Wednesday, May 14, 2008

Bouncing electrons (Part 1)

An electron is simply a very light particle with a negative charge. Usually, they're paired up with atoms, and they give us the full range of chemical reactions. But when electrons are by themselves, they're not really all that complicated, are they? In fact, above Earth's atmosphere, there are plenty of electrons all by themselves. Of course, by "plenty" I mean a near-perfect vacuum, but it's a lot by empty-space standards. There aren't really enough for them to bump into themselves very often. So what could they possibly do besides float leisurely in space?

It turns out that electrons do a lot, because they interact with Earth's magnetic field.

Magnetic fields, if you didn't know, are different from electric fields. Sometimes people get the electric force and magnetic force mixed up because on the surface they're so similar. The electric force, (which manifests in lightning and static electricity) causes like charges to repel and opposite charges to attract. Similarly, the magnetic force (which manifests in magnets and compasses) causes like poles to repel and opposite poles to attract. But magnetic poles do not attract or repel electric charges. They have a much stranger interaction.

The way that magnets work is by creating a magnetic field. The field consists of invisible lines that go from the north pole to the south pole. Technically, Earth's north magnetic pole is actually in the southern hemisphere, so sometimes "north" and "south" are mixed up (to the dismay of geophysicists).

When electrons, or any charged particles, move through the magnetic field, the magnetic field pushes them in a direction that is perpendicular to their motion and perpendicular to the magnetic field. Since the electron is being pushed neither forward nor backwards, it doesn't speed up or slow down. Instead, it simply changes direction, and travels around in circles. This type of motion is called "gyration".
The arrow V shows which direction that the electron (light blue dot) is going. The arrow F shows which way the magnetic field is pushing the electron. Because F is always perpendicular to V, the electron will constantly change directions and travel in the circular green path. The magnetic field in this example is coming out of the screen towards you.

Now, personally, I think it's just amazing that this weird physical force causes electrons to move around in circles of all things (even more amazing when you find that it is a consequence of Relativity). Another weird consequence is that the electron make circles at the same rate, no matter how fast it's going. If it's going really fast, it will simply make larger circles. In Earth's magnetic field, electrons will make hundreds of circles per second, regardless of their speed. This rate is known as the cyclotron frequency.

The electron cannot get too far away from the magnetic field line because it will simply circle back on itself instead. However, it can travel along the magnetic field lines without any resistance at all. The result is that the electrons are free to move along the magnetic field lines (in helix-shaped paths), but cannot jump from one line to another. It turns out that those invisible magnetic field lines aren't just mathematical curiosities, but they actually tell us where the electrons can move.

So where do the electrons end up? If you follow the magnetic field lines, don't they simply hit the earth? Yes, they do! Some of those particles will hit the upper atmosphere, creating colorful displays of light: the aurora. That's why the aurora is most common near the north and south poles--because that's where most of the particles come down and hit the atmosphere.

But not all of the particles hit the earth. Some of them "bounce" back along the magnetic field line! Find out why in Part 2.