40 shades of green - and purple, and red (Part 1)

The St Patrick's Day aurora at its peak at Carnlough, Co Antrim

Ireland and the colour green. The two seem to go hand in hand, inextricably linked, so to speak. And when you travel through the countryside, it's not hard to see why. Green fields abound, dotted with trees covered in green leaves. So much so that Johnny Cash famously sung about walking from Cork to the delightful town of Larne to view those 40 shades of green, the varying verdant hues all across our landscape.

So much are we associated with green that the Irish Tourist Board began a campaign a few years back to mark St Patrick's day called Global Greening. They have persuaded countries all over the world to light up their landmark buildings in green to mark the occasion of the day of Ireland's patron saint.

Belfast's City Hall, lit up green for St Patrick's Day 2014

But 17 March 2015 went one better. Not only were buildings going green to celebrate the day, but the very heavens above the country put on the most incredible auroral display, the best I have yet seen, coming from the largest geomagnetic storm of this 11 year solar cycle.

In my next two blogs, I want to explain a bit about how this stunning display occurred, and also describe what it was like to see it at its peak, when the aurora stretched above me like a 180 dome - above, before, beside me.

How did this display occur?

I just about managed to grab a photo of Sunspot 2297 before it disappeared from view. I took this on 19 March in preparation for the eclipse.

It was two days before St Patrick's Day, on 15 March, that the story of this display began. Sun spot 2297 on the surface of the sun produced a solar flare that sent out a Coronal Mass Ejection towards the earth. First predictions suggested it might arrive in the evening of 17 March and give the earth a glancing blow. There was the chance, according to the forecast, of a G1 geomagnetic storm. Something to keep an eye on, but not something to get hugely excited about just yet in the mid-latitudes.

Even though the plasma from this CME was travelling at 1,000,000 mph, with 93 million miles to travel, it typically takes a few days for a CME to reach the earth. So now, as always, it was a waiting game. What would this CME actually deliver?

The first indication of just that comes when the CME passes the ACE satellite, a satellite located about 1 million miles away from the earth, designed to collect information about the solar wind about an hour before it reaches the earth. On 17 March, this CME arrived at ACE earlier than expected, at around 04.00 hours UK time. This gave us our first glimpse since the solar wind left the sun of what it was now like.

The key factor for any enhanced solar wind is something called the Bz. This is a measure of the magnetic orientation of the solar wind and it is crucial as this is the factor that determines how the CME interacts with the Earth's magnetic field. The Earth's magnetic field points to the north (as anyone who has used a compass will know!) If the magnetic field of the CME is southwards, it opposes the Earth's magnetic field, and this gives the aurora display more of a chance to head south into the mid-latitudes. 

At first, this CME fluctuated between northward and southward orientation. But it was what happened at around 12.00 UK time that was most significant. The Bz switched to a southward orientation (a negative value) - and stayed there for the next 12 hours. It was this sustained negative value that was crucial in producing the amazing displays we were to enjoy later. 

The stats from the ACE satellite from the week of the St Patrick's Day aurora. The 17 March is highlighted in yellow - the remainder of the graph allows you to compare how the enhanced solar wind associated with the arrival of the CME differs from more normal values.

During the day, aurora chasers across the UK were all a buzz with excitement. I was far from the only one to speculate on social media about whether or not we would have green skies over Ireland for St Patrick's Day. The aurora oval (an estimation of how visible the aurora would be across the planet) was looking great for North America during the day. Would it hold out for us until darkness fell?

The aurora oval just before darkness fell over the UK. Things were looking very promising indeed!

The aurora oval just before darkness fell over the UK. Things were looking very promising indeed!

The Kp values for the days around St Patrick's day. I've never seen purple bars since I've started tracking these graphs - no wonder it was an amazing display!

The Kp values for the days around St Patrick's day. I've never seen purple bars since I've started tracking these graphs - no wonder it was an amazing display!

 As the day wore on and the forecasts for the evening became more and more promising, the next thing people turned to was not the space weather, but the terrestrial weather. A blocking high pressure was dominating the UK, drawing in air from the east. But, due to the low winds, this allowed a build up of haze and low level cloud across much of the eastern parts of the UK. The forecasts were not looking too promising for clear views.

But, for once, over to the west we looked like we might have clearer views. Yes, there was still a haze in the sky, especially obscuring the lower atmosphere. But as night fell, the skies above cleared over Ireland. The space weather was right. The terrestrial weather was right. So, many of us Irish aurora hunters headed out into the St Patrick's Night, excited, but still unaware of the stunning displays that we were about to be wowed by over the next few hours. The kind of displays that will make a grown man literally jump up and down, punching the air, hollering with delight and joy at the views going on all around. But that's a story for next time!