The St. Patrick’s Day Geomagnetic Storm... by Sarah Reay

On the 17th March 2015 the Earth experienced a strong geomagnetic storm. This ‘St. Patrick’s Day storm’ was the largest storm in over 10 years, and the largest of the current solar cycle! Sarah Reay, from the BGS Geomagnetism team, expands on the science behind the solar storm...

So what happened?

The storm began at 04:46 UT on the 17th March 2015 when a shock in the solar wind (a stream of charged particles emanating from the Sun) hit the Earth’s magnetic field signalling the arrival of a coronal mass ejection (CME). A CME is a massive burst of charged gas and magnetic field ejected from the Sun’s corona which is carried away from the Sun by the solar wind. The sunspot region responsible for the CME was centrally placed on the solar disc and so the Earth was directly in the line of fire! This CME travelled quickly towards Earth arriving earlier than space weather forecasters had predicted, taking us a little by surprise.

BGS magnetometers, which measure the variations in the strength and direction of the Earth’s magnetic field, recorded the impact of the CME. We observed a rapid variation in the magnetic field signalling the start of the geomagnetic storm. At Eskdalemuir observatory in the Scottish Borders the rapid variation was approximately a fifth of a degree in compass variation (i.e. declination). 

A magnetogram showing the variation in the compass variation (in degrees west of true north) for the three UK magnetic observatories the 17th – 18th March 2015. You can see the shock arrival and storm commencement followed by larger variations later in the day

Shortly after the shock impact those on the night-side of the Earth were treated to a spectacular auroral display. There were various sightings reported across North America, and more unusually, some great sightings of the aurora australis in New Zealand (New Zealand is of similar geomagnetic latitude to the south of UK). If it had been dark, we should certainly have seen the northern lights across the UK. However our night-time was many hours away. The question everyone was asking was how long would the storm last and how strong would it be?

Why the storm was so strong?

One of the main factors that influence how big a magnetic storm will be is the direction and strength of the interplanetary magnetic field (IMF). That is, the magnetic field carried in the solar wind. If this turns southwards it allows much more energy into the Earth’s magnetic field. If it turns northwards it can effectively ‘shut down’ a magnetic storm. This key aspect is, unfortunately, not one space weather forecasters can predict well in advance so it is difficult to know what nature a storm may have in the coming hours.



Solar wind conditions measure by the ACE satellite. The top trace (red) shows IMF turning southward for extended period of time. Orange, yellow and green traces show the change in density, speed and temperature and the CME arrived.

Remarkably in this case the IMF went strongly southwards for well over 12 hours allowing a lot of energy to flow into the Earth’s magnetic field. This produced a major magnetic storm. The Space Weather Prediction Centre (run by NOAA in the USA) has five defined levels of geomagnetic storm activity from G1 to G5 with G5 being the most severe. This storm, at its peak, reached G4 level for several hours. G4 levels were seen globally between 12:00 – 18:00 UT and again between 21:00 – 00:00 UT on the 17th March 2015. Space weather forecasters in BGS and Met Office continually monitored the situation throughout the day and consulted with each other as the storm progressed. 

A snapshot of BGS's activity monitor when global geomagnetic activity was at the G3 storm level on the 17th March 2015.

A northern light show

When a geomagnetic storm is in progress the auroral ovals, usually located near the Arctic and Antarctic circles, broaden and move out towards the equator. That is why during a magnetic storm the aurora can be seen more easily in the UK.

As the geomagnetic storm rumbled on throughout the St Patrick’s Day more people around the world were treated to a spectacular auroral display - that is, if they were lucky enough to find a gap in the clouds. Unfortunately for the UK many places were covered in thick cloud or fog so missed out on this event. However many more were lucky and sightings were reported across Scotland, Northern Ireland, Wales and the parts of England even as far south as Hampshire and Sussex. In Europe, aurora was seen as far south as Germany and The Netherlands.

Model of aurora oval over the northern hemisphere at 21:40UT on the 17th March 2015. The line of auroral visibility in UK is located around the Midlands. Image SWPC NOAA.

The day after the solar wind conditions remained heightened and geomagnetic activity, whilst no longer at the peak of activity, continued at a moderate storm level throughout the 18th March. Once again parts of the UK reported aurora sightings but these were mainly confined to Scotland.

 So how big was this storm?

One way of measuring how large a magnetic storm is by a type geomagnetic index – the Ap index. This is measure of global geomagnetic disturbance. When Ap is greater than 100 (out of maximum of 400) this is classed as a ‘severe storm’ (severe in this case refers to the magnitude of the storm rather than a comment on the possible impact). The St Patrick’s Day storm had an estimated daily Ap of 108. This is the largest magnetic storm of the current solar cycle (which began in 2008). We need to go back 10 years to September 2005 for the last storm with an Ap >100. The last magnetic storm which was bigger than the St Patrick’s Day storm was in November 2004, almost 11 years ago!

Chart showing all the major magnetic storm with a daily Ap greater than 100 since 1980. Notice the large 10-year gap before the St. Patrick’s day storm.

Do you want to keep track of current geomagnetic activity and watch out for the next chance to see the aurora in the UK?

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Sarah Reay
BGS Geomagnetism Team