As followers of AuroraWatch UK will undoubtedly be aware, last night was a pretty good night for seeing the aurora from across the UK. As shown in Figure 1, enhanced solar wind speeds, reaching 600 km/s at their peak, were recorded and were the result of a coronal hole high speed stream (you may see others refer to this as a “CH-HSS” or just “HSS”). This high speed, in addition to a favourable southward pointing interplanetary magnetic field, was responsible for strongly disturbing the Earth’s magnetic field and generating a great auroral display.
These favourable solar wind conditions lead many services, who primarily use NOAA SWPC’s data, to issue alerts of aurora visibility. As shown in Figure 2, the estimated Kp index, a measure of the planetary geomagnetic disturbance, peaked at around 7 (out of a possible 9). This is an elevated level which suggests that an aurora should be visible from across the UK. It is important to note, however, that this is only an estimate and that Kp cannot be truly measured in real-time and is not a very good indicator for aurora alerts.
Additionally, as shown in Figure 3, the OVATION Prime model predicted strong auroral displays with a very good chance of an aurora being visible from the UK. So with these favourable conditions and promising signs from other aurora services, one might expect that AuroraWatch UK would have issued strong indications of aurora visibility too.
Unfortunately, we didn’t.
What went wrong?
Well let’s first start by saying our magnetometers did record geomagnetic activity and throughout the night our alert level was at “Yellow” status. As shown in Figure 4, from around 5pm (UT) until the early hours of the morning, the AuroraWatch UK status was “Minor geomagnetic activity” which indicates that “Aurora is unlikely to be visible from the UK except perhaps the extreme north of Scotland”.
Clearly, however, we got this wrong. The aurora was seen from right across the UK which suggests that we should have at least issued an amber alert “Aurora is likely to be visible from Scotland, northern England and Northern Ireland” or perhaps even a red alert “It is likely that aurora will be visible from everywhere in the UK”.
The main thing to know is that we issue alerts based upon real, measured data. We do not issue forecasts and we do not speculate about what might happen. Our alerts are 100% based on the difference between our recorded magnetic field data (black line in Figure 4) and our “quiet day curve” (blue line in Figure 4). A quiet day curve is an estimate of the geomagnetic activity on a quiet day, i.e. when there is no elevated activity. This estimate changes throughout the year based on many different variables, including temperature at the magnetometer site.
The difference (dH in Figure 5) reached a maximum of 99.4nT – just 0.6nT below the 100nT amber threshold. We were exceptionally close to issuing an amber alert. Unfortunately, we have a threshold set in our code and we just didn’t quite reach it.
There are a few things we could do differently, however, and these would have generated amber alerts. For example, if we measured the difference between the maximum H and the minimum H (a measure of the Earth’s magnetic field in the geographic north-south direction), we would have recorded a change in excess of 140nT and this would have generated an amber alert. Additionally, if we’d set our amber threshold a little lower (e.g. 90nT) then we would have generated an alert too. Or perhaps if we’d tweaked our quiet day curve, we’d have recorded a larger reading then also.
We also note that the data above was recorded at our Lancaster station, which is the primary station we use to generate our alerts. However, had we shifted to using data from our Crooktree station (located near Aberdeen) we would have measured a slightly larger disturbance and generated an amber alert.
So, yes, there are a few things that we could have done differently and on this occasion any one of them would have resulted in us issuing an amber alert. The problem is that the AuroraWatch UK systems are all automated and so it takes events such as these for us to determine how well this automated system is working.
Rest assured we didn’t like missing this alert any more than you. We are going to be looking into this event further and we will be making changes in the near future to help prevent us missing events of this kind again.