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How were the Van Allen radiation belts discovered by Explorer 1?

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geomagnetic field The magnetic field of a bar magnet has a simple configuration known as a dipole field. Close to Earth's surface this field is a reasonable approximation of the actual field.

The discovery of the Van Allen radiation belts was made possible by instrumentation designed by American physicist James A. Van Allen and his associates for the Explorer satellite program.1

Launched on January 31, 1958, Explorer 1 carried a Geiger-Müller counter specifically intended to measure cosmic rays. Data transmitted by this instrument revealed that Earth is surrounded by zones of highly energetic charged particles trapped by the geomagnetic field.234

Specifically, Explorer 1 detected the inner region of these radiation zones, marking the first major scientific discovery made by an artificial satellite. Subsequent data from other U.S. and Soviet spacecraft launched later that year helped distinguish the outer belt, confirming the existence of the two concentric, doughnut-shaped regions that now bear Van Allen's name.253