The long nights of winter in northern latitudes are a good time to look out for auroras, the magical curtains of dancing illuminations that glow like neon lights. This year should be good for spotting these northern lights, as the sun is growing more active in its roughly 11-year cycle of activity.
For thousands of years auroras were a mystery and many people in northern Europe believed they were the souls of the dead. Scientists for a long time had no idea what created the auroras and were put off studying them by the harsh polar winters where auroras blazed brightest. But in 1899 the Norwegian physicist Kristian Birkeland led an expedition to the Arctic to study the Earth’s magnetic field during auroras. It was gruelling work and two of his team died but the results showed that the auroras were linked to electromagnetic currents. Birkeland proposed that energetic charged particles from the sun were guided by the Earth’s magnetic field towards the pole where they collided with the atmosphere and created the aurora. His theory was vehemently criticised by the Royal Society in London. Undeterred, Birkeland financed another polar expedition in 1902 and found that sunspots stimulated auroras.
He then created artificial auroras in his laboratory using terrellas — magnetised metallic spheres that mimicked the Earth’s magnetic field. Each terrella was sealed inside a vacuum tank with high voltages and then beams of cathode rays fired at it, creating a luminous glow of an artificial aurora around the poles of the terrella. Even these ingenious experiments failed to persuade Birkeland’s sceptics. Only decades later, in the 1960s, did measurements from satellites confirm the link between the aurora and disturbances in the Earth’s magnetic field. Today Birkeland is recognised as an outstanding scientist far ahead of his time, and for 20 years his portrait has been on the Norwegian 200-kroner banknote, along with a small picture of his terrella.
