This Is Why … auroras are only found at the poles

Photo by stein egil liland on

If you’ve ever had the pleasure of seeing the northern lights in person, you know that words do not do them justice. The auroras (aurora borealis in the north, aurora australis in the south) are beautiful and haunting, moving across the night sky as ethereal light. The displays are typically green, but occasionally you’ll see hints of red, pink, or purple. Why do we only see them at the extremes of our planet – near the poles – and why these specific colours?

Our planet, as well as others in the solar system, is constantly hit by high-energy charged particles from the Sun, known as the solar wind. Think of it as the price we have to pay for all that heat and light we get. When these particles pass through our atmosphere, some of them smash into the molecules they find there, transferring their energy to these gases. This extra energy then gets released in the form of light, with the different gases in the atmosphere giving off different colours of light. For example, green light is mainly emitted by oxygen.

But why do we typically see this spectacle close to the poles? More physics! Our planet has a magnetic field, which is incredibly useful for navigation. It turns out that it is also incredibly useful for protecting most of the planet from these solar winds. When the charged particles enter Earth’s field, our magnetic protection redirects the charges in such a way that they travel around the planet towards either the north or south pole. Instead of crashing straight through the atmosphere, partially giving energy to the gases there and generating an aurora, partially getting through to the surface, the charged particles are pushed aside like raindrops hitting the top of an umbrella. When they reach the poles, Earth’s field is no longer able to redirect the charges[1], so they plunge through the atmosphere creating this iconic luminous display.

A little physics, a little chemistry, and solar winds that have traveled 150 million kilometres become dancing ribbons of light in our far north and south.

Video by Distill from Pexels

[1] At the poles, Earth’s magnetic field is parallel/antiparallel to the direction of the charged particles’ velocity coming in through the atmosphere. When the field and the velocity are in the same direction, there is no magnetic force applied to the charged particles – they have to be travelling a bit at an angle to the magnetic field in order to experience a redirection force.

Published by joanneomeara

Professor, Department of Physics, University of Guelph

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