The Radiation Belts!

While learning about Earth as a planet, chances are, you have already found in your textbooks a curious figure that looks something like the one on the left.

What it shows is that Earth is surrounded by clouds of invisible particles. Some artist has probably colored the clouds purple and blue. The textbook will then tell you that these are called the van Allen Radiation Belts.

Other textbooks or artists might prefer to show you another view of the 'Belts' that looks something like this one. Here you can easily see Earth surrounded by the two sets of belts in a cut-away perspective view. You can also see Earth's magnetic field drawn with the dashed lines. Both of these views give you a sense of what the Belts look like, but they don't tell you very much about why they are important.

The Belts are formed by clouds and currents of particles that are trapped in Earth's magnetic field like fireflies trapped in a magnetic bottle. Artists like to draw them as though they look like dense clouds of gas. In fact, they are so dilute that astronauts don't even see them, or feel them, when they are outside in their space suits. Because you can't seen them from the ground at all, scientists didn't know they existed until they could put sensitive instruments inside satellites and study these clouds directly. They only had a hunch that something like them existed because they were predicted by certain mathematical models,

The existence of these Belts was one of the very first discoveries made at the dawn of the Space Age in the late 1950's. Back then, scientists such as Prof. James van Allen at the University of Iowa were very interested in particles called cosmic rays, which other physicists had detected from the ground as far back as the 1930's. What Prof. van Allen discovered, however, was that the particles flowing into his sensitive satellite instruments didn't get there helter-skelter. Instead, they seemed to come from only certain regions of space that the satellite traveled through! Later on, more detailed satellite data showed that there wasn't just one Belt, but actually three. The three distinct belts were caused by the flows of two kinds of charged particles: electrons and protons.

The Inner Belt (shown in blue above) between 600 and 3,000 miles (1,000 and 5,000 km) contains high-energy protons carrying energies of about 100 million volts, and electrons with energies of about 1 to 3 million volts. This is the Belt that is a real hazard to astronauts working in space!

The Outer Belt (shown in purple above) between 9,000 and 15,000 miles (16,000 and 24,000 km) consists of mostly electrons with energies of 5 to 20 million volts. This is the Belt that is a hazard to communication satellites whose sensitive circuits can get damaged by the fast-moving particles.

Where do the particles in the belts come from?

Here are more resources about the Belts: