Friday, May 16, 2008

Bouncing electrons (Part 2)

See Part 1

When we last stopped, I had shown that electrons move along magnetic field lines. But if they simply follow the magnetic field lines, they'll eventually hit the Earth. Some do hit the earth, but others actually bounce back in a process called "magnetic mirroring". Why is this?

Ok, so I'm using the same picture. This time, I want you to notice that near the poles, the lines are very close together. The lines spread out much more when they are far away from the earth. When the lines are closer together, this means the magnetic field is stronger. When they are further apart, it means the magnetic field is weaker. It should come as no surprise that Earth's magnetic field is strongest when you are close to Earth.

This is a bit of an oversimplification, but the basic idea is that stronger magnetic fields repel electrons.* Therefore, if an electron is traveling along a magnetic field line, getting closer to Earth's north pole, it will eventually turn around. And then it will follow the magnetic field line all the way to the south pole. But since the magnetic field is stronger near the south pole, the electron will turn around again. As a result, electrons will bounce back and forth from pole to pole. Each bounce happens in a matter of seconds.

One of the results of this bouncing is that some regions of Earth's magnetic field are like traps for electrons (as well as other charged particles). And so we have the Van Allen Radiation belts, where lots of high energy radiation is trapped. Their shape can be described as "toroidal" or "donut-shaped". There are several other important regions above Earth with similar shapes.
Mmmm... radiation donuts.

Ok, so electrons are doing two things at once. They are gyrating and bouncing from pole to pole. But that's not all!

There is a third type of motion caused by Earth's gravity. Electrons, though very light particles, still fall. Only they don't fall. Remember, they're still trapped on magnetic field lines. If they fall down, they will very quickly circle around back up. So perhaps gravity has no effect at all? But it does have an effect! Unlike the magnetic field, gravity actually slows down and speeds up electrons instead of simply changing their direction. And faster electrons make larger circles! One side of the circle (the one closer to Earth) will be larger while the other side will be smaller. The resulting motion will look something like this.
As weird as it sounds, downward gravity causes the electron to "drift" to the side! Specifically, electrons will drift eastward. It takes a few minutes for them to go all the way around the Earth. Positively charged particles will also drift, but in the westward direction. Negative charges drift east, positive charges drift west, and we've got an electric current! This is called the ring current. Scientists measure the ring current to determine how many particles are in space, which tells us something about how the "space weather" is going.

And so, electrons above Earth have three types of motion. They gyrate, making hundreds of circles every second. They bounce from north pole to south pole in a matter of seconds. They drift eastward, going around the earth in a few minutes.

There's one last detail I want to add (and there are always more details), because it is related what I researched. All of the above types of motion conserve energy. The electron doesn't really change its speed much. However, this assumes that Earth's magnetic field is constant. It isn't. A stream of particles called the solar wind is always coming out from the sun. When these particles hit the Earth's magnetic field, they cause the magnetic field lines to vibrate like harp strings. Now, each of the three types of motion occurs at a different frequency. If the harp strings vibrate at a frequency near one of the types of motion, a resonant interaction will occur! For example, if the magnetic field line fluctuates every few minutes, it will resonate with the drift motion. The electrons might move between field lines, or speed up. We think this is the cause of one of the Van Allen Radiation Belts, but we're not sure. To find out, we must take lots of data in various circumstances to see if the evidence all lines up!

*Electrons don't actually slow down when moving into stronger electric fields, they simply transfer some of their forward motion to their circling motion. The technical description of this is that electrons conserve their "magnetic moment" under ordinary conditions.


DeralterChemiker said...

This is fascinating. But I ask myself, "How does he know all this? Can I believe him?" For example, can you tell me how one can prove that electrons bounce from pole to pole within a few seconds? Is it just a calculation without proof? Excuse me for being skeptical, but one electron looks like any other (disregarding spin).

miller said...

That's a good question, and luckily I can answer it!

Most of what I've shown here is directly derived from theory. We have equations to describe motion in magnetic fields. We know these equations are correct because of experiments in labs. I've actually seen this circular motion myself. We know these equations remain correct above Earth because they are consistent with astronomical observations as well as satellite observations.

Of course, theory can only take you so far, and much of the details were discovered after the space age, when we started sending satellites above the earth. The Van Allen radiation belts were only proven to exist when spacecraft went right through it and observed the radiation. Soon, a lot of theory developed to explain all the different details that we observe.

Here are a few examples of how we can test these theories:

1. Magnetometers on the ground can measure directly the vibrations of the magnetic field lines. They can also measure the ring current because all currents create magnetic fields.
2. Spacecraft can directly measure the surrounding magnetic field and other properties of the surrounding plasma.
3. Satellites can send signals to the ground, and sensitive equipment can be used to measure how the signal changes as it travels through the ionosphere.

The theory of electron motion above the Earth makes several predictions that can be confirmed or falsified. Off the top of my head, it should make some predictions about the distribution of electrons and their current, as well as various properties of the waves that travel along magnetic field lines. Beyond that, it gets more complicated.