Electromagnetic pulse
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2008-07-15T19:14:19Z
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{{Cleanup|date=January 2007}}
The term '''electromagnetic pulse''' ('''EMP''') has the following meanings:
# [[electromagnetic radiation]] from an [[explosion]] (especially a [[nuclear explosion]]) or an intensely [[fluctuation|fluctuating]] [[magnetic field]] caused by [[Compton scattering|Compton-recoil]] [[electron]]s and [[photoelectron]]s from [[photon]]s [[scattering|scattered]] in the materials of the electronic or explosive device or in a surrounding [[Transmission medium|medium]]. The resulting [[electric field|electric]] and magnetic fields may couple with electrical/electronic systems to produce damaging current and [[voltage surge]]s. 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 [[Effects of nuclear explosions#Electromagnetic pulse|electromagnetic shockwave]].
# A broadband, high-[[intensity]], short-duration burst of electromagnetic energy.
In the case of a nuclear detonation or an asteroid impact{{huh}}, most of the energy of the electromagnetic [[pulse]] is distributed in the [[frequency]] band between 3 Hz and 30 kHz.{{Fact|date=July 2007}}
==Practical considerations==
[[Image:EMP_mechanism.GIF|right|333px|thumb|The mechanism for a 400 km high altitude burst EMP: gamma rays hit the atmosphere between 20–40 km altitude, ejecting electrons which are then deflected sideways by the earth's magnetic field. This makes the electrons radiate EMP over a massive area. Because of the curvature of earth's magnetic field over the USA, the maximum EMP occurs south of the detonation and the minimum occurs to the north.]]
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 rod]]s or [[Antenna (radio)|antennas]] — will be affected by the pulse. Older, [[vacuum tube]] (valve) based equipment is much less vulnerable to EMP; [[Soviet Union|Soviet]] [[Cold War]]–era military aircraft 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. {{Fact|date=July 2007}}
Many nuclear detonations have taken place using bombs dropped by aircraft. The aircraft that delivered the atomic weapons at [[Hiroshima]] and [[Nagasaki, Nagasaki|Nagasaki]] did not fall out of the sky due to damage to their electrical or electronic systems. This is simply because [[electron]]s (ejected from the air by gamma rays) are stopped quickly in normal air for bursts below 10 km, so they do not 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.{{Fact|date=February 2007}}
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 [[photoinduced charge separation|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 ''[[Operation Dominic I and II|Bluegill]]'' and 410 kt ''[[Kingfish (nuclear test)|Kingfish]]'' detonations (48 and 95 km burst altitude, respectively) in 1962 [http://glasstone.blogspot.com/2006/03/emp-radiation-from-nuclear-space.html], but the vital aircraft electronics then were far less sophisticated than today and did not down the aircraft.
Several major factors control the effectiveness of an EMP weapon. These are:
# The altitude of the weapon when detonated;
# The [[nuclear weapon yield|yield]] of the weapon;
# The distance from the weapon when detonated;
# Geographical depth or intervening geographical features.
Beyond a certain altitude a nuclear weapon 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 certain kinds of nuclear rocket engines. See [[Project Orion (nuclear propulsion)|Project Orion]].
<br style="clear:both;">
===Weapon altitude===
[[Image:High altitude EMP2.GIF|right|333px|thumb|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 an 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 ray|gamma]] and [[x-ray]]s from the primary fission stage [[ionisation|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.)]]
According to an internet primer published by the [[Federation of American Scientists]]<ref>[http://www.fas.org/nuke/intro/nuke/emp.htm Nuclear Weapon EMP Effects<!-- Bot generated title -->]</ref>
: ''A high-altitude nuclear detonation produces an immediate [[flux]] of [[gamma ray]]s from the nuclear reactions within the device. These [[photon]]s in turn produce high energy free [[electron]]s 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 [[coherence (physics)|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 [[Australasia|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 the continental U.S. 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 [[horizon|visual horizon]]. Because of the nature of the pulse as a large, long, high powered, noisy [[voltage spike|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 altitude 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 yield]]s quoted in such scenarios are in the range of 20 megatons. This is roughly 1,000 times the sizes of the weapons the [[United States]] used in [[Japan]] at [[Hiroshima]] and [[Nagasaki, Nagasaki|Nagasaki]].
===Weapon distance===
The major energy in an EMP is electromagnetic, and radiates out from the point of detonation in a sphere. EMP is [[electromagnetic radiation]]. The intensity of these fields decreases 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. [[Image:EMP areas.JPG|right|333px|thumb|How the area affected depends on the burst altitude.]]
{| class="wikitable"
! 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
|-
|colspan="3"| 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 hold as the air is [[ionisation|ionised]] and there are other EMP effects such as a radial (non-radiated) electric field due to the separation of [[Compton electron]]s from air molecules, and other complex phenomena. so its energy = 1/d^2
==Non-nuclear electromagnetic pulse==
[[Image:E-4 advanced airborne command post EMP sim.jpg|thumb|A right front view of a [[Boeing E-4]] advanced airborne command post (AABNCP) on the electromagnetic pulse (EMP) simulator for testing.]]
'''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 (electronics)|coupling]] into the target, [[wave-shaping]] circuits and/or microwave generators are added between the pulse source and the [[Antenna (radio)|antenna]]. A vacuum tube particularly suitable for microwave conversion of high energy pulses is the [[vircator]].
[[Image:USS Estocin FFG-15 moored near EMPRESS I.jpg|thumb|left|[[USS Estocin (FFG-15)]] moored near an Electro Magnetic Pulse Radiation Environmental Simulator for Ships I (EMPRESS I) facility. (Antennae at top of image)]]
NNEMP generators can be carried as a payload of bombs and [[cruise missiles]], allowing construction of [[electromagnetic bomb]]s with diminished mechanical, thermal and ionizing radiation effects and without the political consequences of deploying nuclear weapons.
NNEMP generators also include large structures built to generate EMP for testing of electronics to determine how well it survives EMP.{{Fact|date=February 2007}} In addition, the use of ultra-wideband radars can generate EMP in areas immediately adjacent to the radar;{{Fact|date=February 2007}} this phenomenon is only partly understood.{{Fact|date=February 2007}}
==Modern scenarios==
Typical modern scenarios seen in news accounts speculate about the use of nuclear weapons by [[rogue state]]s or [[terrorist]]s in an attack. These typically involve weapons similar to those used over Hiroshima and Nagasaki. Aerial detonation would require the use of aircraft, or [[surface-to-air missile|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 altitude 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 [http://glasstone.blogspot.com/2006/03/emp-radiation-from-nuclear-space.html].
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 [http://glasstone.blogspot.com/2006/03/emp-radiation-from-nuclear-space.html], which becomes conductive and hence rapidly shorts out the electron [[Compton current]]s 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.
A terrorist EMP attack might profoundly affect any major city; however, because of the high cost of real estate and traffic issues, many major businesses have relocated valuable assets outside of major [[urban area]]s, and have taken other measures to protect themselves. Therefore, the long-term economic and technological impact of such an event might not be as grave as previously imagined, depending on the nature of the original attack. {{Fact|date=March 2007}}
A common scenario is detonation of a device over the middle of the U.S. using long-range missiles available only to major military powers. An offshore detonation at high altitude, by contrast, would present less technical difficulty and would disrupt both an entire coast and regions hundreds of miles inland (''e.g.'' [[Electromagnetic_pulse#Weapon_distance | 120 mile altitude, 1000 mile EMP radius]]). Moreover, a high altitude burst could be positioned over international waters by means of a missile of low accuracy, [http://www.missilethreat.com/archives/id.11/detail.asp launched from a ship], also in international waters. [[Rodong-1 | North Korea]], [[Shahab-2 | Iran]], and [[Ghauri_%28missile%29 | Pakistan]] (for example) have [[Scud#Other_nations | Scud-derived]] missiles of more than adequate capability.
==References==
<references />
==See also==
{{Portal|Electromagnetism}}
* [[High-energy radio-frequency weapons|High Energy Radio Frequency weapons]] (HERF)
* [[Explosively pumped flux compression generator]]
* [[Transient electromagnetic device]]
* [[Electromagnetic environment]]
* [[Electromagnetic propulsion]]
* [[Electromagnetic bomb]]
* [[Electromagnetism]]
* [[Pulsed power]]
* [[Neutron flux]]
==External links==
* [http://www.globalsecurity.org/wmd/library/report/1988/CM2.htm GlobalSecurity.org – Electromagnetic Pulse: From chaos to a manageable solution]
* [http://www.usace.army.mil/inet/usace-docs/eng-pamphlets/ep1110-3-2/toc.htm Electromagnetic Pulse (EMP) and Tempest Protection for Facilities] – U.S. Army Corps of Engineers
* [http://glasstone.blogspot.com/2006/03/emp-radiation-from-nuclear-space.html EMP data from ''Starfish'' nuclear test measured by Richard Wakefield of LANL, and review of evidence pertaining to the effects 1,300 km away in Hawaii, also review of Russian EMP tests of 1962]
*[http://www.freepatentsonline.com/3775596.html Pulse Counter]
[[Category:Electromagnetic radiation]]
[[Category:Energy weapons]]
[[Category:Nuclear weapons]]
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