ASTRONOMY ( HERCHEL)

AURORAE

Neil Bone, The British Astronomical Association, on 11 May 2001

Aurorae are caused by the impact of the solar wind on the earth's magnetosphere. They are formed as circular bands around both the North and South magnetic poles of the earth simultaneously (borealis and australis). The diameter of the bands depends on the intensity of the solar wind. As the diameter increases, which occurs especially on the night side of the earth, the aurora becomes visible further away from the poles.

The solar wind is a stream, of varying density, of charged particles, mostly protons and electrons. Its intensity varies with the 11-year sunspot cycle and indicates the activity of the magnetic field of the sun, which affects the surface, the corona. When the activity is high, the corona of the sun may eject `gusts' of solar wind at speeds up to 1000 km/sec, two to three times the normal speed. Sun spots are cooler regions of the photosphere, the visible surface of the sun, and can produce `flares' and `prominences', and both these phenomena, `gusts' and `flares' result in aurorae.

Aurorae are located at 100 1000 km above the earth's surface and vary in colour and brightness widely. They are most frequent one year before and two years after the maximum of the sunspot cycle. There are very few near the minimum in the cycle. Often another aurora will be displayed 27 days after one is seen, the time of rotation of the sun's surface. News and forecasts of possible sightings are published on the Internet in www.sec.noaa.gov

Power and communication companies require such forecasts since the disturbance of the earth's magnetic field can disrupt satellite operation and communication and also can create power surges in electric distribution systems that can shut them down. The SOHO space craft located between the sun and the earth can give about a day's warning of a gust or flare in the solar wind.

In the Middle Ages, aurorae were interpreted as `signs and portents'. Swords, blood, armies and dragons were imagined in the coloured veils in the night sky; now we only worry whether the soap opera will be received.

Donald Lovell