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Friday, February 18, 2011

Storm in a Teacup - some optimism?

So the big Coronal Mass Ejection (CME) hit and generated... no aurora over the UK.
So with all the commentary and coverage why did we not see aurora?

As my colleague Jim Wild said on Twitter:

Some media coverage suggested that were a certainty last night - not the case! But it was the best chance for a few years!

Why were some optimistic about the chances (including me) and why did we not see anything?

It all comes down to how the CME hit the Earth's magnetosphere. The CME is a large cloud of plasma (electrons and ions) that travels through the solar wind (also a plasma). Besides the particles it also carries with it a magnetic field and this is the important bit.

If that magnetic field is orientated with a significant component pointing southward, something cool can happen when the CME reaches the Earth: the magnetic field from the CME can merge or 'reconnect' with the Earth's magnetosphere.

Reconnection allows energy and matter to transfer from the CME into the magnetosphere. In general the stronger the southward magnetic field of the CME, the stronger the reconnection. Another factor is the speed of the CME; the higher the speed, the bigger the impact and also the stronger the interaction.
Of course the magnetic field in a CME can be very complex, but sometimes we see a nice simple pattern with the field forming a great, closed loop such that for a time the field points north(south) before flipping south(north). Sometimes it is just much more complicated than that - this stuff isn't easy you know.

So what happened with the CME today?

Let's look at the data from ACE, a solar wind monitor sat upstream of the Earth:

These data are provided in real time by the Space Weather Prediction Center, NOAA.

The top panel shows the magnetic field strength (white) and the north-south magnetic field component (red). Straight away we can see that the field strength went up (which is good) but that most of that was pointed northwards (positive), there were a couple of little dips below zero (southward) but you really need it to stay negative for about 40-60 minutes even to generate decent auroral displays at high latitudes.

The fourth plot down is the solar wind speed (yellow). This jumped from a very slow 350km/s to a moderately fast 500km/s when the CME passed ACE. This is not really fast in the great scheme of things; the big storm in 2003 that produced aurora over Boulder, Colorado (USA), was triggered by a CME moving at over 1000 km/s. When Aurora was visible over Lancaster UK in January 2005, the CME was moving at over 800 km/s.

basically, in my opinion, this CME was
a) not moving fast enough
b) did not have enough southward magnetic field

to generate a good display over most of the UK.

Anyway, it was cloudy.

However. Keep on eye on the activity levels as something a bit funny is happening. The speed has ramped up (though its a bit all over the place - instrumental problem?) to close to 700 km/s and the fact there has not been much in the way of southward field makes me wonder if that may be yet to come (though the total field strength is dropping).

So, maybe there might still be some aurora later on tonight if we are lucky.

Of course the really nice thing about this event is that its an indicator of things to come. We are cruising towards solar maximum and things on the Sun are hotting up after a long solar minimum. Chances of UK aurora are going up all the time.

Postscript(3:44pm): A good article for Channel 4 news.
For about 3 hours from 12:30 today the magnetic field in the solar wind (or Interplanetary Magnetic Field) has been negative (add about 40 mins for travel time from ACE).

It started off at around -10 nT before dropping steadily to zero. This was probably enough to trigger some decent activity at higher latitudes but I doubt it will stretch too far south.

So if you are at high latitudes with clear skies and in a suitable time zone (i.e. night) go outside and have a look.

For those of us in the UK, the IMF is still a teensy bit southward and so who knows maybe if things freshen up again we might still get lucky later. Chances are slim though.

Thursday, February 17, 2011

Storm or no storm?

Solar flares and the possibility of seeing the Aurora Borealis over the UK have been in the news today. It stems from the fact that on Tuesday (at about 2am) we had the biggest solar flare that we have had for something like 4 years. It was what we call an X-class X-ray flare. that means the energy flux was greater than 0.001 Watts per square metre. You can see the recent flare on the plot below, which is courtesy of the excellent Space Weather Prediction Center, part of NOAA.

From the right-hand scale you can see it tipped over into the X-class band and it was what we would call an X2 flare.

Associated with this flare was a CME - a coronal mass ejection. This big magnetic cloud is currently travelling through interplanetary space and our best guess is that it is heading straight towards Earth. If and when it hits it could trigger a geomagnetic storm. If the CME has a large southward magnetic field component it might lead to conditions ripe for aurora over mid-latitudes (i.e. the UK).

Note the important words in italics.

It is great that the Sun is being active again as we head up towards the next solar maximum. The long solar minimum was pretty geophysically interesting but at the same time its nice to have a chance of seeing aurora at mid-latitudes; and chances of doing so do go up at solar max.

Clearly it is nice that the aurora is getting a mention in the media but I have been a leeetle bit concerned about some of the coverage. I am concerned (rightly or wrongly) that the impression is that we will see some aurora. To be sure this is the best chance for quite some time and it is an encouraging event but it is not a foregone conclusion. For example take the title of this BBC news story

Aurora Borealis to light up the night sky

The story itself is a bit more balanced but the headline is worrying. On the other hand, here is a really good, balanced story by Paul Rincon, also on the BBC.

Now as of yet the CME from the X2 flare has not hit and so the chances for aurora viewing are still there and still reasonable. Though if it hits in the morning, as solar storm watch predicts, we have to hope that the auroral oval stays at mid-latitudes for quite a few hours.

This may be a bit of a tall order.

Postscript: it is not a strictly linear relationship but it is worth bearing in mind that the last time aurora was visible over the north of England it followed an X7 flare. So I suspect that the best place to see aurora in the UK will be Scotland and Northern Ireland.