High-altitude nuclear explosion

High-altitude nuclear explosions are the result of nuclear weapons testing. Several such tests were performed at high altitudes by the United States and the Soviet Union between 1958 and 1962.

The strong electromagnetic pulse (EMP) that results has several components. In the first few tens of nanoseconds, about a tenth of a percent of the weapon yield appears as powerful gamma rays with energies of one to three mega-electron volts (MeV, a unit of energy). The gamma rays penetrate the atmosphere and collide with air molecules, depositing their energy to produce huge quantities of positive ions and recoil electrons (also known as Compton electrons). The impacts create MeV-energy Compton electrons that then accelerate and spiral along the Earth's magnetic field lines. The resulting transient electric fields and currents that arise generate electromagnetic emissions in the radio frequency range of 15 to 250 megahertz (MHz, or fifteen million to 250 million cycles per second). This high-altitude EMP occurs between 30 and 50 kilometers (18 and 31 miles) above the Earth's surface. The potential as an anti-satellite weapon became apparent in August 1958 during Hardtack Teak. The EMP observed at the Apia Observatory at Samoa was four times more powerful than any created by solar storms, while in July 1962 the Starfish Prime test damaged electronics in Honolulu and New Zealand (approximately 1,300 kilometers away), fused 300 street lights on Oahu (Hawaii), set off about 100 burglar alarms, and caused the failure of a microwave repeating station on Kauai, which cut off the sturdy telephone system from the other Hawaiian islands. The radius for an effective satellite kill for the various prompt radiations produced by such a nuclear weapon in space was determined to be roughly 80 km. Further testing to this end was carried out, and embodied in a Department of Defense program, Program 437.

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