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February 2016,
TOPICAL SCIENCE
Issue 2
The Aurora appears to drift lazily across the night sky, as wispy curtains of greenish light, above the snowcovered mountains of Norway.
Sunstorms and The Aurora The Aurora Borealis, also known as the Northern Lights, is often seen in northern countries, such as Finland, or Norway, or even in the northern parts of Scotland. It is observed as a green or sometimes red glow in the sky, which may look like wispy threads of light, or luminous curtains in the night sky. It can be a stunning sight. A similar phemonenon is observed in the southern hemisphere, near the South Pole, where it is known as the Aurora Australis.
The aurora is caused by charged particles from the sun, interacting with molecules in the earth’s atmosphere, making them glow with beautiful colours. The reason that auroras become visible in Polar regions is because of the earth’s magnetic field. This acts as a shield, protecting most of the earth’s surface from charged particles from space. The magnetic field directs the particles along its lines of force, which converge towards the north and south magnetic poles.
Because of the shape of the Earth’s magnetic field, most of the charged particles enter the earth’s atmosphere above the poles, where they interact with the molecules in the air. Electrons in these molecules are given an energy boost by the charged particles from the sun. This allows them to be raised to a higher than normal energy level. When the electrons drop back to their normal ‘ground state’ energy, they give out particular wavelengths of light. It is unusual for the Aurora Borealis to be visible as far south as Ireland.
Inside this issue: At this time of year, some enthusiasts travel to Nordic countries in the hope of seeing the Northern Lights (Aurora Borealis). This issue of ‘Topical Science’ attempts to explain the science behind this amazing natural phenomenon.
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The Aurora drapes itself in a wide arc of pale green light over a fjord in Norway The Origin of the Auroral Energy The sun has an eleven-year cycle, alternating between active phases and less active periods. Even in its quieter phases, the sun is seething with activity. Like all stars, it is a massive ball of extremely hot gas, held together by gravity. Unlike the gases in earth’s atmosphere, this gas is not composed of molecules, but of ionized atoms. These are atoms that have had their negatively charged electrons ripped off by the intense heat, exposing the positively charged nuclei. Most of the material of the sun consists of the element hydrogen. The nuclei of hydrogen atoms are tiny charged particles called protons. Such a hot ionized gas is known as plasma. We have three states of matter on our planet earth; solid, liquid and gas. Plasma is a fourth state that occurs only at extremely high temperatures, such as in the stars. The sun’s gravity produces enormous pressure in its centre and this, along with the high temperature, causes thermonuclear fusion to occur. In this process, four protons fuse to form helium nuclei, releasing enormous amounts of energy. The mass of a helium atom is slightly less than the combined masses of four protons. A great amount of energy is produced because the ‘missing’ mass is
converted into energy, according to Einstein’s famous equation E = MC^2. Thermonuclear fusion is responsible for the sun’s energy output. The temperature difference between the sun’s interior and its surface causes convection currents, while the sun’s rotation makes the plasma swirl around, with different parts of the sun rotating at different speeds. This turbulent motion of charged plasma creates strong fluctuating magnetic fields, whose lines of force create loops in the sun’s outer atmosphere, known as the corona. Closed magnetic loops cause a slight cooling in places, which appear as darker sunspots on the bright surface. When the sun becomes more active, more sunspots appear. While the magnetic fields are stable, they confine the plasma. But as energy builds up, the loops occasionally break, releasing up to a billion tons of material in what is known as a coronal mass ejection (CME), which sends streams of charged particles out into space. The Coronal Mass Ejection is accompanied by a solar flare of light, which travels, obviously, with the speed of light, reaching Earth in eight minutes.
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However, the charged particles are made of matter and travel more slowly. They take two or three days to reach earth and so we get advance notice of their arrival. Organisations such as NASA detect the solar flare and send out an alert. This is because, whenever there is a coronal mass ejection from the sun, there will be a greater than usual number of charged particles entering the Earth’s atmosphere in the Polar regions during the following days. This can cause quite a spectacular auroral display.
It sometimes happens, following a CME, that the Aurora Boreais becomes visible even as far south as Ireland, in northern counties such as Donegal, or even Sligo. Astronomy Ireland always alerts its members whenever there is a likelihood of an Aurora being seen in Ireland. There are sky-watchers in Donegal who have managed to capture pictures of this rare event in recent years. Check out the website: http://www.donegalskies.com/
Margaret Franklin is a retired chemistry lecturer. This article is based on one published three years ago in her ‘Topical Science’ column in a local newspaper ‘The Westmeath’ Independent’. Margaret has written many articles for ‘Science Spin’ magazine, which is unfortunately now no longer published in hard copy.
This photograph of the Aurora was taken by Paul Halton, from the deck of the Hurtigruten cruise ship, ‘Nordlys’, during an Arctic Voyage organised by Astronomy Ireland, December 2013. A diary of this voyage has already been published in Issuu, entitled: ‘Chasing the Aurora’.
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