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A-roaring display

The dazzling and colourful aurora borealis, or northern lights, observed by many across the UK last weekend was one of the most extreme and long-lasting geomagnetic storms recorded in the last 155 years.

16/05/2024
The aurora over Haddington, Scotland. Credit: Migle Petruskeviciute
The aurora over Haddington, Scotland. Credit: Migle Petruskeviciute

Without getting into the specifics, Coronal Mass Ejections, or CMEs, are common occurrences where a portion of the Sun’s outer atmosphere is ejected into space, caused by rapid changes in its magnetic field. CMEs often occur along with solar flares, which are unleashed from active regions called sunspots. Last week’s active sunspot, now rotating out of view of the Earth-facing solar disc, was particularly active, emitting a series of Earth-directed CMEs from mid-early last week.

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This series of consecutive solar flares and their associated CMEs arrived in Earth’s atmosphere in the early evening on 10 May. Amazingly, last week’s storm is not just any geomagnetic storm. It shares characteristics with a few of the largest storms since 1869; such as the 2003 Halloween geomagnetic storm. 1869 is the year in which geomagnetic global storm index, the aa index, was first used to measure daily geomagnetic activity. The aa is derived from magnetic observatory data.

On the ground, we can continuously and accurately measure magnetic field variations over many years at geomagnetic observatories. In the UK, we have three permanent observatories: Lerwick in the Shetlands, Eskdalemuir in Dumfries and Galloway, and Hartland in Devon. Other, newer magnetic measuring stations can be found in Northern Ireland, Leicestershire and Herstmonceux.

We would expect variations of the local geomagnetic field to be greatest at Lerwick because it is northernmost location. Looking at background conditions with no storms, the horizontal magnetic field intensity at Lerwick varies around 30-50 nanoTesla (nT). On the evening of the 10 May, the peak variations in horizontal field intensity reached 800 nT!

Usually, with smaller geomagnetic storms, the naked eye is unlikely to observe the aurora at latitudes south of Scotland and that’s assuming we have clear, dark skies free from light pollution. With this big geomagnetic storm, which still requires a name, many people in large cities across England were able to observe vivid colours on the 10 to 11 May.

Events like last weekend will become useful case studies for scientists to gain a better understanding of the effect of space weather on the Earth. This will help to improve our capability to forecast severe space weather events which can be even more impactful than the one we have recently experienced. More extreme space weather events may cause damages to power grid systems and operational satellites, affecting things like GPS and mobile networks.

While we are not expecting further significant geomagnetic activity this week, as we are approaching solar maximum large events such as this one are expected in the coming years.

About the author

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Guanren Wang

Geomagnetism scientist

BGS Edinburgh
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