Aurora Australis

Aurora australis captured from Cradle Mountain, Tasmania on 8 May 2014. Copyright: Mark Walsh

Once thought to be portents of doom, auroras are now known to be remarkably common phenomena, caused when electrically charged electrons and protons accelerate down the Earth’s magnetic field lines and collide with neutral atoms in the upper atmosphere – usually about 100 km above the Earth.

These collisions cause the neutral atoms to fluoresce, emitting light at many different wavelengths. The most common aurora colours are red and green, caused by the fluorescence of oxygen atoms, while nitrogen atoms can throw bluish-purple lights into the mix.

During periods of particularly high solar activity, the sun can eject large blobs of plasma called ‘coronal mass ejections’—momentous blasts of material and solar wind that can reach speeds of up to 2,000 km per second. When this material reaches Earth, most of it is deflected by the planet’s magnetic field. But in the process, huge amounts of energy are transferred to our magnetic field, generating intense geomagnetic storms that can last for two or three days – and produce dramatic auroras over successive nights.

The brightest auroras are concentrated in a ring – the ‘auroral oval’ – centred on the Earth’s magnetic pole. This ring of light is usually located above the Antarctic and sub-Antarctic, but enlarges and expands equator-wards during a geomagnetic storm. This explains why auroras are best viewed from our south coasts, and are usually low to the horizon.

Patrolling The Sky
The mounting probability of aurora sightings has helped to lift interest in the recent launch of the Bureau’s online Aurora Patrol Camera—a sophisticated all-sky camera located near Cressy in Tasmania, which enables members of the public to monitor the sky in real time.

The first dedicated ‘aurora cam’ in Australia, the camera has proved highly reliable at picking up even the faintest auroras, comparing favourably with similar cameras operating in the northern hemisphere.

Tasmania’s unique location means that auroras can actually be seen at this phase of the solar cycle as often as every week – although most people would not naturally recognise the phenomenon when it is low to the horizon.

Auroras are generally fickle creatures that do not show themselves to everyone, and I certainly wouldn’t advise anyone to visit Tasmania just to see one. You ideally need a dark night with very little cloud, no bright moon, and no light pollution—preferably on a dark beach or headland.

If you don’t go “wow” when you look up at the Milky Way, you can usually forget about looking for auroras.

Auroras are more likely to occur close to the equinoxes, in late March and late September, but this is a guide only because incredible displays can occur at any time of year.

Auroras are more likely to occur close to the equinoxes, in late March and late September, but Dr Parkinson cautions against using this as a guide because incredible displays can occur at any time of year.

Aurora Patrol Camera - Latest Picture: Tasmania

An all-sky aurora patrol CCD camera located at Cressy, Tasmania provides aurora watchers with real-time guidance on whether auroras are visible from Tasmania. The date (yyyy/mm/dd), time (hh:mm:ss) and exposure time (seconds) are shown in the top left corner of the images. The images are orientated with South to the right and East at bottom. When fog or low cloud is present, patches of green glow occur toward the left (North). This is caused by light pollution from Cressy located several kilometres to the North of the station. Faint red auroras (630.0 nm) are often apparent low on the southern horizon (right) for Kp of 3 or more. Green auroras (557.7 nm) are often recorded low on the southern horizon for Kp of 4 or more. The human eye loses sensitivity to deep red light when dark adapted, so red auroras remain sub-visual unless they are very bright. The green auroras are often as bright as the Milky Way, and they are almost certainly visible to a well-trained observer from locations free of cloud and light pollution. Red auroras can extend toward zenith above Tasmania during the local midnight hours when Kp is 6 or more

Warning: The camera control software is fully automatic and often changes the exposure time, dark frame subtraction and colour balance. Corrupted images are often caused by erroneous dark frame subtraction.

Aurora Example

This is an example of an aurora patrol image recorded during the geomagnetic storm of 29th June 2013. The image is orientated with South to the right and East at bottom. The town of Cressy is located several kilometres to the North of the station. Patches of greenish glow located toward the left are caused by light pollution scattered by clouds. The Milky Way galaxy stretches from the North East (bottom left) to the South-West (top right). The red glow toward the right is red-line aurora and the bright yellow-green glow along the Southern Horizon is bright green-line aurora, not light pollution from Hobart or some other urban centre. The aurora is shining through semi-transparent clouds drifting in the lower atmosphere. The two dark blotches at upper left are probably insects on the outside of the dome.

This example is provided to help aurora watchers understand what they should look for in the images.