Starfish Prime was a high-altitude nuclear test conducted by the United States of America on July 9, 1962, a joint effort of the Atomic Energy Commission (AEC) and the Defense Atomic Support Agency (which became the Defense Nuclear Agency in 1971). Launched via a Thor rocket and carrying a W49 thermonuclear warhead (manufactured by Los Alamos Scientific Laboratory) and a Mk. 2 reentry vehicle, the explosion took place 250 miles (400 km) above a point 19 miles (31 km) southwest of Johnston Island in the Pacific Ocean. It was one of five tests conducted by the USA in outer space as defined by the FAI. It produced a yield equivalent to 1.4 megatons of TNT. An electromagnetic pulse (EMP), also sometimes called a transient electromagnetic disturbance, is a short burst of electromagnetic energy. Such a pulse may occur in the form of a radiated electric or magnetic field or conducted electrical current depending on the source, and may be natural or man-made. The term "electromagnetic pulse" is commonly abbreviated to EMP, pronounced by saying the letters separately (E-M-P). EMP interference is generally damaging to electronic equipment, and at higher energy levels a powerful EMP event such as a lightning strike can damage physical objects such as buildings and aircraft structures. The management of EMP effects is an important branch of electromagnetic compatibility (EMC) engineering. The damaging effects of high-energy EMP have been used to create EMP weapons. These are typically divided into nuclear and non-nuclear devices. Such weapons, both real and fictional, have gained traction in popular culture. An EMP typically contains energy at many frequencies from DC (zero Hz) to some upper limit depending on the source. The whole range of concern is sometimes referred to as "DC to daylight", with optical (infrared, visible, ultraviolet) and ionizing (X and gamma rays) ranges usually being excluded. The highest frequencies are present in Nuclear EMP (NEMP) bursts. These continue up into the optical and ionizing ranges. Some types of EMP event can leave a visible trail, such as lightning and sparks, but these are side effects of the current flow through the air and are not part of the EMP itself. The waveform of a pulse describes how its instantaneous amplitude (field strength or current) changes over time. Real pulses tend to be quite complicated, so simplified models are often used. Such a model is typically shown either as a diagram or as a mathematical equation. Most pulses have a very sharp leading edge, building up quickly to their maximum level. The classic model is a double-exponential curve which climbs steeply, quickly reaches a peak and then decays more slowly. However pulses from a controlled switching circuit often take the form of a rectangular or "square" pulse. In a pulse train, such as from a digital clock circuit, the waveform is repeated at regular intervals. EMP events usually induce a corresponding signal in the victim equipment, due to coupling between the source and victim. Coupling usually occurs most strongly over a relatively narrow frequency band, leading to a characteristic damped sine wave signal in the victim. Visually it is shown as a high frequency sine wave growing and decaying within the longer-lived envelope of the double-exponential curve. A damped sinewave typically has much lower energy and a narrower frequency spread than the original pulse, due to the transfer characteristic of the coupling mode. In practice, EMP test equipment often injects these damped sinewaves directly rather than attempting to recreate the high-energy threat pulses. Minor EMP events, and especially pulse trains, cause low levels of electrical noise or interference which can affect the operation of susceptible devices. For example a common problem in the mid-twentieth century was the interference emitted by the ignition systems of gasoline engines, which caused radio sets to crackle and TV sets to show stripes on the screen. Laws had to be introduced to make vehicle manufacturers fit interference suppressors. At a higher level an EMP can induce a spark, for example when fuelling a gasoline-engined vehicle. Such sparks have been known to cause fuel-air explosions and precautions must be taken to prevent them. A large EMP can induce high currents and voltages in the victim, damaging electrical equipment or disrupting its function.

• published: 08 May 2014
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