Electromagnetic pulse

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^*, 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 works of fiction, EMP has made many appearances, particularly in science fiction. In the cyberpunk sub-genre, EMP is often portrayed as a superweapon that distorts social order by destroying technological artifacts central to a society, or as a potent weapon against mechanical or robotic enemies. See Electromagnetic pulse in fiction.

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, vacuum tube (valve) based equipment is much less vulnerable to EMP; Soviet cold war era military aircrafts often had avionics based on vacuum tubes. 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. This is simply because electrons (ejected from the air by gamma rays) are stopped quickly in normal (dense) air for bursts below 10 km, so they don't get a chance to be significantly deflected by the Earth's magnetic field (the deflection causes the powerful EMP seen in high altitude bursts), but it does point out the limited use of smaller burst altitudes for widespread EMP.

If the B-29 planes had been within the intense nuclear radiation zone when the bombs exploded over Hiroshima and Nagasaki, then they would have suffered effects from the charge separation (radial) EMP. But this only occurs within the severe blast radius for detonations below about 10 km altitude. EMP disruptions were suffered aboard KC-135 photographic aircraft flying 300 km from the 410 kt Bluegill and 410 kt Kingfish detonations (48 and 95 km burst altitude, respectively) in 1962 ^*, but the vital aircraft electronics then were far less sophisticated than today and did not crash the aircraft.

Several major factors control the effectiveness of an EMP weapon. These are:

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

Beyond a certian height a nuke will not produce any EMP, as the gamma rays will have had sufficient distance to disperse. In deep space or on worlds with no magnetic field (the Moon or Mars for example) there will be little or no EMP. This has implications for certian kinds of nuclear rocket engines. See Project_Orion.

Weapon height

According to an internet primer published by the Federation of American Scientists ^*

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.

Weapon yield

Typical nuclear weapon yields 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.

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 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.

Non-nuclear electromagnetic pulse

Non-nuclear electromagnetic pulse (NNEMP) is an electromagnetic pulse generated without use of nuclear weapons. There are a number of devices to achieve this objective, ranging from a large low-inductance capacitor bank discharged into a single-loop antenna or a microwave generator to an explosively pumped flux compression generator. To achieve the frequency characteristics of the pulse needed for optimal coupling into the target, wave-shaping circuits and/or microwave generators are added between the pulse source and the antenna. A vacuum tube particularly suitable for microwave conversion of high energy pulses is the vircator.

NNEMP generators can be carried as a payload of bombs and cruise missiles, allowing construction of electromagnetic bombs with diminished mechanical, thermal and ionizing radiation effects and without the political consequences of deploying nuclear weapons.

Modern scenarios

Typical modern scenarios seen in news accounts 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.

However, the EMP at a fixed distance from a nuclear weapon does not depend directly on the yield but at most only increases as the square root of the yield (see illustration above). This means that although a 10 kt weapon has only 0.7% of the total energy release of the 1.4 Mt Starfish Prime test, the EMP will be at least 8% as powerful. Since the EMP depends on the prompt gamma ray output, which was only 0.1% of yield in Starfish Prime but can be 0.5% of yield in pure fission weapons of low yield, a 10 kt bomb can easily be 5 x 8% = 40% as powerful as the 1.4 Mt Starfish Prime at producing EMP ^*.

The total prompt gamma ray energy in a fission explosion is 3.5% of the yield, but in a 10 kt detonation the high explosive around the bomb core absorbs about 85% of the prompt gamma rays, so the output is only about 0.5% of the yield in kilotons. In the thermonuclear Starfish Prime the fission yield was less than 100% to begin with, and then the thicker outer casing absorbed about 95% of the prompt gamma rays from the pusher around the fusion stage. Thermonuclear weapons are also less efficient at producing EMP because the first stage can pre-ionise the air ^*, which becomes conductive and hence rapidly shorts out the electron Compton currents generated by the final, larger yield thermonuclear stage. Hence, small pure fission weapons with thin cases are far more efficient at causing EMP than most megaton bombs.

Nevertheless, 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.

External links

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