Strong geomagnetic activity was recorded during the night of 13-14 October, resulting in an amber alert being sent out to our subscribers. High activity lasted for several hours and nearly reached our threshold for a red alert (falling just short of the required 200nT, maxing out at 190nT).

Geomagnetic activity over the night of 13-14 October nearly resulted in a red alert. Note: time is in UT (one hour behind BST).

The activity was the result of a Coronal Mass Ejection (CME): a burst of solar material that had exploded off the Sun several days earlier. As can be seen from the ESA/NASA SOHO mission, the halo CME looked quite wispy (it’s that faint white cloud coming off the Sun). Notice the outward (radial) direction of travel, from the spacecraft’s perspective, creates a halo-like appearance around the Sun…hence the name.

The halo CME erupted from the Sun on 09 October before travelling 150 million kilometres to Earth

Being southward pointing (negative red in the top panel of the below plot), the interplanetary magnetic field (IMF) direction contained inside the CME was favourable for aurora. The speed of the CME (purple) was a little slower than we’d have liked though for a really strong auroral display. The very high speed at 20-2200 UT on 13th October was a glitch and not real measurements.

Solar wind data for 13-14 October. Provided by NOAA’s DSCVR satellite.

The southward turning in the IMF actually happened much earlier (at around 0500 UT on 12 October) than we started detecting geomagnetic activity in our magnetometers (1400 UT on 13th October). The consistency of the southward direction allowed a lot of energy to be transferred from the solar wind into the Earth’s magnetosphere (the magnetic bubble surrounding us).

The “hidden” aurora

So conditions in the solar wind were looking good, and geomagnetic activity was being recorded by our magnetometers – yay! Unfortunately, the Great British weather let us down.

Cloud cover at 1700 UT (1800 BST) on 13th October. Image from weatheronline.co.uk

Most of the aurora captures that we did see were obscured by clouds or the rather bright moon. But here are a few of our favourites:

John Purvis captures two auroral arcs, a rare feat from the UK, from Ballykelly, Northern Ireland.

John Purvis captures multiple arcs from Northern Ireland. CC BY-NC-SA 2.0

@aurorawatchuk Couple of brief glimpses tonight from Barassie beach, Ayrshire looking over towards Arran. pic.twitter.com/NIOszGuWb4

— Steve White (@spwhite2) October 13, 2016

“Last night’s aurora” captured by Nigel Barry and shared from Flickr.

Faint so far, and fighting the clouds, but a bit of an #aurora in Lancashire tonight. pic.twitter.com/TddgbxGCuk

— Geraint Johnes (@geraintjohnes) October 13, 2016

Unsettled conditions continue

While not likely to be as strong as last night, unsettled conditions are predicted to continue. If the predictions are true and the sky is clear – so keep an eye out for our alerts.

A reminder about Kp

We saw a lot of discussion last night about the Kp index – which, at one point, was predicted to reach an astonishing level of 8.6 out of 9 (link for past 7 days – prediction for 13th October will be gone come 21st). But predictions aren’t always accurate and the Kp index never actually reached this level with real-time estimates maxing out at 6. So always take the predicted values with a pinch of salt. Also, Kp is not a very good indicator of auroral activity – especially if you’re interested in local (i.e. UK) activity.