The Aurora Borealis in the northern hemisphere (and the Aurora Australis in the southern hemisphere) is one of nature's most spectacular events, perhaps even more inspiring than the St. Thomas Carnival fireworks. (Well, the sounds aren't as impressive.) The photo shown is but one of many different forms that are seen. The display may range from giant flowing blue curtains to crimson arcs. You can see a gallery of the many different types in photographs taken in Alaska by Jan Curtis. You can also see a gallery of auroras from space.
Although the Aurora Borealis can, on rare occasions, be seen as far south as middle America, typically it is seen only in the far northern regions. There is a reason for this. The aurora is the result of high speed charged particles from the sun (called the solar wind) being trapped by the earth's magnetic field. Most of the trapped particles are directed towards the poles where the field is strongest. Let's examine just how this happens.
Recall that the magnetic force exerted on a charged particle is always perpendicular to both the field and the velocity. In class, we saw how a uniform magnetic field will result in a circular path when the particle enters the field perpendicularly. However, most of the sun's charged particles enter the earth's field at an angle and the field is certainly not uniform. The result is that the charged particles move along the field lines in an ever-tighter spiral as they head toward the poles and into the earth's atmosphere.
It is the collision of the charged particles with atoms in the earth's atmosphere that produces the light. Electrons in the atmosphere's atoms are physically bumped into higher quantum energy states. When the electrons fall back to their original energy states, light is emitted. (We will study this phenomenum in some detail near the end of the semester.) Since this happens primarily near the earths' poles, you need to be near the poles to see the aurora. The Exploratorium in San Francisco has self-guided Aurora Tour that provides a few more details.
The Magnetosphere
The earth's magnetic field certainly has an effect on the solar wind. But it could be argued that the solar wind has a greater effect on the earth's field! In the absence of the solar wind, the earth's field would resemble that of a bar magnet with field lines emanating symmetrically from near the geographic south pole and flowing back into the earth near the geographic north pole, as shown in the picture to the right. This is approximately correct near the earth's surface.
However, the solar wind blowing from the sun severely distorts the earth's field into a complex plasma region called the magnetosphere. The high speed particles (averaging about 400 km/s!) constitute a current that creates it own magnetic field. The fields interact in a rather complex way. There are two good sources on the magnetosphere. The University of Oulu in Findland has a Magnetosphere Introduction that provides details of the various regions within the magnetosphere. NASA has a less extensive magnetosphere tutorial, as well.
There's one more point to consider. There's an incredible magnetic storm going on around the earth. The magnetosphere is generally not a quiet place. The magnetosphere plays a vital role in that it shields the earth's surface from the solar wind. (NASA makes a point to keep astronauts and satellites out of the most active regions of the magnetosphere.) Should you ever get the chance to see an aurora, think of all those high speed charged particles that are entertaining you, rather than penetrating you.
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Questions:
About how long does it take for the charged particles from the sun to reach the earth?
What events on the sun are correlated to auroras?
Which gas is responsible for the green color in auroras?
What phenomena gave Kepler the idea of a solar wind?
The Aurora Borealis in the northern hemisphere (and the Aurora Australis in the southern hemisphere) is one of nature's most spectacular events, perhaps even more inspiring than the St. Thomas Carnival fireworks. (Well, the sounds aren't as impressive.) The photo shown is but one of many different forms that are seen. The display may range from giant flowing blue curtains to crimson arcs. You can see a gallery of the many different types in photographs taken in Alaska by Jan Curtis. You can also see a gallery of auroras from space. 
Recall that the magnetic force exerted on a charged particle is always perpendicular to both the field and the velocity. In class, we saw how a uniform magnetic field will result in a circular path when the particle enters the field perpendicularly. However, most of the sun's charged particles enter the earth's field at an angle and the field is certainly not uniform. The result is that the charged particles move along the field lines in an ever-tighter spiral as they head toward the poles and into the earth's atmosphere.