You’ve probably noticed all the stories floating around recently about the Sun’s increase in activity and auroras being visible in places that they usually aren’t. It’s all been pretty exciting. Especially if, like me, you’ve always wanted to see the northern lights and there was a (very small, but still non-zero) chance of the phenomenon being visible in your home town.
In light of this (no pun intended), I decided a blog post about the science behind auroras was in order…
What exactly is happening with the Sun at the moment?
The Sun goes through cycles, each lasting around 11 years. During this cycle, its magnetic field increases and then decreases again. The magnetic field of the Sun is the source of its “activity” – a term which describes solar phenomena like sunspots, faculae and prominences. Activity can also come in the form of coronal mass ejections (CMEs). These are huge bubbles of material with diameters a few times that of the Sun(!), that explode into space, releasing billions of tons of plasma.
A couple of years ago the Sun’s activity was at an exceptionally low and long-lasting minimum, but since then it’s been increasing and we’re heading for a maximum in 2013. This means lots more activity is on the horizon: near a solar minimum we get around one CME a week, near a maximum this increases to two or three per day.
What has this got to do with the northern lights?
The northern lights (aka aurora borealis) are an amazing display of green and sometimes red light seen near to the magnetic north pole, and they’re caused by CMEs. Their southern equivalent occurs near the south pole, and is known as aurora australis.
After a CME erupts from the Sun, it can interact with the solar wind and cause huge interplanetary shock waves that go on to reach the Earth. When particles from the solar wind get to Earth, they are channelled down our planet’s magnetic field lines and end up accelerating towards the magnetic north and south poles. These particles then interact with atoms and molecules in our atmosphere and excite them, causing them to release photons. It is these photons that make up the light we see in the sky during an aurora.
For for information about the CME pictured above, and a video, see here.
Also, this BBC News article has a good illustration showing the solar wind’s interaction with the Earth’s magnetic field.
Images: Top, U.S. Air Force photo by Senior Airman Joshua Strang. Bottom, NASA.