Electromagnetic pulse

An electromagnetic pulse (sometimes abbreviated EMP) is a burst of electromagnetic radiation. The abrupt pulse of electromagnetic radiation usually results from certain types of high energy explosions, especially a nuclear explosion, or from a suddenly fluctuating magnetic field. The resulting rapidly-changing electric fields and magnetic fields may couple with electrical/electronic systems to produce damaging current and voltage surges.
In military terminology, a nuclear warhead detonated hundreds of kilometers above the Earth's surface is known as a high-altitude electromagnetic pulse (HEMP) device. Effects of a HEMP device depend on a very large number of factors, including the altitude of the detonation, energy yield, gamma ray output, interactions with the Earth's magnetic field, and electromagnetic shielding of targets.

The fact that an electromagnetic pulse is produced by a nuclear explosion was known since the earliest days of nuclear weapons testing. The magnitude of the EMP and the significance of its effects, however, were not realized for some time.
During the first United States nuclear test on 16 July 1945, electronic equipment was shielded due to Enrico Fermi's expectation of an electromagnetic pulse from the detonation. The official technical history for that first nuclear test states, "All signal lines were completely shielded, in many cases doubly shielded. In spite of this many records were lost because of spurious pickup at the time of the explosion that paralyzed the recording equipment." During British nuclear testing in 1952–1953 there were instrumentation failures that were attributed to "radioflash", which was then the British term for EMP.
The high altitude nuclear tests of 1962, as described below, increased the awareness of EMP beyond the original small population of nuclear weapons scientists and engineers. The larger scientific community became aware of the significance of the EMP problem after a series of three articles were published about nuclear electromagnetic pulse in 1981 by William J. Broad in the weekly publication Science.

In 1962, the Soviet Union also performed a series of three EMP-producing nuclear tests in space over Kazakhstan, which were the last in the series called "The K Project".Although these weapons were much smaller (300 kiloton or 1.3 PJ) than the Starfish Prime test, since those tests were done over a populated large land mass (and also at a location where the Earth's magnetic field was greater), the damage caused by the resulting EMP was reportedly much greater than in the Starfish Prime nuclear test. The geomagnetic storm–like E3 pulse (from the test designated as "Test 184") even induced an electric current surge in a long underground power line that caused a fire in the power plant in the city of Karaganda. After the collapse of the Soviet Union, the level of this damage was communicated informally to scientists in the United States. Formal documentation of some of the EMP damage in Kazakhstan exists but is still sparse in the open scientific literature.

How the peak EMP on the ground varies with the weapon yield and burst altitude. The yield here is the prompt gamma ray output measured in kilotons. This varies from 0.115–0.5% of the total weapon yield, depending on weapon design. The 1.4 Mt total yield 1962 Starfish Prime test had a gamma output of 0.1%, hence 1.4 kt of prompt gamma rays. (The blue 'pre-ionisation' curve applies to certain types of thermonuclear weapon, where gamma and x-rays from the primary fission stage ionise the atmosphere and make it electrically conductive before the main pulse from the thermonuclear stage. The pre-ionisation in some situations can literally short out part of the final EMP, by allowing a conduction current to immediately oppose the Compton current of electrons).