What causes an Aurora?

Last week saw the bright lights of the Aurora Borealis dancing across the skies of some of Great Britain due to two large solar flares. Aurora comes from the Latin word for sunrise. Although they are commonly known as the Northern Lights (Aurora Borealis) that is only when they appear in the Northern Hemisphere. When in the Southern Hemisphere, the southern lights are known as Aurora Australis. But how is nature’s theatre created?

The Aurora are caused when electrically charged particles from the sun, in the form of a solar wind, enter the atmosphere. The ions are trapped in the magnetic field that surrounds the Earth. The solar wind then causes the magnetic field to move away from the planet then move back pushing the charged particles into the atmosphere. The collisions between the particles and the magnetospheric atoms in the atmosphere leads to the release of energy forming bright lights.

They are usually seen at the Northern or Southern poles due to the acceleration of particles in the solar wind towards the magnetic poles. The solar wind distorts the magnetic field allowing particles to enter the atmosphere at the poles. The charged solar particles interact with gases that make up the atmosphere causing the dancing glow that is associated with the Northern Lights. They are seen most brightly during the spring or autumn equinoxes.

Auroras can appear as different colours, depending on which gases in the atmosphere they interact with. The more common green aurora colour is due to our eye’s sensitivity to green. They appear at lower altitudes with high concentrations of oxygen which reacts with excited nitrogen and radiates green energy. Red appears at high altitudes caused by excited oxygen atoms. The eye is less sensitive to red and there are fewer oxygen atoms at high altitude causing the red light to be faint.

There are two different defined types of aurora. Diffuse auroras are more difficult to see as they are dimmer and don’t have sharp, defined outlines. The bright dancing lights that we associate with the Northern Lights are discrete auroras, caused by parallel acceleration of electrons down the magnetic field.

Next time you look up at the sky hoping to see the pulsating glow of the Aurora Borealis, remember that you are looking at the collisions of particles of the sun with our own atmosphere creating the stunning light show.

Jessica Hewitt-Dean

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Image: Joshua Strang


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