Published: 10 March, 2005

Collison of particles, gases causes aurora borealis

What is the aurora borealis and why does it occur?

This is a great question. Aurora are beautiful shimmering curtains of colored light seen in the night sky close to the Earth's magnetic poles. The Earth has two magnetic poles hence the two names aurora borealis, commonly known as the "Northern lights" and aurora australis commonly known as the "Southern lights".

Aurora are caused by particles from our Sun colliding with gases in our atmosphere. The sun has many storms on its surface. During these storms clouds of solar particles are thrown out into space. The particles travel at very high speeds, up-to 1000 kilometers per second (compared to a jet that flies at a mere 900 kilometers per hour or one quarter of a kilometer per second!). But the sun is 150 million kilometers away from the Earth and even at these speeds it still takes the particles nearly two days to reach us. When the particles arrive at the Earth they are captured by the Earth's magnetic field and guided down towards the two magnetic poles. The particles collide with gases high up in the Earth's atmosphere at a height of around 100 kilometers above the ground i.e you'd need a sky-scraper with 33,000 floors to reach it. Some of the energy from the collision is converted into light. For you to see an aurora it takes around 100 million collisions! A red aurora is the result of collisions between solar particles and nitrogen gas and a green color is caused by collisions with oxygen gas.

Any planet or moon that has both an atmosphere and magnetic poles will have auroras. Auroras have been seen on Saturn, Jupiter, Uranus and Neptune. On Jupiter's moon Io there can be aurora due to collisions between particles and gas emitted by active volcanoes on the moon's surface!

For more details about aurora visit the exploratorium.

To see many beautiful pictures of aurora both on Earth and in space go to the NASA astronomy picture of the day web site.