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The mere mention of the word, "aurora" brings to mind mysterious and spectacular images, and thousands of people travel to the Arctic and Antarctic regions each year to see it with their own eyes. Lighting up the sky at altitudes over 100km from the Earth, the aurora fascinates us with its fantastic beauty. Nikon products play a vital role in researching this incredible phenomenon by helping determine changes in the upper atmosphere, of which little is known.
We asked Makoto Taguchi, Associate Professor at the National Institute of Polar Research (NIPR) to talk about his work in upper atmosphere research.
Spectacular beauty
First of all, please tell us what kind of work is done at NIPR.
Photo of the Antarctic sky taken with the All-Sky Camera (ASC)
NIPR was established in 1973 as one of the Inter-University Research Institutes for researchers belonging to universities and other research institutions. It oversees observation, scientific studies and education concerned with the polar regions. There are a total of five research groups at the institute, and I belong to the group concerned with upper atmosphere physics research. We mainly study physics in the upper atmosphere, the ionosphere and the magnetosphere of the earth. At present, I am studying the dynamics of the upper atmosphere, especially wind and temperature changes that happen when aurora appears.
Exactly how far above the Earth is the upper atmosphere ?
The atmosphere of the earth can be divided roughly into four layers: from the ground to about 10km above the lowest layer is called the troposphere, from there to about 50km is the stratosphere, from there to about 80km is the mesosphere and above that is the thermosphere. The upper atmosphere includes both the mesosphere and the thermosphere. Since part of the thermosphere is ionized (i.e. atoms and molecules split into ions and electrons), it is also called the ionosphere. Above this is the very thin atmosphere connected to outer space. Outside the atmosphere, you find the earth's magnetic field. The area under the force of the magnetic field is called the earth's magnetosphere. This reaches distances of over 60,000km, or 10 to 30 times of the earth's radius (6,378km).
How high does the aurora appear ?
The aurora appears mainly in the upper atmosphere, about 100 to 500km above the ground, centering around the auroral zone (from approximately 60 to 70 degrees in latitude) of the earth's two poles. As you know, the aurora constantly changes its shape and color. Upper atmosphere studies hope to clarify exactly how the atmosphere in the upper sky moves, what energy is produced when the aurora appears and where that energy goes.
Would you explain the mechanics of the aurora ?
In the magnetosphere of the earth, on opposite side from the sun, there is a region called the plasma sheet where large quantities of hot plasma (electrons and protons) are gathered. When magnetic force pulls these electrons and protons to the earth, they enter the atmosphere and hit molecules of oxygen and nitrogen. The energy generated by this collision is emitted as light. This phenomenon creates the aurora, which appears at both the north and south polar regions. You may think this happens only in the winter, but auroras -- including dark ones -- occur all the time. Because there are extended periods of daylight (called midnight sun) at the poles, the aurora is not visible during the summer.
The color of the aurora depends on which substances collide when they enter the atmosphere, is that right ?
That's right. The changes in the aurora's color are caused by the different atoms and molecules that emit light. The color seen most frequently is green, although it appears whitish to the naked eye. The green color emitted is caused by oxygen atoms. Other colors such as red and pink are caused by oxygen atoms and nitrogen molecules, respectively. During observation, we can see the aurora changing dynamically. It brightens rapidly or flickers violently, its beauty spectacular. This phenomenon is called an auroral substorm. It happens when the solar wind (streams of plasma from the sun) shakes the magnetosphere and makes electrons and protons in the plasma sheet fall to the earth in larger quantities than usual.