How is aurora borealis created and why does it occur at the north and south pole?

Asked by: Diana

Answer

The Earth has a magnetic field very much like that of a bar magnet, directed from the geographic South pole to geographic North pole. Charged particles streaming outward from the Sun arrive in the Earth's vicinity. Collisions in the Earth's vicinity slow them down and they subsequently get trapped in spiral paths around the planet's magnetic field lines. A point of note is that the field lines become more dense as they approach a pole. As the lines become more dense the charged particles' path becomes flatter and eventually turns back on itself. A particle spiraling along the field lines from the South pole to the North Pole turns around at the North Pole spiraling back and heading southward. These are in effect magnetic mirrors. Particles trapped in this way accumulate in regions called the Van Allen Belts. There exists two such belts: one containing protons at a height of about 3,000 km and the other containing electrons at about 15,000 km.

One of the consequences of the belts is the aurora borealis and aurora austrealis. This occurs whenever the charged particles enter the atmosphere and excite the atoms in the air creating the colors you see. These are noticeable near the poles where the magnetic field lines dip toward the Earth's surface, carrying the Van Allen belts with them.
Answered by: David Latchman, B.Sc. Physics, University of the West Indies