The Northern lights
Ken Tapping, June 12, 2018
My very first encounter with an aurora was in England, before I came to Canada, at the house of a radio amateur. He was communicating by sending shortwave signals northward, where they reflected off an aurora to radio amateurs all over northern half of our planet. The Morse code bouncing back from the aurora were really strange. The whistle note making up the dots and dashes had a strange "warbling" tone and sounded as though they were echoing in some vast, celestial cathedral. It was much more fascinating than the only sighting I had of an aurora from Southern England, an elusive green glow in the northern sky that went away after a couple of hours. Then I saw the Canadian version.
In Canada, particularly in the North, the aurora can be truly spectacular: flickering curtains, blobs and rays of green and red, and sometimes bright enough to read by. It's not surprising that most northern communities around the world have incorporated the Northern Lights into their mythology and folklore. There is far too much to summarize here, but two contrasting myths are: don't look at the aurora while giving birth or your child will be cross-eyed, alternately, children conceived under the aurora will be exceptional.
Scientists have been trying to work out what an aurora is and how it works for well over a hundred years. We now know roughly what is going on, but there are still lots of fundamental things we do not understand. We know for sure that auroras are caused by interactions between the Sun, the Earth's magnetic field, and the atmosphere.
The story starts with the solar wind, a blast of atomic particles and magnetic fields flowing out from the Sun. It is always there, but varies in speed and density, becoming a solar gale on occasion. Most of the time it is flowing at a few hundred kilometres a second, but sometimes comes at us at thousands of kilometres a second.
The Earth is surrounded by a magnetic field. In the absence of the solar wind it would appear like a huge doughnut, with the holes centred over the North and South Magnetic Poles. However, the solar wind blows it out into a long, teardrop shape.
Particles from the solar wind are constantly penetrating the Earth's magnetic field and getting trapped there. The rubbing of the solar wind over the surface of the Earth's magnetic field makes waves. These run along the magnetic field, down to the ground at northern latitudes. These can be picked up and converted to sound. They sound like "feeding time at a cosmic zoo".
When the solar wind blows extra hard, more of the Earth's magnetic field is pushed back into the tail of the teardrop, where they become increasingly stressed, snap and release their stored energy. The energy pulses propagate back towards the Earth where they generate huge voltages which accelerate particles down into the atmosphere around the magnetic pole, where they collide with atoms of nitrogen and oxygen in the air, making them glow green and red. The sheer variety in auroral displays is probably due to the range of processes involved, and that each one of them may manifest themselves in different ways. Space missions have shown that other planets with magnetic fields have auroral displays too.
The beautiful light show is only a tiny part of what is going on in the upper reaches of our atmosphere. Huge electric currents are flowing, a menagerie of plasma waves are growing and spreading out, and magnetic fields are doing amazing things. It is intriguing to think that some of the most challenging physics in the universe is going on a few hundred kilometres above our heads.
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