The ancient Greenlanders thought the Northern Lights were a sign from heaven that their ancestors were trying to contact the living. The Norwegians saw them as old maids dancing. However, modern science tells us that the Lights are created by charged solar particles ejected from the Sun in a solar wind during explosions or flares. If these particles reach the Earth – a journey that takes two or three days – they are diverted around the planet by the Earth’s magnetic fields.
What does Aurora Borealis mean?
While translated from Latin as ‘northern dawn’, there’s some dispute as to who first coined the expression. Some attribute it to Italian scientist Galileo Galilei who saw the major aurora of 12 September 1621; others claim French astronomer Pierre Gassendi first used the phrase following the same event. Either way, the reference to dawn alludes to the red colour that is seen in the aurora at the lower latitudes at which both men would have observed this impressive display.
What causes the Northern Lights?
Most of the solar wind travels around the Earth and disappears into space. Some particles, however, enter the Earth’s upper atmosphere at its polar regions. As they do so, they collide with atoms and molecules there, which absorb some of the solar particles’ energy. This is referred to as ‘exciting’ the atom or molecule. In order to return to their normal state, the atoms and molecules emit photons, or light particles. The Northern Lights are therefore an actual source of light, not a reflection of sunlight as some people suppose. The colour of the Northern Light depends on which gas the solar particles collide with. In the auroral displays that are most commonly to be seen with the naked eye, oxygen emits a greenish-yellow burst of light, while the photons produced by nitrogen are crimson. This activity takes place in the Earth’s high atmosphere, at 100 kilometres or more above the planet’s surface.
Magnetic versus Geographic Poles
The Earth’s magnetic field lines, which help to create the Northern Lights by diverting solar particles towards the poles, run between its magnetic rather than geographic poles. So, while the geographic North Pole sits permanently at 90° latitude, and is the point at which all the lines of longitude converge, the Earth’s magnetic north pole moves: it is influenced by the perpetually shifting molten iron that makes up the Earth’s outer core. At the time of writing the north magnetic pole is in the Canadian Arctic, and is calculated to be drifting towards Russia at a speed of about 60km per year.
The Auroral Ovals
The convergence of the Earth’s magnetic fields around the magnetic north and south poles lead the Northern Lights to exhibit in auroral ovals, or rings around the top and bottom of the Earth. If you want to see the Northern Lights, you therefore have to be beneath, or within sight of, the auroral oval. The auroral ovals stay in a fairly fixed location in space while the Earth rotates beneath them. When levels of solar activity are low, the northern auroral oval sits between 60° and 70° of latitude; it is wider on the night-time side of the Earth. During a very violent solar flare, however, the auroral oval fattens or bulges and the aurora can be seen from lower latitudes. However fat or thin, the auroral oval is always present but can only be seen in its entirety – as a ring – from space.
The Sunspot Cycle
Sunspots are dark blotches on the Sun’s surface that are cooler than the surrounding regions (though they’re not exactly cool if a sunspot could be suspended in the night sky it would still provide more light than the full moon). Sunspots are temporary phenomena, which contain concentrated magnetic field lines; this intense magnetic activity prevents them heating to the same temperature as the rest of the surface of the Sun.
Solar disturbances are greater when sunspot numbers are high and so, in turn, we see more intense and active Northern Lights when sunspots are common. Sunspots come and go in ‘sunspot cycles’ which peak about every 11 years. During these peaks, solar flares are more common and more energetic, and so the Northern Lights though visible at all stages of the sunspot cycle tend to be more frequent and intense at these times.
Additionally, the years following a peak see increased Northern Lights activity. This is because coronal holes (areas of the Sun that are low in density and have open magnetic fields – the magnetic fields lead out into space rather than looping back into the Sun, and so allow the solar wind to escape from the Sun) form during sunspot cycle peaks, and they too lead to greater auroral activity.
The most recent maximum sunspot cycle peak started in 2011 and has gone on longer than usual. This makes is difficult to predict when the next maximum exactly will be.
The Different Shapes of the Northern Lights
The aurora can be seen as undulating ribbons and shimmering curtains, spiky fingers that creep up from the horizon and dazzling rods of colour that burst like a firework from a single point high in the sky. The different shapes are partly caused by the position of the aurora in relation to the magnetic zenith – the point in the sky that you can see if you look along the line of the Earth’s magnetic field. If the aurora runs across the observer’s magnetic zenith, the observer will see waves or lines converging into a single spot.
If the aurora is some distance away from the observer’s magnetic zenith, it will look more like a two-dimensional curtain or line. It works a bit like perspective when you stand at the foot of a tall building: looking directly upwards, you see the nearest building’s walls converge more sharply than those of buildings some distance away.