As many of you will have seen, there was some pretty big hype last week about a massive solar storm which would mean the Northern Lights would be visible from much of the UK:
What was the reason?
On Wednesday 20 March, a coronal mass ejection (CME) was launched from the Sun. CMEs are huge explosions on the solar surface that carry vast amounts of energy and plasma out into the solar system. If a CME hits the Earth it can, if conditions are right, result in spectacular aurora displays – including from the UK.
Two different computer simulations, or “models”, run by NOAA SWPC and NASA both predicted that the CME would give a “direct hit” to the Earth.
Direct Hit! Both #NOAA & #NASA prediction models agree we have an Earth-directed #solarstorm that will impact around 12pm March 23! This one is dense & strong so it could pack a decent punch! Expect #hamradio & #GPS disruptions on Earth's nightside, plus #aurora to mid-latitudes! pic.twitter.com/H3carfQR5m
— Dr. Tamitha Skov (@TamithaSkov) March 21, 2019
This lead to the Space Weather Prediction Center issuing a G2 (moderate) geomagnetic storm watch, with the note:
Aurora may be seen as low as New York to Wisconsin to Washington state.
(which is roughly the same magnetic latitude as the whole of the UK). Understandably, from this, quite a few sites issued alerts about potential aurora sightings from the UK, e.g.:
🚨 Aurora alert 🚨 You could spot the northern lights this evening in Scotland & Northern Ireland. The best chance will be in clear spells away from towns & cities. If you're lucky enough to spot them, send us a picture using #LoveUKWeather pic.twitter.com/VJd6v8LGTQ
— Met Office (@metoffice) March 23, 2019
So what happened?
As you can see we registered no auroral activity on our magnetometers. Our magnetometers record disturbances in the Earth’s magnetic field which are the result of the aurora. Generally, if there’s aurora visible from the UK, we’ll register that in our plots.
But there simply wasn’t any significant aurora and that’s because the CME didn’t hit Earth – at least not with any power.
What the satellites tell us
Located 150,000 km in front of the Earth, directly between us and the Sun, is the satellite DSCOVR (and a few others, ACE and WIND, for example). DSCOVR gives us about an hour’s warning of what’s coming our way.
The data show that we got a small bump in the solar wind speed (yellow) and density (orange) at around 20:44 (UT) on 24 March (dashed white line). This signaled the arrival of a CME shock. Think of the CME as boat in the water, what we got was the bow wave crashing into us – rather than the boat itself.
We’re still seeing “elevated” (from what we’ve seen in the past few days anyway, but that’s been extremely quiet) solar wind speed but this is not going to produce any aurora activity. It appears the CME has passed us by this time.
Lesson for the future
Predicting space weather is hard!
Predicting terrestrial weather is notoriously difficult, and we’ve been doing that for centuries, with hundreds of thousands of observations, and the best computer modeling available. Space weather prediction is a new field and relies on relatively small amounts of data from just a few different satellites. Sometimes the predictions won’t be accurate, despite the scientific community’s best efforts.
We love to see space weather and the aurora reported on in the national press and we think it’s great that predictions are issued – they allow people to get ready for a potential show. However, we do all need to keep in mind that there are no guarantees with the aurora. The CME might miss us, or might not be as strong as we expected, or its magnetic field orientation might not be favourable for aurora. All of which would mean no aurora.
And then, of course, even if everything does line up perfectly – there’s always the clouds..!