Sunday 29 October 2017

Nordurljuos


Aurora Borealis - Tromso Norway - Canon 400D DSLR =18-55mm EOS lens at F3.5, f=18mm. ISO 1600 - Archie Mendes 2011


“Galileo Galilei (1564-1642) saw the ‘Northern Lights’ from Italy and called them ‘Boreale Aurora’ or ‘dawn of the north’ this was because from Italy the lights were usually seen as red and always to the north.  In the far north the aurora are not associated with dawn, they can appear at any time of night, and are usually yellow-green.  In 1621, the well known astronomer Pierre Gassendi - observing the ‘Northern Lights’ from Paris, turned Galileo’s words about face and called them ‘Aurora Borealis’.

The Aurora Borealis or Northern Lights have intrigued mankind for millennia.  Aristotle described the Aurora as ‘Meteron’ which roughly translates as ‘something appearing in the air’ and my favourite- the allegoric description ‘Capre Saltanti’ translating as ‘jumping goats’.  Sir Edmund Halley (1656-1742) believed they were controlled by the Earth’s magnetic field while the French scientist – Jean-Jacques d'Ortous de Mairan (1678 –1719) claimed they were the result of interactions between the atmospheres of the Sun and Earth.  

From a book written in AD1250 and albeit after the end of The Viking Age (AD800 – AD1100), the ‘Old Norse’ word for the ‘Aurora Borealis’ is norðrljós’.  The pagan Vikings believed the ‘Northern Lights’ were the gateway to Asgard the home of their gods.

'Bivrost' - The bridge between earth and sky or 'reflections of shoals of herring'? - Reykavik 2017 - Canon 600d DSLR EOS 18-55 lens at F3.5, f=18mm. ISO 1600, 5 sec exp.- Kurt Thrust 2017
Following in the footsteps of: F.C. Meyer (1697-1729), Jean Baptist Biot (1774-1862), Christopher Hansteen (1784-1873), Anders Jonas Angstrom (1814-1874), Carl Fredric Fearnly (1818-1890) and Karl Selim  Lemstrom (1838-1904), the Swedish scholar Kristian Olaf Birkeland (1867-1917) promoted the idea that the ‘Northern Lights’ were formed when high energy, so called cathode rays’ that were emitted from the sun were caught in the earth’s magnetic field and forced towards the earth’s atmosphere in the polar regions. He believed that these ‘cathode rays’ or electrons as we now call them, and the atoms and molecules of atmospheric gases interact to create the aurora in two luminous rings one around each pole. Birkeland was so convinced by this idea that as a demonstration experiment, he created a chamber from 10 cm. thick glass from which was evacuated as much air as was possible with the technology available 100 years ago. Inside the chamber was a centrally suspended metal sphere containing an electro-magnet.  A high electric charge was established between the sphere and a cathode positioned in one corner of the chamber. The electrons emitted from the cathode were caught in the magnetic field around the sphere and forced to move towards the surface in the polar regions of the sphere. Through adjustments of the voltage and the electro magnet he was able to create two parallel rings around the sphere, just like the aurora and as his theory predicted.  When I first saw the Aurora from Norway, a working model of his apparatus was on display in the Science Museum at Tromso.

The driving force behind the aurora is the sun. The 'lights' are created by the interaction of the solar wind with the earth's magnetic field and the atoms of gas in the upper atmosphere.  When the sun is active – lots of sunspots and coronal eruptions of charged particles, the aurora puts on dramatic displays of moving luminosity and colour. The green and red colours are associated with excited oxygen atoms and blue with excited nitrogen.
 
Aurora Borealis - Tromso Norway - Canon 400D DSLR =18-55mm EOS lens at F3.5, f=18mm. ISO 1600 - Archie Mendes 2011

The Sami peoples, who herd their reindeer under Scandinavian skies, have more than one word in their language for the aurora  but the most commonly used is ‘guovsahas’- meaning ‘the audible lights’. Having seen them ‘dance’ across the sky I swear I also heard the flutter of the changing colours. There is however no real evidence that the aurora creates sound waves.  As the ‘Northern Lights’ occur at an average height of 105 km above the surface of the earth there is probably insufficient air to enable sound waves to form.

My favourite explanation for the ‘Northern Lights’  may be found in Danish Folklore where it was said that the lights were the result of wild swans flying too far to the north and becoming frozen in ice. Each time they lifted their wings in an attempt to free themselves, the reflections from their wings caused the ‘Northern Lights’ to shine”.  Archie Mendes - visiting astrophysicist at the Jodrell Plank Observatory.

For a modern easy to read scientific explanation of the aurora folow the link to David P Stern's paper:

No comments:

Post a Comment