During a polar night is there any pattern or consistency to where aurorae australis begin? I'm assuming they become visible first in the exosphere and descend down to thermosphere. Do they tend to begin and move from east to west, or west to east or is it entirely random?
Auroras start at a distance of 80-200 km above the Earth's surface.
The color of the aurora will be blue/purple if the air molecules in the area below 120 km are excited.
If the air molecules in the area from 120-180 km are the ones that gets excited the colour will be yellow/green.
Above 180 km gives the red light in the aurora.
The most powerful aurora will in general be 180 degree from the Sun's position and it will move from east to west at the opposite side to where the Sun is.
During powerful auroras the aurora arch can easily cover 270 degrees or more if close to the magnetic poles, if the aurora covers the area away from the magnetic poles the aurora arch will get smaller.
When the charged particles from the Sun interact with Earth's magnetic field a powerful ground current will be induced, this can induce DC power in power lines and severely damage the AC power transformers. Should an event like the one that happened on September the first, 1859 (the Carrington event) hit us today we would have major problems with power supply and satellite communication.
You can read more about all of this here, Aurora and please take a look in the sources there too.
Aurora Australis and Aurora Borealis occur as a result of the upper atmosphere being bombarded with sub atomic particles (protons and electrons) from the solar wind. Solar winds are mass ejections from magnetic storms on the sun, and vary in intensity and frequency. These particles are electrically charged and deflected toward the poles by the Earth's magnetic field.
The aurorae have a tendency to be displaced toward the night time side of the Earth and to move very slowly from west to east because the magnetic poles are slightly off-centre and rotate with the rotation of the Earth. Films on TV often show the aurora speeded up, but usually their movement is only barely perceptible to the naked eye. If you can see the background stars moving, the movie has been speeded up. The ones I saw with the naked eye moved very slowly.
There are two main colours. Green is by far the most common, but occasionally the aurora is red. I have seen both from southern England. The red ones are caused by solar wind particles striking and exciting oxygen atoms in the high, upper atmosphere, causing them to emit a reddish light. These particles also vary in velocity, so the more powerful ones are able to reach further down into the atmosphere and stimulate a much stronger emission of green light from nitrogen atoms. This overwhelms the fainter red light from oxygen, so the aurora looks green. They are not often seen from southern England. It sometimes happens, but only in the winter with a very clear sky.