A-roaring display

Without getting into the specifics, coronal mass ejections (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.

Top 5 tips for viewing the aurora borealis. BGS © UKRI.

This series of consecutive solar flares and their associated CMEs arrived in Earth’s atmosphere in the early evening on 10 May 2024. 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 Hallowe’en 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, Shetland
• Eskdalemuir, Dumfries and Galloway
• Hartland, 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 the northernmost location. Looking at background conditions with no storms, the horizontal magnetic field intensity at Lerwick varies around 30 to 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 effects of space weather on Earth. This will help to improve our capability to forecast severe space weather events that could be even stronger than the one we 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, further large events such as this one are expected in the coming years.

About the author

Guanren Wang

Geomagnetism scientist

BGS Edinburgh

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