why they appear
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| Photo courtesy of:
http://edgeoftheplank.blogspot.com/2011/02/northern-lights-aurora-borealis.html |
The aurora borealis, commonly referred to as the northern lights, appears as curtains of light in the sky in geographical areas of northern latitude. These lights are the result of a series events that start at the sun and end in the night sky.
First, we start with the solar wind. This solar wind is a thin layer of plasma that the sun boils off of its outer layers. This plasma is comprised of protons and electrons. This plasma maintains the properties of the sun's magnetic field even after it leaves the sun
and
encounters a planet. Normally, the particles just move around a planet
that comes across their path. However, the Earth has a magnetic field
of its own; and these particles experience a force from this magnetic
field, which changes their path of motion.Once the solar wind comes into contact with the Earth's magnetic field, 3 things happen. At first, the magnetic field of the wind will bend the magnetic field. Once the particles get closer to the Earth, the magnetic field of the wind and the Earth begin to balance each other out. Next, the magnetic field of the Earth becomes too strong and stops bending. Then the solar wind forms a magnetosphere, which is area around the Earth that is bounded by the solar wind, and the plasma enters this Photo courtesy of: http://odin.gi.alaska.edu/FAQ/#what
magnetosphere.
The third step in this process is the most difficult to understand. Where the solar wind and the magnetosphere overlap, energy is transferred into the magnetosphere through a number of processes. The most effective of these processes is called reconnection, which is best described in Laura Layton's article, "THEMIS Satellites Discover What Triggers Eruptions of Northern Lights":
"The culprit turns out to be magnetic reconnection, a common process that occurs throughout the universe when stressed magnetic field lines suddenly "snap" to a new shape, like a rubber band that's been stretched too far" (2008).
(For a video that animates this particular process, visit http://odin.gi.alaska.edu/FAQ/#what.) This process of reconnection is extremely efficient in transferring energy to the plasma in the magnetosphere. Since the plasma wants to dissipate this energy, it travels to the Earth's upper atmosphere, where it dissipates it in the form of a photon, or a light. This creates the shimmering lights we see on Earth.
The following
video sums up the entire process in somewhat simple terms: