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What are the northern lights?

The northern lights, or aurora borealis, are a spectacular, colourful display of light commonly seen in the night sky in the northern hemisphere. Auroras in the southern hemisphere are known as the southern lights, or aurora australis.

Both the northern lights and the southern lights are polar lights, or aurora polaris, because they occur near Earth's magnetic poles.

AuroraMAX – The Northern Lights, NWT

Aurora borealis over Yellowknife, NWT. (Credits: Canadian Space Agency, University of Calgary, Astronomy North)

Since the International Space Station is at about the same altitude as the polar lights, astronauts see them at eye level! Here is an example.

How auroras form

Auroras occur when charged particles (electrons and protons) collide with gases in Earth's upper atmosphere.

Those collisions produce tiny flashes that fill the sky with colourful light. As billions of flashes occur in sequence, the auroras appear to move or "dance" in the sky.

Earth's magnetic field steers the charged particles towards the poles. The shape of Earth's magnetic field creates two auroral ovals above the North and South Magnetic Poles. That is why auroras occur almost every night in the northern sky, from August to May.

Solar wind

Our planet's magnetic field forms an invisible shield that protects us from the solar wind. From time to time, the solar wind gets stronger and penetrates Earth's magnetic field. The stream of particles interacts with gases in the magnetic field (the magnetosphere), generating magnificent auroras.

Earth's magnetic field spans across thousands of kilometres in space.

As a rule, the more active the Sun is:

When solar activity decreases, the oval returns to its normal position and the auroras become less intense.

Also, because of the solar wind, sometimes the magnetic field lines reconnect on the side of Earth opposite the Sun. They snap back like an elastic band, sending large amounts of energy back towards Earth's poles. This phenomenon, called magnetic reconnection, creates stunning displays of aurora.

A coronal mass ejection (CME) is an enormous expulsion of plasma and magnetic field from the Sun's outermost atmosphere. This animation traces the path of a CME as it leaves the Sun and slams into Earth's magnetosphere. The magnetic field lines reconnect behind the planet and produce the polar lights. (Credits: ESA, NASA)

A colourful phenomenon

Green, red, blue: the northern lights come in a variety of colours.

Go to The colours of the northern lights page to find out more.

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