A disappointing night

/ 2017-07-17

We had such high hopes. Predictions were good, the best we’d seen for a while, and the early stats were promising but the aurora fizzled out before we had a chance to see it.

Our long summer evenings played a small part here. With so few hours of darkness, catching a sighting of the aurora was always going to be a little harder than during those long, cold winter nights. Here in Lancaster, civil twilight ended at 22:23 and began again at 04:12. For those of you further north, darkness was even shorter.

As shown by the black lines below, our activity status was low throughout the hours of darkness. Only just reaching yellow for a few hours. (Remember: we’re in British Summer Time at the moment which is UTC +1 hour.)

AuroraWatch UK activity level for 0700 UT on 16 July 2017 to 0700 UT on 17 July 2017.

But it wasn’t just summer’s fault.

In the graph below, the hours of darkness are again shown by the vertical dashed black lines. The first thing we can see is Bz, shown in red in the top panel, has completely flat-lined. Bz was strongly negative earlier in the day but, during our hours of darkness, sat stubbornly around zero.

Remember that a negative (or “southward”) Bz (the z-component of the interplanetary magnetic field) is very important for the aurora. A negative Bz allows the energy from the solar wind to be transferred into the Earth’s magnetosphere which generates the aurora.

Solar wind data, recorded by the DSCVR satellite, and presented by the NOAA Space Weather Prediction Center (SWPC).

We can also see that the density (orange) and speed (purple) both drop off slightly too. While not quite as important as a negative Bz, these both play a part in generating the aurora.

An underwhelming CME strike

The predictions were that a coronal mass ejection (CME) would strike the Earth at around 22:00 UT on 16 July 2017.

Predicted CME strike (model run by NOAA SWPC) on 16 July 2017. Labels by Dr Tamitha Skov.

But modelling CMEs is hard and fraught with uncertainties. As it turns out the CME arrived a little early and really only gave the Earth a “glancing blow” rather than a knock out punch.

Some did get lucky

Some around the globe were lucky enough to see the aurora, particularly when the CME first arrived on the 16th, but I’ve not yet seen a convincing shot from the UK.

Last night acts as a reminder that predicting aurora is not easy and there are never any guarantees. Sometimes we just have to sit and keep our fingers crossed!

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