The Northern Lights

The lecture on Wednesday 20th February 2013 was given by Alistair Grieve, one of our members who has photographed the lights from northern Norway and was on last year's WAS aurora trip to Finland.   As 2013 is the year of solar maximum, and as another society visit to Finland has been arranged it was felt appropriate to have a lecture on "The Northern Lights."

The evening began with a series of video animations showing the moving aurora (Alistair emphasised this was far too good to be his work! Although the rest of the photography in the presentation was his own).  He began the talk by telling us that there have been reports of the aurora since Old Testament, Greek and Roman times.  These days it is extremely unusual for the phenomenon to be seen in Mediterranean latitudes, and we could speculate that it may have been more common in ancient times.  The frequency and reliability of reports of the aurora increased with the invention of the printing press. 

He gave us a brief history of how ancient civilisations first interpreted what they saw in the sky and talked about some notable astronomers and scientists who made observations and discoveries about the workings of the Aurora.  Galileo was first to name it Aurora Borealis after seeing it in 1619. Edmund Halley noted that the inclination of auroral rays followed the “dip” of a compass needle in 1716.  Nineteenth century observers and scientists such as Carrington, Loomis, Lemstrom, Fritz and Tromholt established that there was some connection with the sun and sunspots and with the earth’s magnetic field, and that the aurora appeared as an oval surrounding the magnetic pole.  But an understanding of the physics only began to emerge in the early 20th century.

Kristian Birkeland organised two scientific expeditions to north Norway and experimented in his laboratory using a Terella (a model of the sun/magnetised-earth system in a vacuum box). His Terella is now in a museum on Oslo, but a smaller working version can be seen in Tromso.  Another Norwegian, Carl Stormer, built on the foundation laid by Birkeland by photographing the aurora, accurately measuring the heights, and working out the complex mathematics of the interaction of charged particles with magnetic fields.

Alistair then went on to tell us about the physics of the aurora – a very simplified version as it has taken some extremely smart people more than a century to work out the details and there’s a great deal we still don’t know.  But in essence, all it takes to produce the phenomenon is the solar wind, a planet with a magnetic field and an atmosphere.  That is why in the solar system Jupiter, Saturn, Uranus, and Earth have observable auroras.

With the help of an inflatable globe Alistair showed how the Auroal Oval (which we can observe) and the Auroral Zone (which is a statistical concept) are centred on the earth’s geomagnetic poles.  There are actually three types of poles in the north and in the south: the “true” poles (the axis of rotation); the “magnetic” poles (to which a compass points); and the “geomagnetic poles” which define the magnetosphere’s location further out in space.  The magnetic poles have moved surprisingly quickly over the years, but the positions of the geomagnetic poles are more stable.

The spectrum of the aurora shows three main colours.  At highest altitude (about 400km) is a red glow produced by the interaction of relatively low-energy particles with oxygen.  Lower down (100-400km) oxygen also produces the characteristic green colour, and at about 100km high-energy particles cause nitrogen to glow purple/violet which is more rarely seen.

Alistair went on to show us some of his own images which showed many of the forms that the aurora can take – arcs, folded curtains, coronas, rays.  He explained that the electrical power associated northern lights is in the order of a Terawatt.  A few people have reported sounds created by the aurora but the origin of sound remains one of the mysteries which have yet to be resolved.  There is still much to be discovered about the Aurora.

Some members asked questions, and others who had seen an aurora were able to recount their experiences.

It was a very interesting talk, and well illustrated, and we thanked Alistair for putting his presentation together for us.

Ja