a “Red Alert” indicating a good chance of seeing the Northern Lights, or Aurora Borealis. This captivating event drew people from all over the country, eager to witness one of nature's most awe-inspiring displays.

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But why is it that opportunities to observe the Northern Lights from the lower latitudes of the UK, France, and Germany are so rare? To truly appreciate the answer to this question, it's important to understand the fascinating science behind the Northern Lights and the 'Northern' aspect that gives them their name.

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What are the Northern Lights?

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The Northern Lights, or Aurora Borealis, are a result of the Sun's immense gravity weakening with increasing distance from its centre, enabling the outermost regions of the Sun's corona to escape as solar wind, which travels towards Earth. The boundary at which the solar wind and corona are distinguished is known as the Alfvén surface. This solar wind is a plasma composed of protons, electrons, and other charged particles, which collide with atoms in Earth's atmosphere and excite the electrons in these atoms to higher energy levels. Upon de-excitation, the energy gained via collisions is released by the emission of light.

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Lucky observers saw the characteristic emerald green hues, which result from oxygen atoms at an altitude of around 100km. Those luckier still may have seen crimson aurorae caused by oxygen atoms at roughly 150km upwards. We observe different colours because the chemical composition of Earth's atmosphere varies with altitude.

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The Northern Lights. Credit: Evan Boyce 

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Why are they (typically) only visible at the poles?

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The solar wind travels at millions of kilometres per hour and engulfs the Earth. Equatorial regions are protected by Earth's magnetic field as it deflects the solar wind. However, the magnetic field converges at Earth's magnetic poles, redirecting the charged particles of the solar wind to these high-latitude regions, such as Scandinavia and Canada. The same effect occurs at the southern magnetic pole, only these lights are named "Aurora Australis."

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The "auroral zone" is the region of Earth's atmosphere associated with this magnetic funnelling of charged particles. It takes the shape of an annulus centred on Earth's north magnetic pole and is usually in the 65°-70° latitude range.

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Why were they visible in the UK last month?

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The “auroral zone” is key to understanding this question. It is by no means a fixed or static region. There happened to be two coronal mass ejections (CMEs) which arrived at Earth on consecutive nights. The much greater intensity of these CMEs can give rise to distortions to the magnetic field lines resulting in what is called a geomagnetic storm. This triggers the expansion of the ‘auroral zone’ to lower latitudes, thus allowing the Northern Lights to be seen by UK observers.

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A graph displaying geomagnetic activity with universal time (UTC). Credit: @aurorawatchuk on Twitter

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How to know when to look?

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AuroraWatch UK is a free service run by the Lancaster University Department of Physics, providing alerts on the likelihood of observing the Northern Lights. This likelihood is based on geomagnetic activity measurements - disturbances in Earth’s magnetic field - from a network of magnetometers called SAMNET (Sub-Auroral Magnetometer Network).

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I will certainly be eagerly awaiting the next “Red Alert” and hoping for clear skies!

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By Joseph Brennan