Aurora is the name given to light emitted when the upper atmosphere is hit by energetic charged particles, principally electrons, travelling along magnetic field lines. Energetic electrons impacting a gas that then emits light is the same process that operates in a flourescent 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.
The aurora results from the interaction of the solar wind and the Earth’s magnetic field. Some energy released in this interaction is transferred to charged particles which travel along the magnetic field lines into both polar regions. The stronger the interaction of the solar wind with the Earth’s magnetic field, the more equator-ward the oval of auroral activity extends.There are thousands of individual colours in the aurora, each resulting from a specific electron cloud energy transition of an excited atmospheric atom, molecule or ion returning toward a ‘ground’ (= lowest) energy state, but three are dominant. Near the lower border a green atomic oxygen emission generally dominates; at an altitude of 250 km a red emission from atomic oxygen dominates; throughout the aurora a violet emission from a molecular nitrogen ion is significant.
The aurora australis is more commonly seen in Australia around the time of maxima in the ~11-year cycle in solar sunspot occurrence. The most recent sunspot maximum occurred in 2000. 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.
Seen from the ground, auroral forms 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 it is at an altitude around 105 km. The aurora may extend hundreds of kilometres above this lower border.
Colours in aurora
The colours in the aurora result from light (‘photons’) from specific energy level transitions in excited atoms, molecules and ions of the upper atmosphere, returning to their lowest energy state. There are thousands of individual colours in an auroral display, but three are dominant in the visible spectrum: green (the brightest) and red are emitted by oxygen atoms, and violet from a nitrogen molecular ion. The combination of green, red and purple emissions may give aurora a ‘whiteish’ appearance.
Shape of auroral formsAurora 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. These may break into small arcs. If rapid horizontal motion of the auroral form is apparent, the form may appear more violet on its leading edge and greenish on its trailing edge. This results from a small delay (less than a second) between the peak intensity of the nitrogen molecular ion emission and the green oxygen atom emission.
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. A red dominated auroral glow will be very diffuse. It will vary in location and intensity very slowly (time scales of half a minute or so). This results from a significant time delay in the emission of light by the atomic oxygen state, which smooths out any rapid variation in where the auroral electrons are impacting on the atmosphere.
Height of aurora
Generally, if an auroral band has an easily discernible lower border, this will be at around 100 to 110 km altitude. Auroral rays may extend above the lower border for hundreds of kilometres. If the lower border has a pinkish edge to it (resulting from an emission of molecular nitrogen), the altitude may be around 90 to 100 km. A diffuse red aurora occurs above 240 km.
During an active auroral display, the intensity variations will be rapid and spectacular. A most dramatic intensity variation is an increase in intensity moving up or along an auroral form. If the intensity variation is moving upward from the lower border, this has resulted from velocity dispersion of the electrons from an equatorial acceleration region. The faster electrons reach the atmosphere first and deposit their energy lower in the atmosphere. This is followed by lower energy electrons which are stopped at progressively higher altitudes, giving an intensity variation moving upward from the lower border. These variations may recur in rapid succession. Movement along the auroral form may result from a rapid variation in the longitude of the acceleration region at the auroral equator. Strong moonlight and city lights reduce the ability to discern auroral features.
The aurora and Tasmania
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 equatorward. At times it expands over Tasmania. Because Tasmania is located equator-ward of the general location of the auroral oval, auroral displays in Tasmania are generally seen in the southern sky. 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. The chance of observing aurora in Tasmania is strongly correlated with the sunspot cycle. Auroral activity over Tasmania peaks near the peak of the sunspot cycle and for the following couple of years. Aurora in the skies over Tasmania peaked around 2002 and will next peak around 2013. Due to its proximity to the general location of the auroral oval, Tasmania is the Australian state with the greatest probability of observing auroral displays. Auroral displays are more common near the equinoxes, but this does not preclude the occurrence of aurora at other times.