Nuclear scientists had long theorized about the possibility of the electromagnetic field of a high-altitude detonation interacting with Earth’s magnetic field. However, the Starfish Prime EMP was much larger than expected. Strong electromagnetic signals were observed from the burst, as were significant disturbances in the magnetic field and Earth currents. Energized electrons and radioactive debris were trapped in the magnetic bands that encircle the planet, creating artificial auroras reaching south of the equator for hours. Unable to escape the magnetic field, this debris would continue to cause problems long after the test.
This is an excerpt from the Report of the Commission to Assess the Threat to the United States from Electromagnetic Pulse (EMP) Attack, Chapter 6, “Transportation Infrastructure”: