The first was initially discovered by accident, as it was done so by a Geiger counter designed to measure the flux of cosmic rays.
The flux of protons crossing a square centimeter of their surface can be as large as 20,000 per second, a massive amount.  The first was initially discovered by accident, as it was done so by a Geiger counter designed to measure the flux of cosmic rays. Some reports indicate that they are separated by as much as 10,000 kilometers, but most scientists agree that they gradually merge.  First detected by the Explorer 1 satellite in 1958, they are shaped by the Earth's magnetic field.  Consisting of two concentric doughnut shaped regions centered roughly on the equator, FTP, identify these sections of the magnetosphere named for the Iowa scientist who discovered them.
The flux of protons crossing a square centimeter of their surface can be as large as 20,000 per second, a massive amount.  
Van Allen belts