Van Allen Belts

Contents

Introduction

Within the magnetosphere there are several distinct populations of neutral particles and plasmas that have been recognized since the 1960's. The Van Allen Radiation Belts were discovered in 1958 during the early days of the Space Age. There are two of these donut-shaped clouds that gird the equatorial regions of the Earth and are roughly symmetric with its equatorial plane. The inner belts extend from approximately 1,000 to 5,000 km and contain very high-energy protons trapped in the Earth's magnetic field. The outer belt extends from approximately 16,000 to 24,000 km and consists mostly of high-energy electrons. Geosynchronous satellites orbit the Earth just outside the limits of the outer belt, and human space activity is confined to the zone within the inner edge of the inner belt.

Within Earth's magnetic field, electrically-charged particles tend to bounce from pole to pole and drift east or west. The van Allen Belt particles do likewise. Instead of smooth donuts, it would be more correct to show the clouds as having sharp poleward "horns" rather than a smoothed rounded shape. At the tips of the horns, particles either collide with the atmosphere and are lost from the Belts, or are reflected back into space along the magnetic field.

Space suited astronauts exposed to the energetic particles in the Van Allen Belts would receive potentially lethal doses of radiation. The particles that make up the Van Allen Belts bounce along the north and south-directed magnetic lines of force to which they are trapped like water flowing in a pipe. At the same time, there is a slow drift of these particles to the west if they are positively charged, or eastwards if they are negatively charged. An electron makes one complete orbit towards the east in about 50 minutes. This is a much faster orbit rate than for satellites because electromagnetic forces are much stronger than gravitational ones.

The Inner Belt (shown in blue left) 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 left) 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.

History

The Van Allen Belts are an example of particles that are trapped in Earth's magnetic field much like fireflies trapped in a bottle. Although artists like to draw them as though they look like dense clouds of gas, they are so dilute that astronauts don't even see them, or feel them, when they are outside in their spacesuits. They are so dilute, in fact, that scientists didn't know they existed until they could put sensitive instruments inside satellites and study these clouds directly. This was one of the very first experiments conducted at the dawn of the Space Age in the late 1950's. Back then, scientists such as Prof. James van Allen at Iowa State University were very interested in particles called cosmic rays, which other physicists had detected from the ground as far back as the 1930's.

As soon as satellites could be built and orbited in space, van Allen and his team put an instrument onboard satellites such as Explorer I and Pioneer 3, which could detect these fleeting particles before they entered the atmosphere. What they discovered when they looked at the data was far different than what they expected to find. As the satellite moved in its orbit, the instruments recorded a changing flow of cosmic ray particles along the orbit, but the changes were not random at all. By mapping out where, in space, the flow was strongest and weakest, they discovered over the course of many satellite studies, that there were clouds of very high energy particles near Earth. These clouds had a definite shape to them, being thickest in the equatorial plane, and thinning out as they approached either pole.

Van Allen was a bit surprised to discover these Belts because he was not really looking for them. He was interested in studying a completely different phenomenon in Nature—cosmic rays. By the time of van Allen's discovery, however, another community of scientists who study the Earth's space environment had already expected that these Belts would be there to find. They weren't really sure just what kinds of particles would be involved nor what their energies might be. One of the leading scientists of this group was Carl Stormer, whose mathematical research had shown that such clouds of trapped particles should exist in space, long before the first rockets actually detected them.

Structure

Careful satellite studies over the last 50 years show that there are actually two kinds of already familiar particles that make up the Belts: electrons and protons. The individual particles carry a lot of energy, and it is convenient to talk in terms of their energies when describing the Belt particles. This is where the story gets a bit interesting. There are two electron belts and one proton belt:

The proton belt is located from about 500 kilometers above Earth's surface and extends to 13,000 km. This Inner Belt contains protons with energies greater then 10 million volts. Scientists currently think that these protons are trapped cosmic ray particles from outside the solar system, or from the Sun itself possibly during severe solar flares.

The low-energy electron belt actually overlaps the volumes of space where the proton belt is located in the Inner Belt. The electrons carry between 1 - 5 million volts of energy, on average.

The high-energy electron belt is located further out than the two overlaping inner belts. Electrons in this Outer Belt carry between 10 to 100 million volts of energy, on average.

Although we have learned a lot about the Belt particles in the last 50 years, there are still some very big questions about them that, as yet, have no answers. Space physicists don't completely understand where they come from, or how their energies can be so "astronomical" compared to either the plasmasphere particles or Ring Current particles. Typical "Belt" particles carry energies between 1 and 100 million volts. The rest of the particles that we can encounter near the earth barely have energies higher than 200,000 volts. This means there is a BIG difference betwen the van Allen Belt particles and the others. It's such a big difference that its like comparing a cat and a dog and asking what their common ancestor might have been.

There is also another aspect to these Belts of particular interest to manned space flight and satellites. Because the magnetic field of Earth doesn't exactly line up with the Earth's rotation axis, the Belts are actually tilted a bit. Their influence is stronger in equatorial regions over South America. This also means that, because the Belts follow the Earth's magnetic field not its geographic shape, they are closer to the ground over South America and the South Atlantic. This means that if you were in a Space Shuttle, Space Station or operating a satellite as it passes over the South Atlantic, you will be closer to the Belts and receive a larger than average dose of radiation from them as their particles penetrate your spacecraft or satellite skin. This region is called the South Atlantic Anomaly. It affects astronaut radiation dosages as well as data and signal transmission quality from all spacecraft passing through this continent-sized region.

External Links

• Carl Stormer - Education and Public Outreach Program, Imager for Magnetopause-to-Aurora Global Exploration (IMAGE), Goddard Space Flight Center, NASA.
• James van Allen - Education and Public Outreach Program, Imager for Magnetopause-to-Aurora Global Exploration (IMAGE), Goddard Space Flight Center, NASA.
• Radiation Belt - History - Educational Web Sites on Astronomy, Physics, Spaceflight and the Earth's Magnetism, David P. Stern, D.Sc.
• Radiation Belt Images & Movies - Radiation Belt Modeling for Living with a Star, Education and Public Outreach Program, Imager for Magnetopause-to-Aurora Global Exploration (IMAGE), Goddard Space Flight Center, NASA.
• Radiation Belts - Educational Web Sites on Astronomy, Physics, Spaceflight and the Earth's Magnetism, David P. Stern, D.Sc.
  
• Radiation Belts & Trapped Particles - Radiation Belt Modeling for Living with a Star, Education and Public Outreach Program, Imager for Magnetopause-to-Aurora Global Exploration (IMAGE), Goddard Space Flight Center, NASA.
• Radiation Effects & Analysis Group - (REAG) performs testing & analysis, investigating the effects of radiation on electronics & photonics. Goddard Space Flight Center, NASA.
• South Atlantic Anomaly - ROSAT, the ROentgen SATellite, Goddard Space Flight Center, NASA.
• Van Allen Belts - Education and Public Outreach Program, Imager for Magnetopause-to-Aurora Global Exploration (IMAGE), Goddard Space Flight Center, NASA.

Preview Image

"Illustration of Earth's Radiation Belts" - This rendering shows the Inner (green) and Outer (purple) Belts, but the sizes of these belts are closer to their real dimensions.  (Source: Geoff Reeves/Los Alamos National Laboratory, NASA.)