Auroras

Magnetic storms often result in the sighting of auroras, colourful displays that appear in the night sky, at places much nearer to the equator than where they are usually seen. Auroras are commonly seen in areas around Earth's polar regions. They are often referred to as the Southern Lights or Aurora Australis in the Southern Hemisphere, and the Northern Lights or Aurora Borealis in the Northern Hemisphere. Auroras are a dynamic and visually striking manifestation of magnetic storms on Earth.

Auroras happen when charged particles from the Sun enter the magnetosphere. Once inside, the geomagnetic field directs them toward the north and south magnetic poles. Travelling at high speeds the particles collide with gas molecules and atoms in the atmosphere, which energises them. A visible glow appears when they release the energy and return to their ground states, much like the way a fluorescent light works.

When a magnetic storm occurs, the auroral zones expand towards the equator from the polar regions, sometimes providing spectacular displays to residents of mid-latitude regions. During intense magnetic activity auroral displays have been reported from as far north as Queensland.

An aurora in the night sky. This aurora is a red glow with a green fringe at the bottom and accompanied high levels of magnetic activity.

A rare aurora over Michelago 50 km
south of Canberra. Reproduced with
permission from Chris Soames

The different colours seen in auroras are produced by different gases in the atmosphere. At high altitudes, light gases like hydrogen and helium, create blue and violet auroras and high-altitude oxygen (about 320km) is the source of the red emission. At lower altitudes (about 100km) oxygen produces a brilliant yellow-green - the brightest and most common auroral colour. Ionized nitrogen produces blue light and neutral nitrogen produces a red glow (but a different hue to high altitude oxygen). Nitrogen can also create the purplish-red lower borders and rippled edges of the aurora.

Topic contact: geomag@ga.gov.au Last updated: May 20, 2013