Auroras

A rare aurora over Michelago 50 km south of Canberra.
Photograph taken by Chris Soames on 31 March 2001.

Geomagnetic 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 the 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 beautiful manifestation of magnetic storms on the earth.

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

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

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 320 km) is the source of the red emission. At lower altitudes (about 100 km) 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 ripple edges of the aurora.

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