EIU Astro

Aurora Borealis – Best Light Show on Earth July 21, 2009

Sergey Zinovichik has his blog at:       http://chemfan.wordpress.com/

Aurora Borealis or commonly named The Northern Lights is a natural display of lights in the North Pole regions of our globe.  The lights appear as flowing streams of color in the sky taking different shapes and contours and often look like a multicolored curtain coming down from outer space.

The light show comes to us courtesy of our Sun.  Electrically charged particles produced on the Sun are ejected in all directions and a great amount of them head toward Earth.  As these particles (called solar wind) encounter the Earth’s outer regions, the magnetic field of the Earth begins to interact with the particles.

What results is a change in energy and direction of the particles: they head along magnetic field lines toward the poles of Earth.  Just like a bar magnet has its field lines returning at the opposite ends so the Earth’s magnetic lines come together around the poles.  This is why the aurora borealis is brightest and most easily seen in the northern regions of our planet.

Image taken from Wikipedia.org

The colors that we see in the aurora get produced from the collision of solar wind with atoms and molecules of our atmosphere.  Every atom and molecule has around them orbiting electrons.  The electrons sit in different energy levels with the highest energy electrons being furthest from the nucleus.

Although the orbits of electrons around the atom are complicated we can represent  their arrangement with a simple picture:

We can see the first important aspect of orbiting electrons: They can only have certain energies analogous to planets orbiting at certain distances from the Sun.

When a fast moving solar wind particle collides with an atom the electrons in the atom “jump up” to a higher energy level.  The electrons in this excited state are usually unstable and will eventually fall back down releasing the energy they absorbed.  The way electrons release energy is in the form of electromagnetic radiation or what we would call light.

Image taken from HowStuffWorks.com

The type of light emitted depends on the type of atom that was excited.  This is because an oxygen atom’s energy levels differ somewhat from a nitrogen or helium atom.  The energy released corresponds to a certain visible color and the color corresponds to the energy that was lost as the electron returned to its previous state.  The colors we see in aurorae are regular because the energy levels in an atom are the same and because electrons can assume only certain quantized energies.

In some aurora red and green colors are prevalent.  This is characteristic of oxygen atoms being excited and releasing energy.   Molecular nitrogen can produce blue or violet colors.  Helium in rare occasions can produce orange light.  Compounds of oxygen and nitrogen can produce their own characteristic light.

When a mixture of different colors comes together the aurora we see is colored white just like the visible light from the sun put together from all the colors of the rainbow appears white.

Image Taken from APOD

The beauty of the Northern Lights can sometimes come down from the polar region and become visible at lower latitudes.  This is due to periods of high solar wind corresponding to unusual activity on the Sun.  In August and September of 1859 The Northern Lights were seen from the Midwestern region of the United State and were very bright.   Worldwide reports of unusual aurora intensity and frequency  that year were confirmed by magnetographs recording high levels of magnetic activity from the Kew observatory in England.

The southern pole of the planet experiences a similar phenomenon called Aurora Australis or Southern Polar lights and this light show is visible only in the southern hemisphere.

If you are fortunate enough to live in a part of the globe that has auroral displays you will probably agree that the light show that nature displays cannot be rivaled by anything humans can create.

Image taken from APOD