Wednesday, February 16, 2011

What Causes Auroras?

The Aurora, often called the northern or southern lights, are caused by interactions between Earth's upper atmosphere and charged particles from the Sun.

People who live at high latitudes, either northern or southern, often enjoy watching brilliantly hued auroral displays. Near the north pole they are called the aurora borealis or northern lights. Near the south pole they are called the aurora australis or southern lights. The aurora are most commonly visible near the poles, but they are on rare occasions also visible at mid-latitudes.

What causes colorful auroral displays?

Physics and Causes of the Aurora
High speed energetic particles collide with atoms in Earth's atmosphere at a height of anywhere from about 50 to a few hundred miles above Earth's surface to cause the aurora. These high speed particles, which are usually electrons, originate from space, specifically from the solar wind, blowing outward from the Sun.

When the electrons from space strike an atom or molecule in Earth's atmosphere, they give one of the electrons in the atom an energy boost. In scientific jargon, the electron jumps to a higher energy level and the atom is in an excited state. After a while, the electron in the excited atom jumps back down to its original lower energy level. It releases this energy as light causing the auroral glow. This process is the same mechanism that causes emission line spectra and aurora are in fact emission line spectra of the atoms in Earth's upper atmosphere.

The color of emission line spectra depends on the chemical composition, and each type of atom produces its own unique pattern of colors. Hence, the different colors in auroral displays originate from different elements in Earth's atmosphere.

Oxygen molecules cause the green Aurora, and oxygen atoms cause the red colors. Blue auroral displays result from nitrogen molecules. Molecular nitrogen and oxygen are the most common constituents of Earth's atmosphere, so these are the most common auroral colors. Atomic oxygen occurs at high altitudes, so aurora usually have red above the green. Mixtures of these colors form other colors.

Why are Aurora Most Visible near the Poles?
In a magnetic field, electrons and other charged particles do not travel in a straight line. Instead the magnetic force causes the electrons to travel in a spiral path around magnetic field lines. The electrons therefore enter Earth's atmosphere near the north and south magnetic poles, which are near the geographic poles. Hence the aurora are most visible near the polar regions. They are only visible at lower latitudes when there is an extremely high level of solar activity hurling electrons towards Earth.

When Are Aurora Most Visible?
It must be dark to see the aurora, so they are more likely to be visible during the winter months when nights are longer. This effect is greater at high latitude. In the summer, nighttime darkness is almost nonexistent at polar latitudes, so it is correspondingly more difficult to see aurora, even though they do occur.

Aurora are also related to space weather activity. When the Sun is more active it throws more electrons towards Earth. Extreme solar storms usually cause extreme auroral displays, which are often visible at lower latitudes.

Solar activity usually follows an 11 year cycle, so aurora are more common during the peaks of this cycle and rare during the valleys of this solar cycle. The years 2008 and 2009 should have marked the beginning of a new cycle. At this writing, however, the new activity cycle does not yet seem to have started.

People living and tourists vacationing at high latitudes should take the time to go out at night and look for the aurora. The southern and northern lights are among nature's most impressive displays.

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