Colouring the Sky – 23rd August 2012

Our August meeting was addressed by Alan Aylward (UCL), who gave us a talk with the title Colouring the Sky. Miriam writes:

We had an eventful start to this meeting as were locked out at BEECHE! Fortunately Carole was able to arrange for us to use a church hall in Pratts Bottom.

The subject of the lecture was aurorae and we were told about ancient sightings and theories, the evolving science and aurorae of other planets.

Observations of aurorae go back a long time and can occur further south than would be expected. In AD 12 a Roman garrison reported a village on fire north of Rome but it was due to the red glow of aurorae. A Chinese manuscript of 651 described portents of doom because of a strange light in the sky. In 1712 aurorae were seen in London.

Through time there have been different theories about aurorae. The Finns called the “Foxes Fire”, in Norway they were thought to be light reflecting from shoals of herring in the sea. Indians believed they were the dead wives of braves scolding their husbands. Another idea was that aurorae were the fire around the edge of a flat earth. Interestingly, there is no mention of aurorae in Viking sagas as they occurred further north in their era.

In the 18th century there were significant auroral events and the Norwegians began tracking where they appeared and it was found that the Aurora Borealis occurs in a ring around the pole. Magnetic recording linked the aurorae to the magnetic pole and geomagnetic activity causes the aurorae to move south.

Aurorae occur when the solar wind interacts with the magnetosphere causing a shower of particles which excite the upper atmosphere and appear like curtains along magnetic field lines. They form in ovals around the poles. It is thought that the magnetic field of the sun intersects that of the earth and this accelerates particles fifteen degrees south of the cusp.

The spectrum of aurorae shows very discrete lines indicating the emission of specific elements, for example: nitrogen and atomic oxygen. The proportion of different elements produce different coloured aurorae. When seen in the UK they tend to appear red due to low levels of energy.

Aurorae are observed by radar and satellites, it is a dynamic situation and they are very difficult to predict.

Other planets also have aurorae. Those around Jupiter are due to Io emitting gas which is ionized and trapped in Jupiterís magnetic field forming a plasma sheet which moves as the planet rotates. Saturn has aurorae which are linked to the solar wind. Neptune also has aurorae which have been observed but not measured.

If you are fortunate enough to observe the Aurora it must be an experience not to be forgotten!

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