Electromagnetic pulse

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In telecommunications and warfare, the term electromagnetic pulse (EMP) has the following meanings:

The electromagnetic radiation from an explosion (especially nuclear explosions) or an intensely fluctuating magnetic field caused by Compton-recoil electrons and photoelectrons from photons scattered in the materials of the electronic or explosive device or in a surrounding medium. The resulting electric and magnetic fields may couple with electrical/electronic systems to produce damaging current and voltage surges See Electromagnetic bomb for details on the damages resulting to electronic devices. The effects are usually not noticeable beyond the blast radius unless the device is nuclear or specifically designed to produce an electromagnetic shockwave.
A broadband, high-intensity, short-duration burst of electromagnetic energy.

In the case of a nuclear detonation or a meteor impact[1], the electromagnetic pulse consists of a continuous frequency spectrum. Most of the energy is distributed throughout the lower frequencies between 3 Hz and 30 kHz.

Source: from Federal Standard 1037C in support of MIL-STD-188 and from the Department of Defense Dictionary of Military and Associated Terms

In the cyberpunk science fiction sub-genre, EMP is often portrayed as a superweapon that distorts social order by destroying technological artifacts central to a society.

1 Practical Considerations
2 Modern Scenarios
3 In fiction
4 See also
5 External links

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Practical Considerations

The worst of the pulse lasts for only a second, but any unprotected electrical equipment — and anything connected to electrical cables, which act as giant lightning rods or antennas, will be affected by the pulse. Older, tube or valve-based equipment is much less vulnerable to EMP. There are a number of websites that explore methods for protecting equipment in the home or business from the effects of an EMP attack.

It is important to note that many nuclear detonations have taken place using bombs dropped by aircraft. The aircraft that delivered the atomic weapons at Hiroshima and Nagasaki did not fall out of the sky due to damage to their electrical or electronic systems. There are many factors contributing to this, but it does point out the limited use of smaller atomic weapons as EMP devices. Several major factors control the effectiveness of an EMP weapon. These are:

The height of the weapon when detonated.
The size of the weapon.
The distance from the weapon when detonated.
Geographical depth or intervening geographical features

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Weapon Height

According to an internet primer published by the Federation of American Scientists [2]

A high-altitude nuclear detonation produces an immediate flux of gamma rays from the nuclear reactions within the device. These photons in turn produce high energy free electrons by Compton scattering at altitudes between (roughly) 20 and 40 km. These electrons are then trapped in the Earth’s magnetic field, giving rise to an oscillating electric current. This current is asymmetric in general and gives rise to a rapidly rising radiated electromagnetic field called an electromagnetic pulse (EMP). Because the electrons are trapped essentially simultaneously, a very large electromagnetic source radiates coherently.

The pulse can easily span continent-sized areas, and this radiation can affect systems on land, sea, and air. The first recorded EMP incident accompanied a high-altitude nuclear test over the South Pacific and resulted in power system failures as far away as Hawaii. A large device detonated at 400–500 km (250 to 312 miles) over Kansas would affect all of CONUS. The signal from such an event extends to the visual horizon as seen from the burst point.

Thus, for equipment to be affected, the weapon needs to be above the visual horizon. Because of the nature of the pulse as a large, long, high powered, noisy spike, it is doubtful that there would be much protection if the explosion were seen in the sky just below the tops of hills or mountains. The circumstances inside the bottom of deeper valleys may be different, and locations with a large mountain range in-between (such as the Rocky Mountains) likely have some protection. Thus a weapon detonated high over Kansas might have only indirect effects on the US West Coast.

The height indicated above is greater than that of the International Space Station and many low earth orbit satellites. Large weapons could have a dramatic impact on satellite operations and communications; smaller weapons have less such potential.

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Weapon Size

Typical Weapon sizes quoted in such scenarios are in the range of 20 Megatons. This is roughly 1000 times the sizes of the weapons the United States used in Japan at Hiroshima and Nagasaki.

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Weapon Distance

Radius in Miles	Circumference	Relative Strength
10	62.83	100% or 1
20	125.66	50% or 1/2
30	188.50	33.3% or 1/3
40	251.32	25% or 1/4

The major energy in an EMP pulse is electromagnetic, and radiates out from the point of detonation in a sphere. EMP is electromagnetic radiation, and all electromagnetic radiation is transverse in nature - the electric and magnetic fields point at right angles to the direction of propagation. This is why the intensity the these fields decrease in proportion to the circumference and distance from explosion. The actual amount of EMP energy deposited per unit area is entirely different, and that falls off as the inverse-square of distance.

In the table shown, the range of deposition of gamma rays in the atmosphere is assumed to be 10 miles, which is appropriate for a 1 megaton burst at an altitude of about 10 miles.

The size of the perimeter of this circle grows in proportion to the radius of the circle, and so the electric field strength weakens as the circle grows. By simple mathematics the electric field strength does not fall as the inverse square law, but is instead a simple inverse linear relationship.

The range of deposition of gamma rays would be smaller for a surface burst because of the greater air density, which shields the initial gamma rays that cause the EMP. Conversely, for a burst at greater altitudes, the range of the deposition would be far greater than 10 miles, because the gamma rays could travel much further in the low density air before being stopped. The actual energy deposited per unit area, if emitted from an isotropic point source, is always governed by the inverse-square law. But the damaging effect of EMP is determined largely by the peak electric field (measured in volts/metre), which falls only inversely with distance. The amount of EMP energy passing through a unit of area is proportional to the square of the field strength. Within the range of gamma ray deposition, these simple laws no longer holds as the air is ionised and there are other EMP effects such as a radial (non-radiated) electric field due to the separation of Compton electrons from air molecules, and other complex phenomena.

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Modern Scenarios

Typical modern scenarios seen in news account speculate about the use of nuclear weapons by rogue states or terrorists in an attack on the United States. These typically involve weapons similar to those used over Hiroshima and Nagasaki. Aerial detonation would require the use of aircraft, or surface launched missiles of limited range (typically a range 100 to 300 miles). The scenarios have the detonations typically occurring within the earth's atmosphere, and likely relatively close to the ground (within a dozen or so miles).

This would limit the EMP effect because the height of the explosion would be much lower than that needed to be above the visual horizon of the entire United States. Also, the power of the weapons would typically be hundreds if not thousands of times smaller than optimum, and thus the effect would be significantly smaller than that of a larger weapon.

Thus, scenarios depicted in science fiction where small weapons create gigantic effects are not completely accurate, often having been exaggerated for the purposes of artistic license and dramatic effect.

A terrorist EMP attack might profoundly affect any major city. However, since the attacks in the United States of September 11, 2001, many major businesses have relocated valuable assets outside of major urban areas, and have taken other measures to protect themselves. Therefore, the long-term impact of such an event might not be as grave as previously imagined, depending on the nature of the original attack.

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In fiction

Spoiler warning: Plot and/or ending details follow.

In the 1983 television drama The Day After, the opening salvo of the Soviet attack is a high altitude nuclear detonation which is seen disabling the electrical systems in buildings and motor vehicles.
In the 1984 television drama Threads, the prelude to a Warsaw Pact nuclear attack against NATO targets is a high altitude nuclear explosion over the North Sea which causes widespread disruption throughout northern Europe.
In the 1995 film GoldenEye, the control key of two EMP generating satellites is stolen, and one is used to destroy a secret Soviet base, in an attempt to cover up the evidence. During the climax of the film, the other satellite is caused to burn up in the atmosphere before it could be discharged; the characters believe it would have knocked out most of London, triggering a "worldwide financial meltdown."
In climax of the 1996 film Escape from L.A. an EMP is used to overload every electronic device on earth.
The 1996 computer game Terra Nova: Strike Force Centauri features EM emitters, which causes all targets within the blast range to stand out on the sensors for a short time; these can also be used to confuse Mark I clones, as their cybernetic sensors are disrupted.
In the 1997 novel Shade's Children, Red Diamond uses an EMP to disable the Submarine's defenses.
In the 1998 film Small Soldiers, an EMP was used to destroy the violent soldier-toys.
In the 1999 film The Matrix, an EMP is used to disable the Sentinels that are hunting the humans.
In the 2000 N64 game Banjo-Tooie, EMP is the name of a magic spell used to temporarily disrupt machinery in Grunty Industries.
In the 2001 film Ocean's Eleven an EMP is set off in Las Vegas to aid in a casino heist.
In the 2000-2002 TV series Dark Angel, a nuclear weapon has been set off above the United States creating an EMP and resulting world economic depression.
In season 4 of the TV series 24, an EMP is set off in Los Angeles.
In Nelson DeMille's novel The Talbot Odyssey, a rogue Soviet faction is planning to detonate a nuclear warhead in space above New York so that the EMP will disable banking systems and cause a breakdown in American society.
In the 2005 film War of the Worlds, an EMP caused by an alien lightning storm is responsible for knocking out the electric equipment before the aliens begin their assault.
In the computer games System Shock (1994) and System Shock 2 (1999), the player can use small-scale, personal EMP weapons in the forms of EMP rifles and EMP grenades; these weapons are effective against cyborg enemies, and even more so against robots, but useless against purely biological enemies.
The computer game Deus Ex (2000) features EMP grenades that can be used to permanently overload robots, or to temporarily disrupt security systems. If the player is caught in an EMP blast, their bio-electric reserves are drained, unless they have the EMP shielding augmentation.
In the 2004-Current TV series Stargate Atlantis, a nuclear weapon has been set off above Atlantis causing an EMP so that they could cloak themselves from the wraith in Episode 1 of Season 2. In Episode 13 of Season 1, an EMP was used to kill the nanites that were killing inhabatants in Atlantis.
In the 2004 Nintendo game Metroid Prime 2: Echoes, EMP-grenade launchers were one of several weapons used by Space Pirate Commandos.
In 1999 the PC game Command & Conquer: Tiberian Sun an EMP weapon is used to bombard the enemy units/base, the 'shortout' effect only lasts several seconds. The expansion pack Firestorm added a mobile EMP tank, that disables units in the vicinity of the unit.
In 2003 the TV Series Code Lyoko, Ulrich uses a small EMP bomb to stop a man possessed by Xana.
In Valve's PC game, Half-Life 2, the character Alyx Vance has a personal EMP which she carries fixed upon the right side of her belt. She uses it to hack Combine panels and open locked doors. In Nova Propekt, she helps the player by hacking the Combine computer system. It is suggested that she uses the EMP to do this, however she is intelligant enough that she may be able to hack many of them without it.
In Blizzard's game, StarCraft, a flying ship call a Science Vessel shoots an "EMP Shockwave" rocket which causes units to lose charged energy without disabling mechanical units. "By generating a massive, short-ranged electromagnetic pulse, the Explorer can disable shields and special electronics on any nearby units. Although the vessel creating the EMP is not affected, any friendly units within the radius of the EMP pulse are subject to its effects." (StarCraft Manual 41)