As Stormwatch team-member Steve commented to us a couple of days ago, solar storms are like buses. None for ages and then three turn up at once. This time however, we were ready for them and thanks to all of you who took the time to click on our data we were able to predict the arrival time to within 7 hours – our closest prediction yet! The Stormwatch alert warned of a storm heading in a direction that would take it within 3 degrees of Earth and that it would arrive at around 9 UT (GMT) on the 18th February.
At around 01:00 UT on the 18th, the ACE spacecraft (which sits around a million miles upstream of the Earth in the solar wind), saw a simultaneous increase in magnetic field strength, solar wind speed, density and temperature. A classic signature of a CME as detailed by Christian on this very blog a few days ago. You need to add on about an hour to these times to get the arrival time at Earth.
The ACE data for the 18th looked like this;
The heliospheric imagers on the STEREO spacecraft ‘see’ a solar storm by imaging sunlight that is scattered off the hot cloud of plasma. The more particles there are in the solar wind, the brighter it looks in our cameras. The leading edge of the storms we observe therefore corresponds with the sudden jump in solar wind density seen by ACE at around 01:00.
We are currently asking you to scale the middle of the storm trace in ‘Trace It – Incomming’ (keep doing this, it’s the best thing to do!) as it is the easiest part to track. We need to get a few more storms under our belt before we know exactly how much lead time we need to add to improve our predictions but it’s looking like 7-12 hours based on two events.
So, will this storm cause any auroral activity? Well I’d be surprised if there wasn’t anything. The efficiency with which the solar wind can dump energy into our atmosphere depends on the orientation of the solar wind’s magnetic field with respect to the Earths. If it has a northward component (Bz is positive or greater than 0) then the solar wind and Earth’s fields are similar and, like laboratory magnets, like poles repel each other and not much interaction occurs (there may be some if the magnetic field is complex, I’m just talking about the idealised situation here). If there is a southward component however (Bz negative, less than 0) the two fields can connect (space scientist call this magnetic reconnection) allowing the solar wind to enter the Earth’s magnetic field over the north and south poles where it accelerates particles into the Earth’s atmosphere, exciting the gasses there which then glow – generating the aurora.
The longer and more extreme the period of southward Bz, the more likely there is to be auroral activity and the further equatorward this activity is likely to occur.
If you look back at the snapshot of data from the ACE spacecraft (top panel, red line), you will see that initially the storm had a northward component (+ve Bz) but that this subsequently swung southwards indicating that there may be auroral activity. Keep an eye on the real-time feed at;
http://www.swpc.noaa.gov/ace/MAG_SWEPAM_24h.html
if you want to know how conditions evolve from now on.
The skies above Oxfordshire are once again their uniform grey colour so if you have clear skies, let us know if you see any aurora in the next few days. Midnight is a good time to be looking since this is when you will be closest to the energetic particles being thrown into the upper atmosphere by the Earth’s magnetic tail as it snaps back after being stretched to breaking point by the solar wind.
Congratulations on a very successful prediction. Let’s hope we have a few more before the STEREO spacecraft move too far from the Earth. We want to hone our skills before we start making space weather predictions for Venus, Mars Jupiter and Saturn later in the mission, but more about that later… 😉
Thanks once again for all you time, efforts and enthusiasm,
Chris.
The Solar Stormwatch Blog 