What causes the aurora
From the Sun to your sky in four steps: solar wind, magnetic field, falling particles and glowing oxygen.
1. The Sun blows
The Sun constantly sheds a thin stream of charged particles, the solar wind, at 400–800 km/s. Fast streams from coronal holes and huge clouds from coronal mass ejections (CMEs) are what make strong aurora; a CME takes one to four days to reach us.
2. Earth’s magnetic field catches it
The solar wind carries a magnetic field. When that field points south (negative Bz), it connects with Earth’s field and pours energy into the magnetosphere, our magnetic bubble. Energy builds up on the night side and is released in bursts called substorms: the sudden brightenings and dancing curtains.
3. Particles rain down near the poles
Electrons accelerated down the field lines hit the upper atmosphere in a ring around each magnetic pole, the auroral oval. The stronger the disturbance (higher Kp), the wider the oval grows towards the equator.
4. The air glows
- Green (most common): oxygen at about 100–250 km.
- Red: oxygen above 250 km, often the only colour seen far from the oval in big storms.
- Purple and pink fringes: nitrogen, at the lower edge of bright curtains.
Why spring and autumn are better
Around the equinoxes Earth’s magnetic field is tilted favourably to the solar wind, so storms are more frequent in March and September–October (the Russell–McPherron effect). Combine that with dark nights and you have the best aurora seasons. Track it live on the aurora forecast.