(both pics courtesy of http://www.geo.mtu.edu/weather/aurora/)

AURORA BOREALIS: THE MYTHS AND THE FACTS

    The aurora borealis, more commonly known as the northern lights, is a phenomenon that has fascinated human kind for ages, and will likely be the source of study for years to come. It represents an atmospheric and astronomical event that can be easily and frequently observed by any casual observer on this planet, granted that they are at the right place at the right time. Science has provided us with explanations and reasons as to why auroras occur in our atmosphere, but still questions remain unanswered. This web site is dedicated to offering a brief enlightenment into the world of aurora in our atmosphere, through its history, scientific fact, and future research.
 
(pic courtesy of http://www.uit.no/npt/nordlyset/waynorth/02.en.html)
    In ancient times, the northern lights were commonly the subject of folklore and mythology. Many different cultures and religions have adopted the phenomena into their writings, some with more logical explanations than others. Middle-Age Europe offered a wealth of different view. Some believed the presence of the northern lights to be on omen from the heavens, with the color dictating the meaning.(1)Others yet believed it to be the breath of ascended warriors enjoying combat for ages to come. Naturally, these ideas were not proven with any form of science, as last time we checked there were no ancient warriors hanging around in earth's outer atmosphere.

    Oddly enough, though, even with today's technology and scientific prowess, people still hold heavy superstitions and misconceptions about aurora, some of which will be addressed later on in this page. Fact remains that the presence of aurora can be explained as an interaction of solar wind with the earth's ionosphere. The question is now exactly what do those two terms mean individually, let alone how they interact with one another. Before getting down to the nitty-gritty of aurora science, let's examine the principles of solar wind and the ionosphere.


(pic courtesy of http://www.seds.org/billa/tnp/sol.html)

    Solar wind is essentially particle radiation emitted from the sun itself. How does this work, you ask? Eugene Parker answered that question in 1958 through a series of different experiments and theories. It was originally believed that the sun's corona at great distances would be essentially zero in both pressure and density, but this idea was not consistent with the amount of heat conducted from the sun.(2) It was found that a low density stream of particles, the majority of them being protons and electrons, leaves the sun at an average speed of 450 km/sec.(This value actually increase to 750 km/sec at the sun's poles)(3)

    This solar wind propagates to and beyond our planet earth, where it has a variety of interesting effects. It is responsible for the shape of the earth's magnetic field, the trail of comets visible to us here on the surface, and of course, the northern lights.(4) It should be mentioned, though, that while a baseline level of this particle radiation exists, it tends to vary with the amount of sunspots on an 11 year cycle. The solar wind, as mentioned earlier, does only take a portion of the responsibility for the aurora, the other major contributor being the earth's ionosphere, which shall be discussed next.


(pic courtesy of http://www.uit.no/npt/nordlyset/waynorth/03.en.html)
    The earth's atmosphere has distinct regions of different compositions and densities as altitude increases. Naturally, the chemistry of each of these layers has unique properties to them as well. The region known as the ionosphere exists at altitudes greater than 80km. The ionosphere bears properties of both gas and plasma, due to the fact that electrons exist for short periods of time.(5) This layer is credited with much of the electromagnetic phenomenon viewed from the earth's surface, such as AM radio skip. Now that these two principles have been discussed, it's on to what you've actually visited the sight for…….an explanation of how aurora occur!

Magnetic Field Surrounding Earth

(pic courtesy of http://www.uit.no/npt/nordlyset/waynorth/07.en.html)

    Alright, a quick recap here before we proceed. We've established that there is a source of particle radiation aimed towards the earth, that there is a magnetic field surrounding the earth, and that there is an atmospheric region of charged particles that has both the properties of plasma and gas. The entire process begins with the particle radiation, after 2 to 5 days travel through space, reaching the earth's magnetic field. As mentioned earlier, the magnetic field itself can be affected by the solar wind, which is the case in this situation. The daylight side of the earth's field is compressed, while the night side's field is pulled out into a long tube shape, or tail.(6) The current of particle radiation bends around the earth much the way water would flow around marble placed in a stream…..one side is heavily exposed(daylight), while the opposite side experiences a vacuum of the flowing material(night).

(top) Normal Magnetic Tail, (bottom) Magnetic Tail During Aurora

(pic courtesy of http://www.uit.no/npt/nordlyset/waynorth/05.en.html)

    It is on this "vacuum" side that the interesting phenomena occur. When the solar wind strengthens to a certain point, the vacuum side or tail begins to loose its cylindrical shape. The particles that had passed around the earth come flying through the vacuum space on the night side. Eventually, enough strength and instability will cause the particles to curve in to the point that they actually collide with charged particles in the ionosphere.(7) The gasses and charged particles conduct the electrical energy from the particle radiation, thus causing aurora we observe by emitting light.(To further understand the principle of charged particles and this phenomenon, please consult a chemistry text.) Keep in mind, now, that this is all occuring at distances greater than 80km from the earth's surface, meaning that it can be observed from both sides of the ionosphere.

    It is the actual observation of the northern lights that is the next topic for discussion. Naturally, due to the tilt of the earth, one is more likely to see aurora at one pole or the other depend upon what season it is. Keep in mind, though, that the magnetic poles associated with the phenomenon are not located at the same place as the geographic poles on maps. Thus, there is an oval, centered about the magnetic poles, that defines where aurora will occur.(8) Please note that these ovals do not coincide to the very ends of the geographic world as we know it. Areas within the oval are much more likely to see aurora then areas outside the oval, although there is always a chance of being able to see them at locations quite far from the poles.

Auroral Viewing Ovals

(pics courtesy of http://dac3.pfrr.alaska.edu/~pfrr/AURORA/FAQ.HTM)

    Contrary to common belief, the aurora, while containing many different colors of light, does not cover the entire spectral band. Upon analysis, distinct bands are noticeable in an aurora's spectrum. The first man to map the colors of aurora was Lars Vegard.(9) He mapped out the more common colors, and found that reds occurred most frequently.


(pic courtesy of http://www.uit.no/npt/nordlyset/waynorth/07.en.html)

    One idea presently under heavy debate is whether or not aurora make audible sounds when they occur. Many eyewitnesses have claimed to have heard sounds similar to wobbling a thin piece of sheet metal, and other a distinct hissing noise. Researchers are currently examining the idea that sounds are actually transmitted via either electromagnetic waves transduced into acoustic ones, direct transmission of audible sound, or brush discharge.(discharge through pointed objects)(10) The biggest opponent to these ideas state that the event is taking place at 80km above the earth's surface, and the noises observed would not be in sync with changes in the lights as eye witnesses have stated, simply because sound would take a noticeable amount of time to travel that distance.(11) It will likely be some time before research in this area is concluded, as a large pool of witnesses is needed.

    Well I hope you've enjoyed our brief little excursion into the world of aurora. Now that explanations have been given, you sense of sight will be satisfied by the collection of pictures and links found below. Feel free to browse around…..there is a wealth of information about aurora both on the web and in libraries, and science still has many unanswered questions to toy with. Enjoy you're intellectual journey, wherever it may lead you!

References
(1)  http://www.uit.no/npt/nordlyset/waynorth/02.en.html
(2)  http://www-spof.gsfc.nasa.gov/Education/wsolwind.html
(3)  http://www.seds.org/billa/tnp/sol.html
(4)  http://www-spof.gsfc.nasa.gov/Education/wsolwind.html
(5)  http://www.haarp.alaska.edu/haarp/ion1.html
(6)  http://www.uit.no/npt/nordlyset/waynorth/04.en.html
(7)  http://www.uit.no/npt/nordlyset/waynorth/04.en.html
(8)  http://www.dac3pfrr.alaska.edu/~pfrr/AURORA/FAQ.HTM
(9)  http://www.uit.no/npt/nordlyset/waynorth/07.en.html
(10)  http://members.tripod.com/~auroralsounds/
(11)  http://www.dac3pfrr.alaska.edu/~pfrr/AURORA/FAQ.HTM
 

Links
Here are some links I found to both interesting and fun to look at.
  The Aurora Page  .....Lots of neat links to some pretty interesting pics, as well as to scientifiic data

  Auroral Sounds. .......Good information on current research in Auroral sounds, as well as an MPEG of an aurora

 The Northen Lights ....Web site from a research group inside the northern oval!.....Good info, neat pics, and a
                                   Quicktime movie on Aurora Borealis

More Pics  (all pics below courtesy of http://www.geo.mtu.edu/weather/aurora/)

Aurora pic from space shuttle

 

Auroral pics from Alaska