Definition
An aurora is a natural light show that happens when electrically charged particles, sped up by energy from the solar wind and Earth's magnetic field, pour down along magnetic field lines toward the polar skies and crash into oxygen and nitrogen gas in the atmosphere, making them glow.
Particles from Space and Earth's Magnetic Shield
When electricity runs through a neon sign, the gas inside the tube absorbs the energy and instantly glows. An aurora works the same way. The whole night sky becomes one giant, natural neon sign.
The Sun doesn't just give off light and heat. It also blasts out huge numbers of tiny electrically charged particles in every direction. Scientists call this stream of particles the solar wind. If these fast particles hit Earth's surface head-on, life here would be in serious danger.
But Earth has an invisible shield. Our planet is one big magnet, so a magnetic field wraps around it in a huge bubble. This magnetic shield deflects most of the dangerous particles. Still, some of the solar wind's energy leaks into the magnetic field and speeds up the charged particles trapped inside. These particles then travel along the magnetic field lines and funnel down into the skies in a ring around the magnetic poles.
A Light Party of Gases in the Sky
Once the charged particles ride Earth's magnetic field down to the high polar sky, they slam into the atmosphere at incredible speed. There they smash into air molecules like oxygen and nitrogen.
At the moment of impact, the air molecules take on a burst of energy and become "excited." Think of a kid who just had too much sugar. But they quickly want to settle back to their normal state. As they calm down, they let go of that extra energy as a glowing flash of light.
The color depends on which gas gets hit and how high up the crash happens. Oxygen at about 100 to 200 kilometers (60 to 125 miles) up gives off the soft green glow you see most often. Oxygen above 200 kilometers (125 miles) glows a vivid red. Nitrogen mostly adds blue and purple, and it tints the lower edge of the "curtain" pink, finishing the mysterious veil.
A Closer Look
Auroras aren't just an Arctic thing. The same beautiful lights appear over Antarctica too, where they're called the aurora australis, or southern lights. That's because particles pour in through both openings of Earth's magnetic field.
In physics, this glow is explained by energy level transitions in atoms. An electron in an atom gains energy from a collision and jumps up to a higher energy "step." When it drops back down to a lower step, it releases the difference as a packet of light called a photon. It's a quantum mechanical process.
Here's one more important point: auroras are not caused by cold winter weather. They aren't sunlight bouncing off ice crystals when temperatures drop below freezing. They happen more than 100 kilometers (60 miles) above the ground, near the edge of space. How big and bright an aurora gets depends not on the weather but on the strength and speed of the solar wind, and on solar activity like coronal mass ejections.
Common misconceptions
Auroras are icy reflections caused by the brutally cold weather and snowfields of the polar regions.
Auroras have nothing to do with weather or temperature on the ground. They're a physical effect that happens in space, more than 100 km (60 miles) up, when charged particles meet Earth's magnetic field and atmospheric gases.
Where you meet it
When a coronal mass ejection, a huge blob of plasma blasted out of the Sun's atmosphere, heads toward Earth, it can trigger a much bigger and brighter aurora storm one to three days later. It can even be seen at lower latitudes than usual.
Other planets with strong magnetic fields and atmospheres, like Jupiter and Saturn, have giant auroras at their poles too. Space telescopes can observe them.
In one sentence
An aurora is a cosmic light show. Charged particles sped up by the solar wind's energy follow Earth's magnetic field to the poles and crash into the atmosphere, making it glow.