How Light Works: What causes the northern lights?

Added: 19/08/21

The arctic circle, illuminated in fairy blooms of green, blue and red, has always been a source of folklore and legend. But the physics and chemistry that explains the phenomenon is equally magic…

The ribbons of coloured light we see is caused by electrically charged particles released by the sun, entering the Earth’s upper atmosphere at a high speed. The sun is constantly emitting a stream of electrically charged particles, called the solar wind.

As the Earth makes its orbit around the sun, some of the particles from the solar wind are intercepted by the planet. Of these, the vast majority are deflected by the Earth’s magnetic field and continue travelling through deep space. The small number of particles that make it through the Earth’s magnetic field, cluster around the North and South poles.

But where does the light come from? The charged particles clash with the atoms and molecules high up in our atmosphere, and the atoms get excited.

They then begin to decay - essentially to release the excess energy they had accumulated - to their original make up, and as they do so, they emit distinctive colours of light. The frequency of lightwave emitted, and the colour we see, is determined by which type of elements they encounter in the atmosphere. The most common colour seen in the Northern Lights is green. Green light is emitted when the particles interact with oxygen atoms.

The red light we sometimes see is also caused by oxygen atoms. But these oxygen particles are higher up in the atmosphere, producing lower energy red light. Fascinatingly, this red light is always produced, but we don’t always see it as our eyes are significantly less sensitive to red light than green. You might sometimes notice an orange haze around the striking green light in the Aurora - that’s why! 

One of the other major elements in the Earth’s atmosphere is nitrogen. The particles from the solar wind have to collide with nitrogen atoms at a much higher speed in order to react with them. Nitrogen emits purple light when they decay, but this is rarely seen.

The northern lights are most active during the Equinox and Solstice in March, and in September. Auroras usually occur in a band called the annulus centred on the magnetic pole. But occasionally a larger flare of charged particles from the sun, called a Coronal Mass Ejection, can cause the annulus to expand. The aurora becomes visible at lower latitudes and can be seen in the northern regions of the UK.





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