While learning about 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 blue. The textbook will then tell you that these are called thevan 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 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 asProf. 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. If you loojk below, you will see a typical sketch of what the satellite paths looked like for Pioneer 1 and 3, and how the data was translated into a simple picture of the belts.
This is Van Allen's sketch of the inner and outer zones of the radiation belt made after Pioneer 1 and 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. The two lines that go from the upper left to the lower right are the paths of the satellite. Where, along the paths, would the instruments detect 10,000 particles per second? 1000? 10?
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.
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. Here is what this belt looks like if you were to slice in like a watermellon:
The low-energy electron belt actually overlaps the volums 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, 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.
Here is what the electron belts look like if you could slice them along a line of constant geographic longitude from north to south. Can you see where the Inner Electron Belt is Located? The Outer Electron Belt?
If we could look at these Belts from another angle in space, here is what they would look like: (TBD get from Friedlander)
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.
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.
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 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.
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. Here is a close-up of the Inner Belt according to a theoretical model:
The dotted line shows the surface of Earth. The dashed line shows the altitude of the Space Station, and the colored contours show where the Belt radiation is strongest (yellow) and weakest (blue). If you were orbiting at an altitude of 300 kilometers and happened to pass across the region to the east of South America, you and your instruments would get a dose of radiation! Here is what a map of this dosage would look like:
The red area shows where the radiation is the most intense, and blue where it is least intense.
Humans have been affected by Belt particles, though not as severly as some people might believe. To see how this works, we need to learn a little about how to measure the biological effects of radiation exposure.
Scientists measure radiation exposure by using a unit called theRad which stands for 'RAdiation-equivalent Dose' not 'radical'. If a piece of biological tissue is exposed to 1 Rad of radiation, this means a specific amount of energy as been delivered to the tissue. This energy can either do nothing, or it can cause damage. The damage it does is worse if it happens to be particles like electrons and protons, than if it is just electromagnetic radiation. So, to take the kind of radiation into account, a new unit is used called the Rem. When your parents visit their doctor and have to take a chest X-ray, this is worth about 0.020 Rem's - a unit we can also write as '20 milli-Rems' or 20 mRem. The place where you live has different kinds of naturally-occuring radiation from the ground and air, and in one year you get about 350 mRem of dosage. There is little you can do about this, but some places are worse to live than others. If you lived in the Rocky Mountains of Colorado, your exposure might be 380 mRem. If you lived near the sea coast, it might be only 300 mRem. Some people live in areas of India where the soils are rich in radioactive uranium. Their dosages are over 700 mRem per year!
Space Shuttle and Space Station astronauts inside their crafts receive about 2 mRems of additional dosage each time they pass through the SAA. In one day they can accumulate 30 mRem of dosage. 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 actual radiation exposures inside the Apollo space capsule were not much more than the total dose received by Space Shuttle astronauts.
This fact counters some popular speculations that the moon landings were a hoax because astronauts would have instantly died as they made the travel through the belts. In reality, 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'. These were caused by very energetic particles streaking through the fluid in the eye and colliding with retinal cells, leaving behind the appearance of a luminous, but fleeting, trail of light. Similar streaks have been reported by astronauts in the Space Shuttle and other near-earth missions during the most intense 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 the early 1960's before they were banned, humans temporarily created a new belt in a 'notch' region betwen the Inner and Outer Belts. This temporary belt eventually dissipated, but its traces could still be detected in ca 1980's.
Forces and Motions:
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)
Reading to be Informed Questions
5) Why is it difficult to study space plasmas?
9) Why do scientists have such a hard time understanding the origin of the van Allen Belts?
11) What is at least one aspect of space plasmas near earth that scientists currently can not explain?
14) How do charged particles move in magnetic fields in space?
16) Why dont particles traveling in opposite directions collide and disrupt the flows?
17) Are the van allen belts a danger to astronauts?