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The Science Behind the Aurorae

by Dani Johnson

November 22, 2013

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Donate My last post was about how various old mythologies explained the Northern and Southern Lights, so it seems appropriate to talk about the amazing science behind the aurorae, next. In short, the phenomena is caused by charged particles that stream out from the sun (solar wind) and interact with the Earth's magnetic field lines and collide into Earth's atmosphere, creating the stunning light show we're familiar with.

The solar wind is a constant stream of high-speed charged particles (plasma) being blown from the corona of the Sun and travels at a velocity of at least 400km/sec (250 mi/sec). It can be viewed as an extension of the corona into interplanetary space, and it reaches to the farthest ends of our Solar System. Explosive events called Corona Mass Ejections (CMEs) will propel material from the Sun at speeds of around 1,000 km/sec (620 mi/sec), and these cause the most brilliant aurorae.

When charged particles, in general, hit a magnetic field they will accelerate and sometimes even change direction. The charged particles that are blown off of the Sun are already at a ridiculously high-speed, and when they hit the magnetic field surrounding the Earth they are plunged down into the atmosphere at the poles. It is when these accelerated, high-speed charged particles collide with things in our atmosphere that we get a light show.

I Remember playing with the Wooly Willy magnetic toy as a child and noticing that the metal filings in the toy did not cling to the "magic wand" in an orderly manner, they seemed to be chaotic and unpredictable. I always wished that I had better control over the filings, but little did I know that the filings are actually following a magnetic field line radiated by the magnet inside of the wand. A really cool experiment you can do at home to see the magnetic field lines is to get a bar magnet and place it under a sheet of plastic and sprinkle iron filings on top of it. I have provided an image below for clarity, but if you want to see some really cool experiments without actually having to do them yourself, check out this article from Evil Mad Scientist.

Above Image: Magnetic field lines around a series of cubed magnets placed together in a line. Image Credit: oskay Source: flickr; Sourced for this blog in 11/2013. Image is available under the Attribution 2.0 GenericLicense.

The Earth can be thought of as a giant bar magnet with magnetic field lines radiating in a similar pattern from the North pole and the South pole.

Above Image: A schematic diagram of Earth's magnetosphere. Earth is the circle near the middle and the plasma tail is denoted in yellow. [larger image] Image Credit: Larry Lyons/UCLA Source:; Sourced for this blog in 11/2013. Image is available because NASA is a government entity and its images and websites are not copyrighted.

When the high energy particles strike certain atoms in our atmosphere it causes them to jump to a higher energy. The excited atoms can only stay at the higher energy level for so long, and when they return to their original state they give off a specific color. Atomic oxygen gives off red, molecular nitrogen gives off blue, and molecular oxygen gives off green. Red is usually higher in the atmosphere, followed by green, the most abundant, and then blue, the rarest.

Our eyes are more sensitive to green light, and we can't see very well in the dark, so auroras look completely different to the naked eye than they do on camera. Bob King did a very informative article on Universe Today where he compares photos that he took of aurorae with what they look like on camera and what they looked like to him in real life. Most of the time our eyes will only see a whitish sheen with hints of greens, pinks or purples. That's not to say that it isn't an amazing spectacle, though, and I still very much wish to see the aurora in person.

Solar activity will hit the peak of its 11-year cycle at the end of this year, and the auroras are supposed to be some of the best of the decade. The farther north or south you are, the better your chances are of seeing the aurorae. For those of you fortunate enough to live in the northern United States, you probably see them pretty regularly when the conditions are right. For those of us who aren't so lucky, there are quite a few phenomenal places to view the lights this year if you're willing to travel, and there are even touring agencies that want to help their customers get the best viewing experience of their lives; not without a hefty fee, though. The best places to see the lights are in some parts of Alaska, Canada, Greenland, Northern Scandinavia and Northern Russia, although it is possible to see them a bit farther south. Here is a great article from CNN that goes into detail on locations and prices to see the lights.

I had a great time researching this article, but I'll admit that most of the science behind the aurorae goes way over my head. There are some great articles for those who may be interested in reading the nitty-gritty details. One of my favorite places to go for great astronomy information is Astronomy Cast, they have a wonderful podcast about aurorae that has been transcribed into a text article. Another great place to go for more information is this FAQ from the Geophysical Institute of the University of Alaska Fairbanks. There are a lot of great places to go, I have listed the ones I've used as reference for this article below. If you know of other places that I haven't mentioned, feel free to mention them in the comment section below.

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by Dani Johnson

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