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Aurora from 2002 in Poker Flats, Alaska. Credit: Dr. Scott Bounds
The aurora borealis is sometimes called the Northern Lights. Auroras are natural light displays in the sky, usually observed at night, particularly in the polar regions. They typically occur in the ionosphere. They are also referred to as polar auroras. Auroras seen near the magnetic pole may be high overhead, but from further away, they illuminate the horizon as a greenish glow or sometimes a faint red, as if the sun were rising from an unusual direction.
The aurora borealis was so named after the Roman goddess of dawn, Aurora, and the Greek name for north wind, boreas, by Pierre Gasendi in 1621. The aurora borealis most often occurs near the equinoxes; from September to October and from March to April. Its southern counterpart, the aurora australis or the southern polar lights, has similar properties, but is only visible from high southern latitudes in Antarctica, South America, and Australasia. Auroras can be spotted throughout the world. They are most visible closer to the poles because of the longer periods of darkness and the magnetic field.
The aurora borealis is a result of the emission of photons in the Earth’s upper atmosphere(above 80 km), from ionized nitrogen atoms regaining an electron, and oxygen and nitrogen atoms returning from an excited state to their natural ground state. They are ionized by the collision of solar wind particles being funneled down, and accelerated along, the Earth’s magnetic field lines. The energy is lost in the form of a photon that causes light. Oxygen causes green or brownish-red, depending on the amount of energy absorbed. Nitrogen emits blue light if the atom regains an electron after it has been ionized. Red light if returning to its ground state.
The aurora borealis is commonly seen near the north pole, but can bee seen in more temperate climates if a magnetic storm temporarily expands the auroral oval. These large magnetic storms are most common during the peak of the eleven-year sunspot cycle or during the three years after that peak. The ultimate energy source of the aurora is the solar wind flowing past the Earth.
The aurora borealis is a fascinating occurrence. Here is an article that will give you all of the information that you want about the aurora borealis. Here on Universe Today we have a great article about the aurorae. I mentioned that the aurorae are more visible after the sunspot cycle. Astronomy Cast offers a good episode about those sunspots.
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