This is the first blog post from Dan Billett, a Ph.D. student in Lancaster’s Space and Planetary Physics group. Dan has just joined the AuroraWatch UK team and is interested in public outreach so you will, no doubt, be hearing more from him in the future! — Nathan.
On Monday 5th September, NOAA’s Space Weather Prediction Center posted this update referring to a coronal hole high speed stream (CS HSS); a fast stream of solar material which formed a good connection with the Earth’s magnetic field. What has this connection meant for us? Aurora sightings from the UK, of course!
AuroraWatch UK alert status
Over the last few days we have had a few spots of geomagnetic activity (we wrote about the first here) with activity reaching our amber level on more than one occasion, suggesting that aurora might have been visible in Scotland, the north of England and possibly elsewhere. As of writing this post (5th Sept), activity has spiked yet again to the amber level.
The K index (a measure of geomagnetic fluctuations) managed to get all the way up to 5 on the 4th Sept, indicating the presence of a geomagnetic storm.
Solar Wind data
Now we come to looking at what the solar wind has been doing over the last few days. Although the z (north-south) component of the interplanetary magnetic field (IMF) shown by the red line has not been that strongly negative (which is one of the best indicators of aurora) for long, notice that the solar wind speed (given by the purple line) has nearly doubled over the course of barely a day, reaching a top speed of around 800km/s (typically around 300-400km/s).
This significant uptick in speed is because of the fast material being ejected out of the CS HSS arriving at Earth, after its 93 million mile journey. It’s slowed up a little bit now, but these high speeds make up for the short periods of negative IMF Bz to make some aurora still happen.
Not the best few days recently with rain and clouds being much the norm throughout the UK, but the force of the coronal hole high speed stream seems to have pushed some aurora through anyway during times of clear skies:
— Promote Shetland (@PromoteShetland) September 3, 2016
Ythan Estuary, Scotland
— Allan McIntosh (@allanmcintosh12) September 3, 2016
— PK Perspective (@PKPerspective) September 5, 2016
Ballykelly, Northern Ireland
Sightings yet to come?
The immediate effect of this coronal hole is now passing with the solar wind speed dropping steadily. However, the Sun rotates with a period of approximately 27 days and so, if we’re lucky, we may just see the same coronal hole, and its high speed stream, again in just under a month. A lot of things can change in that time though, so we can’t be sure.