Aurora is the name given to light emitted when the upper atmosphere is hit by energetic charged particles, principally electrons from the solar wind, which travel along the Earth's magnetic field lines.
When these energetic electrons collide with gases such as oxygen and nitrogen in the atmosphere, the gases emit light, producing predominantly green, red and violet colours. The combination of green, red and violet emissions may give aurora a white appearance – the process is similar to that occurring in a fluorescent tube or neon light.
Aurora occurs in an oval around the magnetic poles in both hemispheres. It is called aurora australis in the Southern Hemisphere and aurora borealis in the Northern Hemisphere.
Seen from the ground, aurora are often aligned east-west and appear in the form of a shimmering curtain. If a sharp lower border to the ‘auroral form’ can be observed, the aurora is at an altitude of around 105 km. The aurora may extend hundreds of kilometres above this lower border.
Colours of aurora
There are thousands of individual colours in the aurora, each resulting from the movement of an atmospheric atom, molecule or iron from a high energy state to a lower energy state. At lower altitudes (about 100–110 km up) green emissions from atomic oxygen dominate, while at about 250 km up, red emissions from oxygen dominate. Throughout the aurora violet emissions from a molecular nitrogen ion are significant.
Shape of auroral forms
Aurora usually form a band aligned in a magnetic east-west direction. If sufficient numbers of energetic electrons are impacting the upper atmosphere, bands may have shimmering rays extending upwards from them. These rays define magnetic field lines along which the auroral electrons travel into the atmosphere.
The twisting of auroral rays and bands results from the dynamic interaction of electric currents and magnetic fields in the upper atmosphere. In active displays, multiple bands may be visible, and these may break into small arcs.
The active phase of an auroral display will last 15 to 40 minutes and may recur in 2 to 3 hours. Auroral band features may persist all night.
Where to see aurora
The aurora australis is more commonly seen in Australia around the time of maxima in the ~11-year cycle in solar sunspot occurrence. Tasmania is the Australian state from which the aurora australis is most commonly seen, as it is closest to the normal location of the auroral oval.
The global distribution of auroral activity is an oval around the magnetic poles in both hemispheres. As the level of magnetic disturbance of the Earth’s magnetic field increases, the oval of auroral activity expands equator-ward. At times it expands over Tasmania. The most dramatic displays will most likely be observed when the aurora is overhead at around midnight. The earlier in the evening an auroral display is seen in the southern sky, the more likely it is that the display will be more spectacular and more overhead (or even to the north) at around midnight.
The chance of observing an aurora in Tasmania, on a clear night, averages out at around 1 to 2 per cent and is strongly correlated with the sunspot cycle. Auroral displays are more common near the equinoxes (late March and September), but this does not preclude the occurrence of aurora at other times.
Time-lapse of an aurora at Macey Hut, near Mawson station, 2011 (Credit: Chris Wilkinson)