Three hundred and sixty seven nights

The long wait for aurora displays in solar minimum

Three hundred and sixty seven nights.

That’s how long it took.

Long enough for another birthday to pass and for me to be a year older, if not wiser. Long enough for our planet to complete an entire journey around the Sun. Long enough for the golden hues of autumn to give way to the stark whites of winter snow; long enough for the riot of spring flowers to push back against the cold; long enough for the rich, verdant greens of summer to reign supreme across our land. And long enough for us to be right back into the depths of autumnal orange once more.

It was, in other words, a very long time to wait.

Sadly, in the throes of solar minimum, I’m getting very used to that. Little did I know I’d have to wait a year and two days from when I last saw the aurora dance in the skies above Northern Ireland for me to get a glimpse of an auroral pillar and sub storm once more. It is, party, due to the cloud that plagues us so much here, meaning there are many displays we miss out on. But it is, mostly, due to one simple fact: aurorae are few and far between when the Sun decides to settle down into a comfortable and largely inactive minimum.

So, it was with some anticipation that I greeted a precious chance of another possible viewing as an enhanced solar wind was due to arrive at our planet. On a Friday night, no less (always easier to stay up late then). And there were clear skies forecast (never a given in Northern Ireland). And I was due to be up north at my favourite astro photography haunt, the Giant’s Causeway.

Could I, after all this time, be lucky once more?

Yes. Yes, I could. And here’s the story of that night.

Coronal holes - solar minimum’s heroes

The Sun goes through an 11 year cycle from maximum, down through minimum, and back up to the peak once again. And it would seem right now that we are in the doldrums of minimum. Little did I know back only 3 or 4 years ago, when I was seeing more than a dozen shows per year, that so soon after glimpses would be fleeting and often subdued. Why is that?

During solar maximum, the Sun’s surface tends to be covered with sun spots. These active regions on its surface can launch out a salvo or two of Coronal Mass Ejections, unimaginably large explosions of super-heated solar plasma that hurtle towards our planet and slam into our atmosphere a few days later. Most of the really big displays I’ve ever seen have been courtesy of these CMEs.

During solar mimiumum, however, sun spots become very rare. Thankfully there’s a second source of enhanced solar wind that we can rely on. Time for Coronal Holes to step up to the plate.

These are cooler areas on the Sun’s surface that have open magnetic field lines. This, crucially, allows plasma from the Sun to shoot out into the Solar System. And - if the hole points towards us - this can give increased chances of aurora in the mid-latitudes.

Coronal holes tend to be more common in solar minimum, so it is to these features that we turn in hope of sending us some northern lights action. The bigger ones can last a few months, meaning that they can survive a rotation around the Sun (it takes 28 days to complete a rotation) and revisit the Earth. This makes them more predictable than CMEs. That said, they also tend to produce less impressive displays, displays that often fail to ignite much by way of activity in the mid-latitudes.


Is it possible?


And so, enter the hero of our show, the Coronal Hole that swung around and faced up on 7th November 2018.


The Coronal Hole during its last three passes, along with the K index it produced (a measure of the strength of aurora is produced). You can see the K index was elevated at 21.00 - just when we were photographing the substorm at the Giant’s Causeway.


This large Coronal Hole had survived a couple of solar rotations. During October, it had stirred up some auroral action; but its September visit was more subdued. What would November hold?

Forecasts for the mid-latitudes were understated. Tamitha Skov, the forecast guru for many of us aurora chasers, was suggesting a slightly elevated chance of minor solar storm conditions over the weekend. I’ve learnt enough through this solar minimum not to get too worked up about forecasts like this. I’ll be aware of them - but I won’t be clearing my calendar a couple of days out in sure and certain anticipation of a great show. And this one was no different.

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It was due on a Friday night; the weather forecast was to be good; I was going to be up north anyway. So it was worth keeping an eye on things on Friday to see how it played out.

Is it probable?

Once we get the heads up of a possible aurora, the next stage is to keep an eye on the stats when the High Speed Stream of solar wind from the Coronal Hole (CH HSS) begins to arrive. For me, that meant having a look on Friday afternoon to see how things were looking.

Data coming in from the ACE satellite (about an hour’s journey time from our planet for the solar wind) tells us what the characteristics of the solar wind are as they approach. And, when I looked on Friday lunchtime, they were good.


Annotated graph from the ACE satellite showing the solar wind the day up to the display.


The speed of the solar wind was slightly elevated (at around 450 km/s compared to ambient levels of around 350 km/s). The Bt was slightly elevated too, heading upwards of 5 nT (and it would peak at around 10 nT).

But what caught my eye was the negative Bz (a negative Bz allows the solar wind to interact more favourably with our Earth’s magnetic field, allowing the aurora to sink further south towards the UK). It had been slightly negative all day - if this continued, it would definitely allow for a better chance of viewing that evening.

The signs were positive mid afternoon. But could the stats remain so into the evening?

In the meantime, I came home from work and went to pick my nephew, David, up from the airport. He was flying in from England to spend a weekend on a photography one-to-one workshop with me. And when I picked him up, it wasn’t taking time to rain. As we dashed for the car, getting soaked in the process, I’m quite sure he was more than a little bit sceptical of my reassurances that not only would the skies clear, but in a few hours we would be standing at the Causeway with the distinct chance of us seeing the aurora.

We headed home for dinner before heading north. Another chance to check the stats - and they continued to be good. Such a long stretch of negative Bz has, in my experience, always been a good sign of activity later. We were game on!


Is it here?

Up north we went, the rain stopped and the cloud clearing all the way. By the time we reached the Causeway, the skies were almost totally clear. And, as our eyes got used to the dark - there it was. That unmistakable lighter band along the horizon. As a seasoned aurora chaser, I knew exactly how this would look on camera - and I couldn’t wait to see it!

We got into position just in time. About 15 minutes after we arrived, the sub storm hit. ‘Can you see it getting brighter?’ I said to David. He could. And then, before our very eyes, a small but noticeable batch of pillars appeared to the western flank of the aurora.

Aurora substorms are times when the energy of the aurora is elevated and they tend to produce more structure and pillars. During big displays, they can fill your field of view with pillars and arcs of light. This time - it is solar minimum after all - it was an altogether more subdued affair. But a pillar is a pillar, and I’m claiming it - especially after waiting so long since the last one!


The Scandanavian line magentometers - real time indication of the onset of the aurora substorm. [Source: flux.phys.uit.no/cgi-bin/mkstackplot.cgi}


Quickly, the intensity faded, and the pillars vanished from view. For the rest of the evening, the aurora sat there as a slight enhancement along the horizon. We managed one or two more shots in the dark before calling it a night. Meantime, the Bz had swung north, and the chances of anything more than night were fading.

So, it was far from the most amazing of displays. But my nephew had managed to see his first aurora (he had travelled to northern Norway last year in the hope of seeing it only to be clouded out the whole time) and he managed to photograph his first ever aurora at the stones of the Causeway. In these lean times of “aurora austerity”, I’ll call that a win!


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Here are a few of the non-aurora images we captured during the workshop weekend.