While learning about the Earth as a planet, chances are, you have already found in your textbooks a curious figure that looks something like this:
What it shows is that Earth is surrounded by clouds of invisible particles, which some artist probably has colored purple and gold. The textbook will then tell you that these are called the van Allen Radiation Belts. The '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 looked 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 spacesuits. They are so dilute, in fact, that scientists didn't know they existed until they could put sensitive instruments inside satellites near 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.
What were they? Where did they come from?
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. A typical sketch of what the satellite paths looked like for Pioneer 3, and how the data was translated into a simple picture of the belts is shown below:
This is Van Allen's sketch of the inner and outer zones of the radiation belt made after Pioneer 3 data returns, as the sketch was presented in a paper by J. A. Van Allen and L. A. Frank, in the science journal Nature in 1959.
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, though they weren't really sure just what kinds of particles would be involved nor what their energies might be.
To learn more about why they are such a hazard to astronauts and satellites, let's take a closer look at them!
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 beltis 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 beltactually overlaps the volums of space occupied by the proton 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, and in the above figure it is colored purple. Electrons in this Outer Belt carry between 10 to 100 million volts of energy, on average.
A Modern Mystery:
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.
Even today, space physicists don't fully 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 consist of electrons and protons, with energies between 1 and 100 million volts. Energetic ring current particles, by comparison, hardly carry more than 150,000 volts of energy.
As we saw in our section on plasma motion, 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 shape. At the peak 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. There is also another aspect to these Belts of particular interest to manned space flight and satellites. Because the magnetic field of Earth is shifted off the rotation axis, in space, its 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.
Humans have been affected by Belt particles, though not as severly as some people might believe. Space Shuttle and Space Station astronauts inside their crafts receive about 2? mRems of additional dosage each time they pass through the SAA. Over the course of a week, this adds up to 7 x 30 = 210 mRems which is just below the dosage you get at ground-level in a single year (about 350mRem). Apollo astronauts, however, were forced to traverse the most intense regions of the Belts in their journey to the Moon. Fortunately, the travel time was only about 30 minutes so their exposures were not much more than the total dose received by Space Shuttle astronauts (TBD). This fact counters some modern speculations that the moonlandings were a hoax because astronauts would have instantly died as they made the travel through the belts. They may have experienced minor radiation poisoning if they had been in their spacesuits on a spacewalk, but no spacewalk was ever scheduled for these very reasons. The shielding provided by the Apollo space capsule walls was more than enough to shield the astronauts from all but the most energetic, and rare, particles. Still, the astronauts reported seeing 'shooting stars' (TBD?). These were caused by very energetic particles streaking through the fluid in the eye and leaving behind a luminous, but fleeting, trail of light. Similar streaks have ben reported by astronauts in the Space Shuttle and other near-earth missions during solar storms. It is not known what the long-term consequences of these kinds of brief exposures are upon astronauts, but prospective travelers to Mars will no doubt see many more of them!
Humans have also made a significant impact upon the Belts through older programs of nuclear testing. In fact, during tests in ...., humans temporarily created a new belt in a 'notch' region just beyond the proton belt. This temporary belt eventually dissipated, but its traces could still be detected in ca 1980's. (Relevant AAS Banchmark....Activity 21....Figure 21)
Reading to be Informed Questions
Because of the level of detail, this activity is only recommended for Grade 9-12.
1) Where do scientists think that the Ring Current particles originate? Why?
2) Do the items mentioned in the essay constitute a system?
3) What are the major exchanges of matter and energy that are descibed within the magnetosphere?
4) How do scientists study the Ring Current from the ground?
5) Why is it difficult to study space plasmas?
6) Explain how a radar gun could be used to study invisible clouds of plasma?
7) Why does the ionosphere reflect radio waves? Under what specific conditions would it reflect radio waves with a frequency of 1 gigaHertz?
8) How are the plasmasphere and ionosphere related to each other?
9) Why do scientists have such a hard time understanding the origin of the van Allen Belts?
10) What is the process called 'magnetic reconnection' and why is it important in space physics?
11) What is at least one aspect of space plasmas near earth that scientists currently can not explain?
12) How do scientists develop explanations for processes operating in space?
13) How do terrestrial experiments help scientists construct better models for the magnetosphere?
14) How do charged particles move in magnetic fields in space?
15) Why do positive charges circulate westward and negative charges eastward?
16) Why dont particles traveling in opposite directions collide and disrupt the flows?
17) Are the van allen belts a danger to astronauts?
From David Stern 'Exploration of the Magnetosphere' Mirroring particles from David Stern.
Drifting particles by David Stern.
Trapped particles. from SPENVIS.
Trapped proton belt. From AP-8 MAX (SPENVIS)
Trapped electron belt. From AE-8 MAX (SPENVIS)
South Atlantic Anomaly. From AP-8 MAX (SPENVIS)
SAA and electron distribution. From AE-8 MAX (SPENVIS)
Trapped particle motions. From http://www.estec.esa.nl/wmwww/wma/rad_env.html
SAA. From http://www.estec.esa.nl/wmwww/wma/rad_env.html
SAA close up in orbit. From http://www.estec.esa.nl/wmwww/wma/rad_env.html
SAA at surface. Intensity map. From http://www.estec.esa.nl/wmwww/wma/rad_env.html
(Picture 1. See Credits below)
(Picture 2. See Credits below)
It once was thought that the Earth was surrounded by near-empty space, in which the Earth's magneticfield would trace a pattern resembling that of a bar magnet (orange lines). However, the first American spacecraft, Explorer 1 (shown here), discovered a belt of energetic particles trapped in thefield and streaming back and forth above the Earth. It was the first of two such zones, the Van Allen belts, to be found. (Picture 4. See Credits below)
Simulated Van Allen Belts generated by plasma thruster in tank #5 Electric Propulsion Laboratory at the Lewis Research Center, Cleveland Ohio, now John H. Glenn Research Center at Lewis Field. (Picture 5. See Credits below)
(Picture 6atozscience )