Editing Nuclear electromagnetic pulse (section)

==Generation==
Factors that control weapon effectiveness include altitude, [[nuclear weapon yield|yield]], construction details, target distance, intervening geographical features, and local strength of the Earth's magnetic field.

===Weapon altitude===
[[File: High altitude EMP.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 to 0.5% of the total weapon yield, depending on weapon design. The 1.4 Mt total yield 1962 [[Starfish Prime]] test had a gamma output of 0.1%, hence 1.4 kt of prompt gamma rays (the '''blue''' '[[ionization|pre-ionisation]]' curve applies to certain types of [[nuclear weapon design|thermonuclear weapons]], for which [[gamma ray|gamma]] and [[X-ray]]s from the primary fission stage ionize 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).<ref>Louis W. Seiler, Jr. [https://apps.dtic.mil/sti/pdfs/ADA009208.pdf ''A Calculational Model for High Altitude EMP''] {{Webarchive|url=https://web.archive.org/web/20170429000435/http://www.dtic.mil/get-tr-doc/pdf?AD=ADA009208 |date=2017-04-29}}. Air Force Institute of Technology. Report ADA009208. pp. 33, 36. March 1975</ref>{{r|Glasstone_1977}}]]

According to an internet primer published by the [[Federation of American Scientists]]:<ref name="fas">{{cite web |title=Federation of American Scientists. "Nuclear Weapon EMP Effects" |url=https://fas.org/nuke/intro/nuke/emp.htm |access-date=2016-06-04 |url-status=dead |archive-url=https://web.archive.org/web/20150101064654/https://fas.org/nuke/intro/nuke/emp.htm |archive-date=2015-01-01}}</ref>

: A high-altitude nuclear detonation produces an immediate [[flux]] of gamma rays from the nuclear reactions within the device. These [[photon]]s 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 [[coherence (physics)|coherently]].

: The pulse can easily span continent-sized areas, and this radiation can affect systems on land, sea, and air. ... 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 [[line-of-sight propagation|visual horizon]].<ref name="fas"/>

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 such as occurred during Operation Fishbowl. The damaging effects on orbiting satellites are usually due to factors other than EMP. In the [[Starfish Prime]] nuclear test, most damage was to the satellites' solar panels while passing through radiation belts created by the explosion.<ref>{{cite web |last=Hess |first=Wilmot N. |title=The Effects of High Altitude Explosions |publisher=[[National Aeronautics and Space Administration]] |date=September 1964 |id=NASA TN D-2402 |url=https://www.futurescience.com/emp/Hess-Wilmot.pdf |access-date=2015-05-13 |url-status=live |archive-url=https://ghostarchive.org/archive/20221009/http://www.futurescience.com/emp/Hess-Wilmot.pdf |archive-date=2022-10-09}}</ref>

For detonations within the atmosphere, the situation is more complex. Within the range of gamma ray deposition, simple laws no longer hold as the air is ionized and there are other EMP effects, such as a radial electric field due to the separation of [[Compton electron]]s from air molecules, together with other complex phenomena. For a surface burst, absorption of gamma rays by air would limit the range of gamma-ray deposition to approximately {{convert|10|mi|order=flip||}}, while for a burst in the lower-density air at high altitudes, the range of deposition would be far greater.{{citation needed|date=August 2016}}

===Weapon yield===
Typical [[nuclear weapon yield]]s used during [[Cold War]] planning for EMP attacks were in the range of {{convert|1|to|10|MtonTNT|lk=on|abbr=on}}.<ref>{{cite report |title=THREAT POSED BY ELECTROMAGNETIC PULSE (EMP) TO U.S. MILITARY SYSTEMS AND CIVIL INFRASTRUCTURE |author=Committee on National Security {{!}} Military Research and Development Subcommittee |date=1997-07-16 |type=Transcript |docket=H.S.N.C No. 105{{ndash}}18 |publisher=[[United States House of Representatives]] {{!}} [[105th United States Congress]] |location=Washington, D.C. |page=39 |language=en |df=dmy-all |url=https://commdocs.house.gov/committees/security/has197010.000/has197010_1.HTM#39 |access-date=2022-08-11 |url-status=live |archive-url=https://web.archive.org/web/20220811182704/http://commdocs.house.gov/committees/security/has197010.000/has197010_1.HTM |archive-date=2022-08-11}}</ref>{{rp|page=39}} This is roughly 50 to 500 times the size of the Hiroshima and Nagasaki bombs. Physicists have testified at United States Congressional hearings that weapons with yields of {{convert|10|ktonTNT|abbr=on}} or less can produce a large EMP.<ref name="hasc">{{cite report |title=ELECTROMAGNETIC PULSE THREATS TO U.S. MILITARY AND CIVILIAN INFRASTRUCTURE |author=Committee on National Security {{!}} Military Research and Development Subcommittee |date=1999-10-07 |type=Transcript |docket=H.A.S.C. No. 106{{ndash}}31 |publisher=[[United States House of Representatives]] {{!}} [[106th United States Congress]] |location=Washington, D.C. |page=48 |language=en |df=dmy-all |url=https://commdocs.house.gov/committees/security/has280010.000/has280010_0.htm#48 |access-date=2022-08-11 |url-status=live |archive-url=https://web.archive.org/web/20220531152332/http://commdocs.house.gov/committees/security/has280010.000/has280010_0.htm#48 |archive-date=2022-05-31}}</ref>{{rp|page=48}}

The EMP at a fixed distance from an explosion increases at most as the square root of the yield (see the illustration to the right). This means that although a {{convert|10|ktonTNT|abbr=on}} weapon has only {{percentage|7|1000|1}} of the energy release of the {{convert|1.44|MtonTNT|abbr=on}} Starfish Prime test, the EMP will be at least {{percentage|8|100}} as powerful. Since the E1 component of nuclear EMP depends on the prompt gamma-ray output, which was only 0.1% of yield in Starfish Prime but can be {{percentage|5|1000|1}} of yield in low-yield pure [[nuclear fission]] weapons, a {{convert|10|ktonTNT|abbr=on}} bomb can easily be {{math|1=''5'' * ''{{percentage|8|100}}''=''{{percentage|40|100|}}''}} as powerful as the {{convert|1.44|MtonTNT|abbr=on}} Starfish Prime at producing EMP.<ref name="glasstone.blogspot.com"/>{{Unreliable source?|date=December 2022|reason=Blog with annotations (original research?)}}

The total prompt gamma-ray energy in a fission explosion is {{percentage|35|1000|1}} of the yield, but in a {{convert|10|ktonTNT|abbr=on}} detonation the triggering explosive around the bomb core absorbs about {{percentage|85|100|1}} of the prompt gamma rays, so the output is only about {{percentage|5|1000|1}} of the yield. In the [[nuclear fusion|thermonuclear]] Starfish Prime the fission yield was less than 100% and the thicker outer casing absorbed about 95% of the prompt gamma rays from the pusher around the fusion stage. [[Nuclear weapon design#Fusion|Thermonuclear weapons]] are also less efficient at producing EMP because the first stage can pre-ionize the air<ref name="glasstone.blogspot.com"/>{{Unreliable source?|date=December 2022|reason=Blog with annotations (original research?)}} which becomes conductive and hence rapidly shorts out the [[Compton current]]s generated by the [[nuclear fusion|fusion]] stage. Hence, small pure fission weapons with thin cases are far more efficient at causing EMP than most megaton bombs.{{citation needed|date=August 2016}}

This analysis, however, only applies to the fast E1 and E2 components of nuclear EMP. The [[geomagnetic storm]]-like E3 component of nuclear EMP is more closely proportional to the total energy yield of the weapon.<ref name="empcnir">{{cite web |title=Report of the Commission to Assess the Threat to the United States from Electromagnetic Pulse (EMP) Attack. Critical National Infrastructures |author=Electromagnetic Pulse (EMP) Commission |url=https://apps.dtic.mil/sti/pdfs/ADA484672.pdf}}</ref>

===Target distance===
In nuclear EMP all of the components of the electromagnetic pulse are generated outside of the weapon.<ref name="fas"/>

For [[high-altitude nuclear explosion]]s, much of the EMP is generated far from the detonation (where the gamma radiation from the explosion hits the upper atmosphere). This electric field from the EMP is remarkably uniform over the large area affected.{{r|Glasstone_1977}}

According to the standard reference text on nuclear weapons effects published by the U.S. Department of Defense, "The peak electric field (and its amplitude) at the Earth's surface from a high-altitude burst will depend upon the explosion yield, the height of the burst, the location of the observer, and the orientation with respect to the [[geomagnetic field]]. As a general rule, however, the field strength may be expected to be tens of kilovolts per metre over most of the area receiving the EMP radiation."{{r|Glasstone_1977}}

The text also states that, "...{{nbsp}}over most of the area affected by the EMP the electric field strength on the ground would exceed 0.5''E''<sub>max</sub>. For yields of less than a few hundred kilotons, this would not necessarily be true because the field strength at the Earth's tangent could be substantially less than 0.5''E''<sub>max</sub>."{{r|Glasstone_1977}}

(''E''<sub>max</sub> refers to the maximum electric field strength in the affected area.)

In other words, the electric field strength in the entire area that is affected by the EMP will be fairly uniform for weapons with a large gamma-ray output. For smaller weapons, the electric field may fall at a faster rate as distance increases.{{r|Glasstone_1977}}