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 #northernlights 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:
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.