Thanks to everyone for their efforts in tracking our first real-time Earth-directed solar storm. We have been very impressed by the care with which you have all been analysing our data. So, what happened? Well, from your clicks, we predicted a storm would reach Earth around 07:00 on the 13th of December. Meantime the official spaceweather forecast issued by NOAA gave the following warning;

Space Weather Message Code: WARK04
Serial Number: 1681
Issue Time: 2010 Dec 12 1708 UTC
WARNING: Geomagnetic K-index of 4 expected
Valid From: 2010 Dec 12 1710 UTC
Valid To: 2010 Dec 13 0700 UTC
Warning Condition: Onset

The K-index is a measure of the amount of disturbance in the Earth’s magnetic field (as it is buffeted by the solar wind). A little later NOAA sent confirmation that the Earth’s field was indeed being affected;
Space Weather Message Code: ALTK04
Serial Number: 1488
Issue Time: 2010 Dec 12 1742 UTC

ALERT: Geomagnetic K-index of 4
Threshold Reached: 2010 Dec 12 1741 UTC
Synoptic Period: 1500-1800 UTC
Station: Boulder
Active Warning: Yes

(I’d encourage you to keep an eye on the professional forecast issued by NOAA at http://www.spaceweather.gov)

So, it seems that something arrived at Earth around 14 hours before our prediction. The effect wasn’t very big but was this disturbance caused by the same storm that we were tracking? I think so, and I’d like to talk you through why I think that.

It is true to say that not much happened at Earth as a result of this storm but just imagine standing in front of someone who was about to throw a baseball at you from a distance. The difference in angle that the pitcher would have to throw that ball between it sailing harmlessly by and alternatively hitting you on the nose is very small but the two scenarios would have dramatically different consequences! OK so this analogy breaks down when you consider that the storm expands as it travels out from the Sun and is very large by the time it reaches the distance of Earth but the principle stands. It doesn’t have to miss by much for it to have little or no effect at all. To stretch this analogy a bit further, I think this storm just brushed by our ear! It’s also difficult to tell ahead of time just how effective a storm will be (in my now horribly tortured analogy, we don’t know whether it’s a baseball or a beachball – OK, that poor analogy has suffered enough, I’ll stop hurting it now).

The reason I think our timing was out for this event was that we ask you to scale the middle of the features in the j-maps (keep doing this, it’s the best way!) so this would mean we are not tracking the absolute front of the storm (which would arrive at Earth a little earlier). Once we have a few predictions under our belt, we’ll know what this offset is (and 12 hours is not a bad estimate from the experience we have so far) and will compensate accordingly.

But how do we know that there wasn’t much of an effect at Earth? Well, one way is to look at data from the Advanced Composition Explorer (ACE) spacecraft. This sits about a million miles upstream from Earth in the solar wind and ‘tastes’ the wind as it goes by. A solar storm usually contains a giant bubble of magnetic field that sweeps up material ahead of it while material from the solar atmosphere expands into the void behind. ACE has many detectors but a useful summary of solar wind conditions over the last seven days can be found at;

http://www.swpc.noaa.gov/ace/MAG_SWEPAM_7d.html

This is a rolling summary of solar wind conditions just upstream of the Earth. These plots are a somewhat confusing collection of wiggly lines that, between them, can tell us if anything unusual is happening. The top panel contains the total magnetic field strength (Bt) and the size of the component that is aligned to the Earth’s magnetic field (Bz). If a magnetic bubble goes by, we’d expect Bt to go up and if Bz was negative at the time (southward) it would mean that the solar storm and Earth’s magnetic fields were aligned in opposite directions – ideal for the two fields to merge and let the solar wind into the Earth’s atmosphere to cause an aurora. On the afternoon of the 12th, the total field does indeed increase in strength but the Bz component is positive for the most part, meaning that the Earth and solar wind magnetic fields were aligned – remember that two aligned magnets repel each other while opposites attract. Not surprising then that there wasn’t much activity.

As a storm goes by ACE, we’d also expect a sudden increase in the solar wind speed and density as a storm front passes (two of the lower panels are marked speed and density). While the speed and density did increase, they didn’t go up by very much and there were no sudden jumps indicating that the storm was not travelling fast enough to generate a shock ahead of it.

Another way of investigating whether the Earth’s magnetic field was rattled by the event is to look at measurements by the aurorawatch team at;

http://www.dcs.lancs.ac.uk/iono/aurorawatch/rt_activity

Their page shows the current state of the Earth’s magnetic field over northern Europe. When a solar storm hits Earth, the very least it does is cause the Earth’s field to be compressed and wobble. This shows up on the ground by very small compass movements. The aurorawatch team run a series of sensitive magnetic field monitors (called magnetometers) and present their data as a series of graphs. Basically, a flat line means a quiet magnetic field and a wiggly line means that the Earth’s field is being buffeted by the solar wind. On the day of our prediction the aurorawatch plots showed little or no disturbance to the Earth’s magnetic field over Europe (in contrast to the activity seen in Boulder, Colorado).

So, while this was not exactly ‘the perfect storm’, I think it did enough to show that our techniques work and that you are all doing a fantastic job in your careful analysis of the STEREO HI data.

In my geekier moments (which occur far too often to be healthy) I like to think that the pioneering space-weather forecasters of today are not only helping us to protect our technological infrastructure here on Earth but are also blazing the trail for the future exploration of the solar-system. Your efforts with solar stormwatch are an important part of this, and I’m really grateful for your time, careful analysis and enthusiasm in helping us with our work.