Just a week ago I wrote of the joys of unexpectedly bumping into the aurora borealis on the north coast. "Let's do it again soon, old friend," I signed off, fully expecting a wait of a few months, perhaps longer. 

Little did I know that, as I typed those words, I would once again find myself out in the wee small hours aurora chasing - one week to the day from my last time! This time I headed to the west coast of Co Antrim, to the picturesque village of Glenarm.

As ever, it was a combination of watching the aurora stats and keeping an eye on the live updates from Aurora UK on Facebook that prompted me that some action might be on the cards. As ever, on Facebook, I have had some people post saying they wish they'd seen this or wondering how they can know if the aurora is happening.

In this blog, I want to show you something of the data behind the display last night. As I began to chase auroras, I started to try to teach myself how to read these charts so that I could better forecast any possible activity for myself. Here's a bit of an insight into what I was looking for last night in the hope that it might help you begin to decipher the mysteries of the aurora for yourself!

The first thing to say is that there are no guarantees with the aurora. But that's not to say it's down to pure chance. If you know how to read the charts and data, you can know when the probabilities of a display are better. Here's how I go about it.


The possibility

Although there can be auroras that catch everyone by surprise, in most cases we can know if one is a possibility by monitoring the sun. For the case of Monday night, there had been a solar emission from a Coronal Hole which raised the chance that we would see action. The forecast had been primarily for it to impact on Sunday night, but there was a chance of some residual activity spilling over into Monday. So we were on the look out - it might be happening!

The magnetometers

When I know that there's the chance of action, the next thing I do is keep an eye on the magnetometers. The aurora occurs as a result of the way the sun's magnetic field interacts with the earth's magnetic field. We have a series of magnetometers positioned at different latitudes that measure the magnetic fluctuations that result when an aurora display is happening. That's what this graph shows. Basically, we're looking for two things:

  • the bigger the fluctuations on the graph, the better
  • the further south those fluctuations are registering, the better

Here is the action on the magnetometers during the 23 February. The magnetometers had been twitching during the day, but into the afternoon and early evening, they really started to move around. This was  something well worth keeping an eye on indeed!

Figure 1 - the magnetometers

Reports from the field

When I see the magnetometers looking like this, I go looking for real time reports of fellow aurora spotters who are out on their own field trips. Typically, in the first instance, these are the fine folk of Scotland who post in Aurora UK on Facebook. If I see action there, I'm able to judge the likelihood of me seeing it in Northern Ireland. As they are further north, they will get stronger displays - if they are barely picking anything up, then there's very little chance I will further south. If, however, things are kicking off for them big style, then I consider heading out (weather permitting, of course!) Last night the reports started coming in of pretty impressive displays from Scotland. That was enough to entice me to head out on the hunt myself!

Taking it further - the stats behind the magnetometers

If you want to develop your knowledge further, then you need to wrap your mind around things like the IMF and the Solar Wind, as this will introduce you to the all important Bz. Now it might start to get a bit more complicated here, but here are some other graphs that it may well be worth your while studying a bit. Again, these are from 23 February and the section highlighted in yellow is when I was out 

Figure 2 - the solar wind and IMF

The graph below shows the Bz and Bt from earlier on 23 February - you can see the red Bz line is negative for large parts of the day, setting us up for the display that evening. 

Figure 3 - the IMF data for earlier on 23 February - you can see it was negative for a large portion of the afternoon

From midnight on in figure 2, you can see again that the red Bz line dropped into negative values. By then, I was back home in bed. But for those who stayed out and braved the very strong (earthly) wind and gales, and who managed to have gaps in the clouds, they were treated to some amazing strong displays with intense colours on camera and vertical structures clearly visible. Nice as it would have been for me to stay out, I really needed my beauty sleep last night. But I went home pretty happy with the results of my hunt anyway!

Summary

  1. Know when an aurora is possible (is there a Coronal Hole or Coronal Mass Ejection (CME) headed our way?)
  2. Watch the magnetometers to see if things are twitching.
  3. Look out for reports in the field to see what's actually happening.
  4. Watch the values for the solar wind and especially the Bz element of the IMF - is it negative?

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