Monday, August 29, 2011

Van Allen Radiation Belts

The Earth actually has trapped belts of charged particles surrounding it in a torus. These belts are called, you guessed it, the Van Allen radiation belts. These belts were actually discovered right here at the University of Iowa, my physics class is even held in a building named after Van Allen.
This is a torus

There are really only two belts of radiation, the inner and outer belts (pretty easy to remember). Even though these two belts compose the Van Allen radiation belt, they are very different. The outer belt is mostly composed of electrons, with a few protons and oxygen ions thrown in for good measure. Most of the outer belts particles are trapped from solar winds and cosmic rays, but the unique composition of ions suggests that there are several other sources. The inner Van Allen belt is a mixture of protons and electrons with far more protons than the outer belt. Unlike the outer belt, the inner belt particles come mainly from cosmic rays impacting the upper atmosphere. Both belts are held in place by the Earth's magnetic field.
Simulation of Van Allen belt on solar wind.

The number of charged particles in the Van Allen belt are fluctuating all the time, more so in the outer belt than the inner belt. The outer belt is farther away from the center of the Earth's magnetic field, and is thus more subject to fluctuations in that field, or to fluctuations in solar activity. One moment the outer belt could be massively charged, and the next moment most of its particles could have flown away.
It has been recently discovered that the Van Allen belts have trapped antimatter. There is a significant quantity of antiprotons, orders of magnitude more than thought, trapped by the Earth's magnetic field.

Van Allen radiation belt

I can imagine that many people would question the importance of these belts, and so they should (it's always good to question). Even if they don't seem like it, these belts have a massive impact on our daily lives. The highly charged particles pose a significant threat to anything that we do in space, some of the highly charged protons in these belts can penetrate more than 140 mm of solid lead. Humans who stay in the radiation belts for extended periods of time have to be concerned with radiation poisoning, but you might still ask what impact this has on your life. Humans aren't often orbiting the earth, but satellites are. Most satellites have to orbit somewhere in either of the radiation belts, and this can pose a serious threat to their hardware. Any number of sensors and computer components can be overloaded by the plasma surrounding our planet, so extensive measures must be taken to assure that these satellites don't go offline and crash your GPS device. There is a safe zone between the two radiation belts, as the above picture might indicate.

A rather interesting proposition has been made. Two physicists, Robert Hoyt and Robert Forward, have come up with something that they call HiVOLT (high voltage orbiting long tether). The basic idea is that a highly charged tether would be attached to satellites to deflect particles into the Earth's atmosphere where they would be harmlessly dissipated. I have no idea if this would work or not, but it is fascinating.

No comments:

Post a Comment