Aurora was the Greek goddess of the dawn, that delicate flush of light that appears along the eastern horizon to herald the return of the Sun to the sky. But the Greeks were not sure what to make of these strange lights that occurred irregularly at night towards the Arctic and it was Galileo Galilei that first used the phrase boreale aurora to describe them, “the northern dawn.” A few years later the French astronomer Pierre Gassendi modified it to aurora borealis and the name is still in use today, with the southern equivalent known as the aurora australis.
The cause of these mysterious lights remained a mystery into the beginning of the twentieth century, although astronomers knew there was some connection between aurorae, magnetism and the Sun. Sir Edmond Halley noted that he saw his first aurora in 40 years in 1716, once sunspots had returned to the Sun at the end of the Maunder Minimum, and several scientists, including Anders Celsius and Alexander von Humboldt noted that aurora and magnetic disturbances occurred at the same time. It was not until the Norwegian scientist Kristian Birkeland spent several years studying the phenomenon during 1902-03 that the cause of aurorae was accurately described, and even then it was not proved until 1960’s when the early rockets were sent into space with a magnetometer to measure the Earth’s magnetic field in space.
There is a constant stream of energetic particles flowing from the Sun that we call the solar wind. Most of this solar wind is deflected around the Earth by our magnetic field. It’s a good thing we have this invisible shield at work, for without it the solar wind would gradually erode our atmosphere away and we would be as dry and dusty as Mars. Sometimes the Sun will spit out more particles than usual, in the form of a solar flare from a sunspot, or from a weak part of its surface called a coronal hole, and these ejections are called coronal mass ejections. These concentrated blobs of energetic particles race outwards into the solar system and slam into any body that happens to be in the way.
When they meet the Earth they can get caught in out magnetic field and funnelled down into the regions where the magnetic poles extend from the planet. This is why we get rings of aurorae around the Polar Regions, but they are not geographically centred on the North or South Pole. Both north and south will see the aurora at the same strength, with the weakest part of the auroral ring towards the midday sun and the thickest part towards the midnight side of the Earth. From the ground we may see these displays as curtains of light, moving slowly across the sky, constantly changing, maybe even bright beams reaching up out of them. From space, the orbiting astronauts can see them standing up right, following the lines of magnetic flow nearly perpendicular to the surface of the planet.
The energetic particles funnelled down towards the surface begin to interact with the oxygen and nitrogen in our atmosphere at the level of the Ionosphere, above 85km above the surface, causing it to glow in green, then red. Blue aurorae are also seen and purple colours may show for particularly strong events. This injection of energy can cause the oxygen and nitrogen to bond into nitrates, natural plant fertiliser, which then falls to the ground and near the poles it can be trapped in between snow layers. Scientists can then examine ice cores to measure nitrate levels in the past as an indication of significant aurorae events in the distant past. This is how we know the Carrington Event of 1859 was one of the biggest auroral events seen in recent centuries.
The closer you are to the magnetic poles, the more likely you are to see aurorae, but that is not the only factor to consider; you also have to wait for the Sun to eject a large amount of charged particles, or plasma, and it doesn’t do this on a regular basis. At present the Sun has a roughly eleven year cycle of activity where it forms many sunspots at its peak. Sunspots are caused by tangled knots of magnetic lines that extend out from the surface of the Sun. As they are cooler than the surrounding gaseous material they appear black, but they are still very hot, and the bigger the sunspots the more likely they are to produce solar flares. Then those flares have to be aimed more or less straight towards us, which doesn’t always happen either.
Fortunately we now have a fleet of spacecraft monitoring the Sun 24 hours a day for us from space, so we can monitor for flares and coronal mass ejections and also learn how the Sun works. The SOHO spacecraft has been watching since 1996 and as a bonus has found nearly 3000 comets. Stereo A and B, launched in 2006, watch from either side of the Sun to give us a view of the sides we can’t see. And the Solar Dynamics Observatory (SDO) joined them in 2011.
Within the interactive online article Southern Exposure: The growing, glowing allure of the aurora australis there is a nice animated series of graphics explaining how the aurorae are formed.
If you are a beginner at monitoring the Sun NASA’s Spaceweather.com is a good place to start.
The Carrington Event was documented in Stuart Clark’s “The Sun Kings” and I can recommend it as a good summary of the how we learnt about how the Sun works as well as the implications that future auroral storms could have on us today.
An in depth look at how Kristian Birkeland uncovered the true nature of the aurora is described in “The Northern Lights” by Lucy Jago.