Aurora is the collective name given to the photons (light) emitted by atoms, molecules and ions that have been excited by energetic charged particles (principally electrons) travelling along magnetic field lines into the Earth’s upper atmosphere. Aurora results from the interaction of the solar wind with the Earth’s magnetic field.

The amazing color displays and formations are produced by the solar wind – a stream of electrons and protons coming from the sun — as it collides with gases in the upper atmosphere. These collisions produce electrical discharges which energize atoms of oxygen and nitrogen causing the release of various colors of light. Earth’s magnetic field channels these discharges toward the poles. Variations in sunspot activity or the occurrence of so-called ‘coronal holes’ can often considerably enhance the auroral discharge adding to the intensity and duration of the displays.

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. Known as ‘Aurora borealis’ in the north, auroras occur in the upper atmosphere of both poles and are occasionally visible from middle latitudes as a dark red glow near the poleward horizon.

The chance of observing auroras is strongly correlated with the sunspot cycle. Auroral activity over Antarctica peaks near the peak of the sunspot cycle and for the following couple of years. Auroral displays are more common near the equinoxes, but this does not preclude the occurrence of aurora at other times. One can only observe auroral displays at the South Pole during the six months from March to September; the rest of year the Pole experiences 24 hours of sunlight.
Although auroras appear in many forms — pillars, streaks, wisps and halos of vibrating color — they’re most memorable when they take the form of pale curtains which seem to float on a breeze of light. Most commonly, auroral glows 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.

Auroral displays appear in many colors with pale green and pink the most common. However, different shades of red, yellow, green, blue, and violet have all been observed. The brightest auroral color is generally a green light emitted by excited oxygen atoms. A red diffuse glow results from another oxygen atom transition. A purple color results from a transition in a Nitrogen molecular ion. The mixture of the major green, red and purple emissions may combine to give aurora a general ‘whitish’ appearance. The color variations are a product of the altitude of the storm, and the density and composition of the ions at that altitude. The folding effect results from the electric field induced on either side of the auroral curtain by the electrons.

Generally, if an auroral band has an easily discernible lower border, this will be at around 60 to 70 miles in altitude. Auroral rays may extend above the lower border for hundreds of miles. If the lower border has a pinkish edge to it (resulting from an emission of molecular Nitrogen), the altitude may be around 50 to 60 miles. A diffuse red aurora occurs above 150 miles.