Van Allen Belts
When Explorer I, America's first satellite, was launched in early 1958, it contained a payload of miniaturized instruments designed to study the regions surrounding the earth in space. James Van Allen, an expert on cosmic rays and skilled with electronic miniaturization technology, had designed the small instrument package that orbited over six hundred miles above the Earth.
One of the most intriguing findings of Explorer I involved radiation levels surrounding the Earth. As the satellite ascended, cosmic ray levels as measured on its Geiger counter rose to a peak, dropped off suddenly to zero, and then returned to high levels once again. Explorer 3 gave the same peculiar readings. Carl McIlwain, one of Van Allen's graduate students, suggested that a high enough level of radiation could jam a counter and send the apparent counting rate to zero. Van Allen speculated that the satellites had found a large band of radiation too intense for the detector to register. If true, this was discouraging news because it meant that humans would not survive a space flight without heavy shielding.
Van Allen designed a lead-shielded Geiger counter for Explorer 4. As feared, it indicated high levels of radiation rather than giving a "zero" reading. As the satellite orbited the earth many times, the shape of the radiation belt was established using information it relayed to the ground. The radiation formed a band around the Earth at the equator, but curved toward the Earth's surface near the polar regions. The overall shape of this band was not unlike a fat doughnut with the hole along the Earth's axis, following the theorized shape of the Earth's magnetic field.
Measurements taken by the moon-bound Pioneer 3 later revealed two distinct belts. The inner belt reached its peak of radiation at about 2,000 miles (3,380 km) from the Earth, while the outer one was located at an altitude of about 10,000 miles (16,090 km). These belts of radiation came to be known as the Van Allen belts since it was Van Allen's interest in cosmic rays that led to the findings. At the time, however, the term magnetosphere was accepted as the formal name of the belts.
Scientists felt that the belts were probably composed of charged particles that originated in the Sun and became trapped in the Earth's magnetic field, spiraling about the lines of magnetic force from pole to pole. This theory was confirmed in 1958 when charged particles from an atmospheric nuclear test temporarily formed an "induced" radiation belt, following the behavior predicted for such particles.
Today little doubt about the origin of the Van Allen belts remains, although the reason for the existence of two belts is not completely understood. The solar wind, in interaction with the belts, creates the beautiful auroras near the poles during periods of high solar activity, usually around sunspot maximum. The interaction between the magnetic field and the upper atmosphere also may have an effect on the ozone holes found high in the Earth's stratosphere.
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