The sun emits a continuous stream of charged particles called the solar wind. This wind interacts with the magnetic field of the Earth and produces large electrical currents. These currents flow along the magnetic field lines down into the upper atmosphere surrounding the north and south magnetic poles (see diagram). These currents cause the atmospheric gases to glow like the gas in a fluorescent tube. A very large quantity of energy is deposited in the upper atmosphere during an auroral display. For example, about 1,000,000 kW, the equivalent to the power capacity of a large power plant, is needed to power a medium-strong auroral light of about 10 x 1,000 km. It should also be noted that only 1% of the energy of the precipitating particles actually goes into the production of visible light !
Where do aurora occur? Auroras occur around the Earth's north and south geomagnetic poles in regions known as auroral ovals. The auroral ovals are constantly in motion, expanding towards the equator or contracting towards the pole, and constantly changing in brightness. Occasionally the aurora may be seen as far south as the south of England. However, this would be a rare event - with maybe 1 or 2 displays every 10 years. This plot shows the typical location of the auroral oval during conditions of moderate activity. Click here to see what the weather is like in space today.
The gases in the aurora glow with characteristic colours. The emissions are the results of processes where atoms or molecules are excited by collisions with precipitating particles, returning to the ground state by emitting photons of a characteristic frequency or set of frequencies. This photograph shows a view of a green-rayed auroral arc. Green is the most common colour in the aurora and is caused by atomic oxygen at altitudes of between about 100 and 200 km.
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