What causes the northern lights?
Quick answer
The northern lights, or aurora borealis, happen when charged particles from the Sun stream toward Earth, get guided by Earth’s magnetic field toward the polar regions, and collide with gases in the upper atmosphere. Those collisions make the gases glow: oxygen gives green and red, and nitrogen gives blue and purple.
The light show starts about 150 million kilometers away. The Sun constantly gives off a stream of charged particles called the solar wind, and sometimes blasts out much larger clouds of them during solar storms.
How it works, step by step
- The Sun releases charged particles, mostly electrons and protons.
- Earth’s magnetic field deflects most of them, but channels some toward the regions around the North and South Poles.
- The particles collide with oxygen and nitrogen about 100 to 300 km (60 to 190 miles) above the ground.
- The gases release energy as light, like the glow inside a neon sign.
Why the colors differ
| Color | Caused by | Typical height |
|---|---|---|
| Green (most common) | Oxygen | About 100–250 km |
| Red | Oxygen, very high up | Above about 250 km |
| Blue and purple | Nitrogen | Lower edges, often under 120 km |
| Pink | A mix of red and blue at the bottom of curtains | Varies |
Green is the most common because our eyes are most sensitive to it and the oxygen that produces it is plentiful at that height.
Where and when to see them
- Where: Under the “auroral oval” around the Arctic: northern Norway, Sweden, Finland, Iceland, Greenland, northern Canada and Alaska.
- When: Long, dark nights from September to March, away from city lights, on a clear night. Activity often peaks around the equinoxes.
- How strong: Forecasts use the Kp index from 0 to 9. Higher numbers mean the aurora may be visible farther from the poles.
During strong solar storms, auroras can reach much farther south. In May 2024, a powerful storm made them visible across much of Europe and the US. The Sun runs on an 11-year cycle, and the recent peak means strong displays remain more likely than usual for the next few years.
The particles start at the Sun’s surface, which is about 5,500°C. Here’s how hot the Sun is in every layer.
Sources
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