AURORA: A SHORT NOTE

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WHAT IS AN AURORA?

Auroras — ever-shifting displays of colored ribbons, curtains, rays, and spots — are most visible near the north (aurora borealis) and south (aurora australis) poles as charged particles (called ions) streaming from the sun (the solar wind) interact with Earth's magnetic field.

HOW ARE AURORA FORMED?

When energetic charged particles enter the earth's atmosphere from the solar wind, they tend to be channeled toward the poles by the magnetic force which causes them to spiral around the magnetic field lines of the earth. They are energetic enough to ionize air molecules, so a considerable number of atoms and molecules are elevated to excited states. When they make the transition back to their ground states they emit light characteristic of the atoms and molecules. Red and green light emitted from oxygen atoms is a constituent of the light seen at the poles. Atmospheric nitrogen also plays a role. An example of the colors that might be visible can be found by observing the nitrogen spectrum. Near the north pole the light show is called the aurora borealis and near the south pole it is called aurora australis.

A polar satellite captured images of aurora over the South Pole of the Earth. UV photographs of Jupiter indicate that auroral phenomena occur in its polar regions. Images of Saturn aurora show a very active pulsating pattern.

COLOUR OF AURORA:

Auroras happen when ions in the solar wind collide with atoms of oxygen and nitrogen in the upper atmosphere. The atoms are excited by these collisions, and they typically emit light as they return to their original energy level. The light creates the aurora that we see. The most commonly observed color of aurora is green, caused by light emitted by excited oxygen atoms at wavelengths centered at 0.558 micrometers, or millionths of a meter. (Visible light is reflected from healthy (green) plant leaves at approximately the same wavelength.) Red aurora are generated by light emitted at a longer wavelength (0.630 micrometers), and other colors such as blue and purple are also sometimes observed.

POLAR AURORA:

Alaska is known as a good place for seeing the polar aurora, also known as "Northern Lights." Originally the phenomenon was named "Aurora Borealis," Latin for "northern dawn," since in the lower 48 states or in mid-Europe it may appear (on the rare occasions when it does) as a glow on the northern horizon, as if the sun was rising from the wrong direction. But the southern hemisphere has the same phenomenon, with the glow coming from the south, so scientists prefer to call it simply the "Polar Aurora."

    Most visitors to Alaska never get to see an aurora, because they come in the summer, when skies are rarely dark enough. Alaskans claim that only around August 16 does the sky get dark enough to see stars, which is when aurora stands out. After that date, your best bet is to go to Fairbanks--and since the brightest auroras occur around midnight (or later, due to Alaska's time zone), you might have to stay up a long time. Perhaps it is better then to ask the night clerk at your hotel to ring you up if a good display becomes visible.

AURORA FROM SPACE STATION:

You've seen amazing images of the Aurora Borealis and its counterpart, the Aurora Australis here on Environmental Graffiti in the past, but now you have a chance to see them as never before: from space. If you think the view down on Earth is incredible beyond words, wait until you see what these natural light phenomena look like from the International Space Station and through the lens of the Hubble Telescope.