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).
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.
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.
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.
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.
— Steve White (@spwhite2) October 13, 2016
“Last night’s aurora” captured by Nigel Barry and shared from Flickr.
— 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.