Aurora borealis above Bear Lake in Alaska. Credit: {link url=""}U.S. Air Force photo by Senior Airman Joshua Strang{/link}

The Sun is hotting up, and we can see the results right here on Earth. Across the northern hemisphere, fantastic light displays have been visible of late, and the frequency of these events is set only to increase as the Sun heads toward a peak in its magnetic activity.

In light of this (no pun intended), I decided a post about what is going on during an aurora was in order.

What exactly is happening with the Sun at the moment?

A coronal mass ejection that occurred on 1st August 2010 and caused a spell of aurorae that summer. Credit: {link url=""}NASA/STEREO{/link}

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 charged particles, or plasma.

A few 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 early 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.


This BBC News article has a good illustration showing the solar wind's interaction with the Earth's magnetic field.

This post is a slightly modified version of one that appeared at my old blog in August 2010.