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The Aurora Explained


  1. The aurora has a curtain-like shape, and the altitude of its lower edge is sixty or seventy miles, about ten times higher than a jet aircraft flies.

  2. Auroras occur along ring-shaped regions around the north and south geomagnetic poles. Fairbanks and Alaska are good places for aurora watching because it is under this region, in the north, where people see aurora borealis, or northern lights. The southern aurora is known as aurora australis.

  3. Like a neon sign, auroral light is produced by a high-vacuum electrical discharge. It is powered by interactions between the sun and the Earth. The light glows from atoms and molecules in the earth's upper atmosphere.

  4. The sun is a ball of gases that is so hot that its outermost part blows away as the solar wind. Consisting of charged particles, this tenuous gas takes about three dyas to reach Earth. Because the earth's magnetic field prevents the solar wind from penetrating our atmosphere, its solar particles stream around our planet, encasing earth and its magnetic field within a comet-shaped cavity called the magnetosphere.

  5. The solar wind powers the gigantic electrical discharge process, causing the magnetosphere to behave as a generator that produces up to ten million megawatts of electrical power.

  6. The upper atmosphere contains, at the lower edge of the aurora, a thin and partly ionized layer called the ionosphere. Reflected by the ionosphere, radiowaves can propagate great distances by bouncing between it and the ground.

  7. Auroral displays indicate that the ionosphere and our protective atmosphere are being energized by the electric power generated in the magnetosphere. As these electrical currents are discharged in the ionosphere, many phenomena are produced, including the visible emissions we recognize as the aurora and magnetic storms.

  8. Auroras are similar to color television images. In the picture tube, a beam of electrons, controlled by electric and magnetic fields, strikes the screen, making it glow in colors that vary with the screen's phosphor. Auroral color depends on the type of atoms and molecules struck by the energetic particles, particularly electrons, that rain down along earth's magnetic field lines in the discharge process. Each atmospheric gas glows with a specific color, depending on whether it is ionized or neutral, and on the energy of the particle hitting the atoms and air molecules.

  9. The brightest and most common auroral color, a brilliant yellow-green, is produced by oxygen atoms at an altitude of roughly 60 miles. High-altitude oxygen atoms (about 200 miles) produce rare, all-red auroras. Ionized nitrogen molecules produce blue light; neutral nitrogen molecules create purplish-red lower borders and ripple edges.

  10. Auroral intensity varies from night to night and during a single night, with best viewing usually from late evening through the early morning hours. Strong auroras can be seen in the continental U.S., particularly in the north during sunspot maximum years. The number of sunspots (a sign of solar activity) varies according to an eleven-year cycle. A few years after a maximum sunspot year (such as 1991), auroras in high-latitude are more numerous. There's also a slight tendency for more auroras in spring and fall.

  11. The magnetosphere protects us from direct effects of the solar wind, but auroras can seriously disrupt radio communications, radio navigation, some defense-related radar systems, and power transmission lines. Current created by changing magnetic fields accompanying aurora causes corrosion in pipes, including the trans-Alaska pipeline.