Layers of the atmosphere
The atmosphere is the band of gases that surrounds the earth. Scientists have identified five layers based on temperature. Moving upwards from the earth, these are the troposphere (where weather occurs), stratosphere (where jets fly), mesosphere, thermosphere and exosphere.
The ionosphere and auroras
The upper part of the mesosphere, and most of the thermosphere, is also known as the ionosphere, 80–400 kilometres above the earth’s surface. The air becomes noticeably ionised (electrically charged), caused mainly by ultraviolet light from the sun.
At a height of about 100 kilometres (known as the E-region of the ionosphere), the air becomes thin enough, and the ionisation strong enough for electric currents to flow along the earth’s magnetic field lines. It is in this region that auroras occur – beautiful undulating bands of light, visible in the night sky.
To understand how they form it is useful to think of the earth as a giant magnet, with magnetic field lines arching from one pole to the other. Auroras occur when charged particles in the solar wind (the stream of electrons and ions ejected from the sun) collide with the magnetic field (magnetosphere), causing electrons to be accelerated into the atmosphere around the north and south poles. In the northern hemisphere this phenomenon is known as the aurora borealis, and in the southern hemisphere as the aurora australis.
Auroras and the sun’s cycle
For reasons that are not entirely understood, the sun goes through a cycle of increased magnetic activity every 11 years. Quiet times are followed by more active times – the solar wind is fast moving and disturbed, and there are large flares and numerous sunspots. Auroras are much more likely to form during these active times.
The aurora australis
The glow of the aurora australis, or southern lights, is usually seen south of the subantarctic islands, but when the sun is more active the aurora can be seen from mainland New Zealand. At this time, people in the southern South Island may see spectacular and colourful displays of the southern lights on clear nights.
The aurora can even be seen north of Auckland during active times. The next period of maximum solar activity should occur in 2012 or 2013. This will be a good time to look out for the waving curtains and moving rays.
In the 1860s five survivors of a shipwreck spent 20 months on the subantarctic Auckland Islands before being rescued. They had a hard time, but one midwinter night they were diverted by a spectacular display in the sky.
‘We saw before us a most magnificent spectacle. It was a Southern aurora in all its pomp of splendour … The stars paled before the sheaves of fire of different colours which rose from the horizon, and sprang toward the zenith, swift as lightnings’. 1
In the mid-1950s New Zealand scientists used radar to send radio waves into the auroral zone of the ionosphere, from Bluff and Slope Point on Southland’s south coast. Reflections of these waves from disturbances in the E-region were picked up on the radar receivers. Known as radio auroras, they are much more common during the sun’s most active period. The New Zealand observations and similar ones from around the north magnetic pole helped to show that radio auroras, like visible auroras, were caused by very fast electrons and ions moving down the earth’s magnetic field lines towards the poles.