An electromagnetic pulse, otherwise known as an EMP, is an electromagnetic radiation surge that can couple with electrical systems. This coupling often results in damage to the current or voltage of a device due to the resulting impact on both electric and magnetic fields. Often, this burst is the result of an explosion caused by nuclear energy producing a fluctuating magnetic field with sudden impact. An electromagnetic pulse can also be caused by a simple short-duration blast of electromagnetic energy from a broadband, high-intensity device.
In terms of military application, electromagnetic pulses are caused by the detonation of a bomb hundreds of miles above the Earth's surface. When used as a weapon, this is called a High-altitude Electromagnetic Pulse device. In order for this effect to be used, the detonation must fall within the parameters of three distinct criteria: altitude of detonation, yield of the energy dispersed, and full interaction with the natural magnetic field of the Earth. Additional problems can occur when a target is shielded with anti-electromagnetic pulse protection.
In the earliest days of nuclear testing, scientists identified the effects of an electromagnetic pulse. However, researchers were unaware of the full magnitude of the effect, resulting in a slow realization of its weapons applications. Scientists led by Enrico Fermi expected some sort of pulse from the first United States nuclear explosion test in 1945. Thus, all electronic equipment was shielded from the electromagnetic pulse.
With the completion of high altitude nuclear testing, conducted in 1962, electromagnetic pulses were further understood. In July of that year, a 1.44 megaton nuclear weapon was detonated 250 miles (about 400 kilometers) above the Earth's surface in the Pacific Ocean. Known as Starfish Prime, the bomb caused major electrical damage to Hawaii, located 898 miles (about 1,445 km) away. This further stimulated research into electromagnetic pulses.
A nuclear electromagnetic pulse requires a specific series of events in order to take place. This process has been defined by the International Electrotechnical Commission. These pulses act far faster than traditional high voltage events such as lightning, making protection difficult. Gamma radiation from a nuclear detonation causes the atoms in the upper atmosphere to lose electrons. Essentially, these electrons push the Earth's magnetic field out of the way in the same manner as a geomagnetic storm.
An important facet of electromagnetic pulses, is the fact that modern technology is far more susceptible to negative effects than older technology. Devices connected to electric cables essentially act like lighting rods, attracting the pulse. Vacuum tube technology, used heavily during the 20th century, was more likely to survive a blast. With the replacement of these electrical devices by solid state equipment, the vulnerability of electronics is much more prevalent.