Abstract: An electromagnetic pulse (commonly abbreviated EMP) is a burst of electromagnetic radiation. The abrupt pulse of electromagnetic radiation usually results from certain types of high energy explosions, especially a nuclear explosion, or from a suddenly fluctuating magnetic field. The resulting rapidly-changing electric fields and magnetic fields may couple with electrical/electronic systems to produce damaging current and voltage surges.
In military terminology, a nuclear warhead detonated hundreds of kilometers above the Earth's surface is known as a high-altitude electromagnetic pulse (HEMP) device. Effects of a HEMP device depend on a very large number of factors, including the altitude of the detonation, energy yield, gamma ray output, interactions with the Earth's magnetic field, and electromagnetic shielding of targets.
The fact that an electromagnetic pulse is produced by a nuclear explosion was known since the earliest days of nuclear weapons testing. The magnitude of the EMP and the significance of its effects, however, were not realized for some time.
During the first United States nuclear test on 16 July 1945, electronic equipment was shielded due to Enrico Fermi's expectation of an electromagnetic pulse from the detonation. The official technical history for that first nuclear test states, "All signal lines were completely shielded, in many cases doubly shielded. In spite of this many records were lost because of spurious pickup at the time of the explosion that paralyzed the recording equipment." During British nuclear testing in 1952–1953 there were instrumentation failures that were attributed to "radioflash", which was then the British term for EMP.
The high altitude nuclear tests of 1962, as described below, increased the awareness of EMP beyond the original small population of nuclear weapons scientists and engineers. The larger scientific community became aware of the significance of the EMP problem after a series of three articles were published about nuclear electromagnetic pulse in 1981 by William J. Broad in the weekly publication Science.
Main article: Starfish Prime
In July 1962, a 1.44 megaton (6.0 PJ) United States nuclear test in space, 400 kilometres (250 mi) above the mid-Pacific Ocean, called the Starfish Prime test, demonstrated to nuclear scientists that the magnitude and effects of a high altitude nuclear explosion were much larger than had been previously calculated. Starfish Prime also made those effects known to the public by causing electrical damage in Hawaii, about 1,445 kilometres (898 mi) away from the detonation point, knocking out about 300 streetlights, setting off numerous burglar alarms and damaging a telephone company microwave link.
Starfish Prime was the first successful test in the series of United States high-altitude nuclear tests in 1962 known as Operation Fishbowl. The subsequent Operation Fishbowl tests gathered more data on the high-altitude EMP phenomenon.
The Bluegill Triple Prime and Kingfish high-altitude nuclear tests of October and November 1962 in Operation Fishbowl finally provided electromagnetic pulse data that was clear enough to enable physicists to accurately identify the physical mechanisms that were producing the electromagnetic pulses.
The EMP damage of the Starfish Prime test was quickly repaired because of the ruggedness (compared to today) of the electrical and electronic infrastructure of Hawaii in 1962.
The relatively small magnitude of the Starfish Prime EMP in Hawaii (about 5600 volts/metre) and the relatively small amount of damage done (for example, only 1 to 3 percent of streetlights extinguished) led some scientists to believe, in the early days of EMP research, that the problem might not be as significant as was later realized. Newer calculations showed that if the Starfish Prime warhead had been detonated over the northern continental United States, the magnitude of the EMP would have been much larger (22 to 30 kilovolts/metre) because of the greater strength of the Earth's magnetic field over the United States, as well as the different orientation of the Earth's magnetic field at high latitudes. These new calculations, combined with the accelerating reliance on EMP-sensitive microelectronics, heightened awareness that the EMP threat could be a very significant problem.
Soviet Test 184
Main article: The K Project
In 1962, the Soviet Union also performed a series of three EMP-producing nuclear tests in space over Kazakhstan, which were the last in the series called "The K Project". Although these weapons were much smaller (300 kiloton or 1.3 PJ) than the Starfish Prime test, since those tests were done over a populated large land mass (and also at a location where the Earth's magnetic field was greater), the damage caused by the resulting EMP was reportedly much greater than in the Starfish Prime nuclear test. The geomagnetic storm–like E3 pulse (from the test designated as "Test 184") even induced an electric current surge in a long underground power line that caused a fire in the power plant in the city of Karaganda. After the collapse of the Soviet Union, the level of this damage was communicated informally to scientists in the United States. Formal documentation of some of the EMP damage in Kazakhstan exists but is still sparse in the open scientific literature.
The concept of the explosively pumped flux compression generator for generating a non-nuclear electromagnetic pulse was conceived as early as 1951 by Andrei Sakharov in the Soviet Union, but nations have usually kept their most recent work on non-nuclear EMP highly classified until the technology was old enough for similar ideas to be conceived by physicists in other nations.
Characteristics of Nuclear
The case of a nuclear electromagnetic pulse differs from other kinds of electromagnetic pulse (EMP) in being a complex electromagnetic multi-pulse. The complex multi-pulse is usually described in terms of three components, and these three components have been defined as such by the international standards commission called the International Electrotechnical Commission (IEC).
The three components of nuclear EMP, as defined by the IEC, are called E1, E2 and E3.
The E1 pulse is the very fast component of nuclear EMP. The E1 component is a very brief but intense electromagnetic field that can quickly induce very high voltages in electrical conductors. The E1 component causes most of its damage by causing electrical breakdown voltages to be exceeded. E1 is the component that can destroy computers and communications equipment and it changes too quickly for ordinary lightning protectors to provide effective protection against it.
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 and downward tilt of Earth's magnetic field over the USA, the maximum EMP occurs south of the detonation and the minimum occurs to the north.
The E1 component is produced when gamma radiation from the nuclear detonation knocks electrons out of the atoms in the upper atmosphere. The electrons begin to travel in a generally downward direction at relativistic speeds (more than 90 percent of the speed of light). In the absence of a magnetic field, this would produce a large pulse of electric current vertically in the upper atmosphere over the entire affected area. The Earth's magnetic field acts on these electrons to change the direction of electron flow to a right angle to the geomagnetic field. This interaction of the Earth's magnetic field and the downward electron flow produces a very large, but very brief, electromagnetic pulse over the affected area.
Physicist Conrad Longmire has given numerical values for a typical case of the E1 pulse produced by a second generation nuclear weapon such as those used in high altitude tests of Operation Fishbowl in 1962. According to him, the typical gamma rays given off by the weapon have an energy of about 2 MeV (million electron volts). When these gamma rays collide with atoms in the mid-stratosphere, the gamma rays knock out electrons. This is known as the Compton effect, and the resulting electrons produce an electric current that is known as the Compton current. The gamma rays transfer about half of their energy to the electrons, so these initial electrons have an energy of about 1 MeV. This causes the electrons to begin to travel in a generally downward direction at about 94 percent of the speed of light. Relativistic effects cause the mass of these high energy electrons to increase to about 3 times their normal rest mass.
If there were no geomagnetic field and no additional atoms in the lower atmosphere for additional collisions, the electrons would continue to travel downward with an average current density in the stratosphere of about 48 amperes per square metre.
Because of the downward tilt of the Earth's magnetic field at high latitudes, the area of peak field strength is a U-shaped region to the equatorial side of the nuclear detonation. As shown in the diagram at the right, for nuclear detonations over the continental United States, this U-shaped region is south of the detonation point. Near the equator, where the Earth's magnetic field is more nearly horizontal, the E1 field strength is more nearly symmetrical around the burst location.
The Earth's magnetic field quickly deflects the electrons at right angles to the geomagnetic field, and the extent of the deflection depends upon the strength of the magnetic field. At geomagnetic field strengths typical of the central United States, central Europe or Australia, these initial electrons spiral around the magnetic field lines in a circle with a typical radius of about 85 metres (about 280 feet). These initial electrons are stopped by collisions with other air molecules at an average distance of about 170 metres (a little less than 580 feet). This means that most of the electrons are stopped by collisions with air molecules before they can complete one full circle of its spiral around the Earth's magnetic field lines.
This interaction of the very rapidly-moving negatively-charged electrons with the magnetic field radiates a pulse of electromagnetic energy. The pulse typically rises to its peak value in about 5 nanoseconds. The magnitude of this pulse typically decays to half of its peak value within 200 nanoseconds. (By the IEC definition, this E1 pulse is ended at one microsecond (1000 nanoseconds) after it begins.) This process occurs simultaneously with about 1025 other electrons.
There are a number of secondary collisions which cause the subsequent electrons to lose energy before they reach ground level. The electrons generated by these subsequent collisions have such reduced energy that they do not contribute significantly to the E1 pulse.
These 2 MeV gamma rays will normally produce an E1 pulse near ground level at moderately high latitudes that peaks at about 50,000 volts per metre. This is a peak power density of 6.6 megawatts per square metre.
The process of the gamma rays knocking electrons out of the atoms in the mid-stratosphere causes this region of the atmosphere to become an electrical conductor due to ionization, a process which blocks the production of further electromagnetic signals and causes the field strength to saturate at about 50,000 volts per metre. The strength of the E1 pulse depends upon the number and intensity of the gamma rays produced by the weapon and upon the rapidity of the gamma ray burst from the weapon. The strength of the E1 pulse is also somewhat dependent upon the altitude of the detonation.
There are reports of "super-EMP" nuclear weapons that are able to overcome the 50,000 volt per metre limit by the very nearly instantaneous release of a burst of gamma radiation of much higher energy levels than are known to be produced by second generation nuclear weapons. The reality and possible construction details of these weapons are classified, and therefore cannot be confirmed by scientists in the open scientific literature.
The E2 component is generated by scattered gamma rays and inelastic gammas produced by weapon neutrons. This E2 component is an "intermediate time" pulse that, by the IEC definition, lasts from about 1 microsecond to 1 second after the beginning of the electromagnetic pulse. The E2 component of the pulse has many similarities to the electromagnetic pulses produced by lightning, although the electromagnetic pulse induced by a nearby lightning strike may be considerably larger than the E2 component of a nuclear EMP. Because of the similarities to lightning-caused pulses and the widespread use of lightning protection technology, the E2 pulse is generally considered to be the easiest to protect against.
According to the United States EMP Commission, the main potential problem with the E2 component is the fact that it immediately follows the E1 component, which may have damaged the devices that would normally protect against E2.
According to the EMP Commission Executive Report of 2004, "In general, it would not be an issue for critical infrastructure systems since they have existing protective measures for defense against occasional lightning strikes. The most significant risk is synergistic, because the E2 component follows a small fraction of a second after the first component's insult, which has the ability to impair or destroy many protective and control features. The energy associated with the second component thus may be allowed to pass into and damage systems."
The E3 component is very different from the other two major components of nuclear EMP. The E3 component of the pulse is a very slow pulse, lasting tens to hundreds of seconds, that is caused by the nuclear detonation heaving the Earth's magnetic field out of the way, followed by the restoration of the magnetic field to its natural place. The E3 component has similarities to a geomagnetic storm caused by a very severe solar flare. Like a geomagnetic storm, E3 can produce geomagnetically induced currents in long electrical conductors, which can then damage components such as power line transformers.
Because of the similarity between solar-induced geomagnetic storms and nuclear E3, it has become common to refer to solar-induced geomagnetic storms as "solar EMP." At ground level, however, "solar EMP" is not known to produce an E1 or E2 component.
For a more thorough description of E3 damage mechanisms, see the main article: Geomagnetically induced current
for Nuclear EMP
Older, vacuum tube (valve) based equipment is generally much less vulnerable to EMP than newer solid state equipment. Soviet Cold War–era military aircraft often had avionics based on vacuum tubes due to both limitations in Soviet solid-state capabilities and a belief that the vacuum-tube gear would survive better.
Although vacuum tubes are far more resistant to EMP than solid state devices, other components in vacuum tube circuitry can be damaged by EMP. Vacuum tube equipment was damaged in 1962 nuclear EMP testing. Also, the solid state PRC-77 VHF manpackable 2-way radio survived extensive EMP testing. The earlier PRC-25, nearly identical except for a vacuum tube final amplification stage, had been tested in EMP simulators but was not certified to remain fully functional.
Many nuclear detonations have taken place using bombs dropped by aircraft. The B-29 aircraft that delivered the nuclear weapons at Hiroshima and Nagasaki did not lose power 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 air for bursts below roughly 10 km (about 6 miles), 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), thus the limited use of smaller burst altitudes for widespread EMP.
If the aircraft carrying the Hiroshima and Nagasaki bombs had been within the intense nuclear radiation zone when the bombs exploded over those cities, 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.
During nuclear tests in 1962, EMP disruptions were suffered aboard KC-135 photographic aircraft flying 300 km (190 mi) from the 410 kt (1,700 TJ) Bluegill Triple Prime and 410 kt (1,700 TJ) Kingfish detonations (48 and 95 km (30 and 59 mi) burst altitude, respectively) but the vital aircraft electronics were far less sophisticated than today and the aircraft were able to land safely.
Generation of Nuclear EMP
Several major factors control the effectiveness of a nuclear EMP weapon. These are
- The altitude of the weapon when detonated;
- The yield and construction details of the weapon;
- The distance from the weapon when detonated;
- Geographical depth or intervening geographical features;
- The local strength of the magnetic field of the Earth.
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, such as Project Orion.
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 a gamma 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 and x-rays from the primary fission stage 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.)
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 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 visual horizon. Because of the nature of the pulse as a large, 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 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 the 1962 tests. The damaging effects on orbiting satellites are usually due to other factors besides EMP. In the Starfish Prime nuclear test, most satellite damage was due to damage to the solar panels from satellites passing through radiation belts created by the high altitude nuclear explosion.
Typical nuclear weapon yields used during Cold War planning for EMP attacks were in the range of 1 to 10 megatons (4.2 to 42 PJ) This is roughly 50 to 500 times the sizes of the weapons the United States used in Japan at Hiroshima and Nagasaki. Physicists have testified at United States Congressional hearings, however, that weapons with yields of 10 kilotons (42 TJ) or less can produce a very large EMP.
If one compares explosions with different yields, the EMP at a fixed distance from a nuclear weapon would not increase at the same rate as the explosion yield, but at most only as the square root of the yield (see the illustration to the right). This means that although a 10 kiloton weapon has only 0.7% of the total energy release of the 1.44-megaton Starfish Prime test, the EMP will be at least 8% 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 0.5% of yield in pure fission weapons of low yield, a 10 kiloton bomb can easily be 5 x 8% = 40% as powerful as the 1.44 megaton 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 kiloton 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-ionize 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.
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.
A unique and important aspect of nuclear EMP is that all of the components of the electromagnetic pulse are generated outside of the weapon. The important E1 component is generated by interaction with the electrons in the upper atmosphere that are hit by gamma radiation from the weapon — and the subsequent effects upon those electrons by the Earth's magnetic field.
For high-altitude nuclear explosions, this means that much of the EMP is actually generated at a large distance from the detonation (where the gamma radiation from the explosion hits the upper atmosphere). This causes the electric field from the EMP to be remarkably uniform over the large area affected.
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."
The same reference book also states that, "... over most of the area affected by the EMP the electric field strength on the ground would exceed 0.5Emax. 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 0.5Emax."
(Emax 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; but for much smaller weapons, the electric field may fall off at a comparatively faster rate at large distances from the detonation point.
It is the peak electric field of the EMP that determines the peak voltage induced in equipment and other electrical conductors on the ground, and most of the damage is determined by induced voltages.
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 ionised and
there are other EMP effects, such as a radial electric field due to the
separation of Compton electrons 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 10 miles,
while for a burst in the lower-density air at high altitudes, the range of
deposition would be far greater.
Non-nuclear electromagnetic pulse (NNEMP) is an electromagnetic pulse generated without use of nuclear weapons. There are a number of devices that can 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, cruise missiles and drones, allowing construction of electromagnetic bombs with diminished mechanical, thermal and ionizing radiation effects and without the political consequences of deploying nuclear weapons.
The range of NNEMP weapons (non-nuclear electromagnetic pulse bombs) is severely limited compared to nuclear EMP. This is because nearly all NNEMP devices used as weapons require chemical explosives as their initial energy source, but nuclear explosives have an energy yield on the order of one million times that of chemical explosives of similar weight. In addition to the large difference in the energy density of the initial energy source, the electromagnetic pulse from NNEMP weapons must come from within the weapon itself, while nuclear weapons generate EMP as a secondary effect, often at great distances from the detonation. These facts severely limit the range of NNEMP weapons as compared to their nuclear counterparts, but allow for more surgical target discrimination. The effect of small e-bombs has proven to be sufficient for certain terrorist or military operations. Examples of such operations include the destruction of certain fragile electronic control systems of the type critical to the operation of many ground vehicles and aircraft.
USS Estocin (FFG-15) moored near the Electro Magnetic Pulse Radiation Environmental Simulator for Ships I (EMPRESS I) facility (antennae at top of image).
Information about the EMP simulators used by the United States during the latter part of the Cold War, along with more general information about electromagnetic pulse, are now in papers under the care of the SUMMA Foundation, which is now hosted at the University of New Mexico.
The SUMMA Foundation web site includes documentation about the huge wooden ATLAS-I simulator (better known as TRESTLE, or "The Sandia Trestle") at Sandia National Labs, New Mexico, which was the world's largest EMP simulator. Nearly all of these large EMP simulators used a specialized version of a Marx generator. The SUMMA Foundation now has a 44-minute documentary movie on its web site called "TRESTLE: Landmark of the Cold War".
Post–Cold War Nuclear
EMP attack Senarios
The United States military services have developed, and in some cases have published, a number of hypothetical EMP attack scenarios.
The United States EMP Commission was authorized by the United States Congress in Fiscal Year 2001, and re-authorized in Fiscal Year 2006. The commission is formally known as the Commission to Assess the Threat to the United States from Electromagnetic Pulse (EMP) Attack.
The United States EMP Commission has brought together a group of notable scientists and technologists to compile several reports. In 2008, the EMP Commission released the Critical National Infrastructures Report. This report describes, in as much detail as practical, the likely consequences of a nuclear EMP on civilian infrastructures. Although this report was directed specifically toward the United States, most of the information can obviously be generalized to the civilian infrastructure of other industrialized countries.
In written testimony delivered to the United States Senate in 2005, an EMP Commission staff member reported:
The EMP Commission sponsored a worldwide survey of foreign scientific and military literature to evaluate the knowledge, and possibly the intentions, of foreign states with respect to electromagnetic pulse (EMP) attack. The survey found that the physics of EMP phenomenon and the military potential of EMP attack are widely understood in the international community, as reflected in official and unofficial writings and statements. The survey of open sources over the past decade finds that knowledge about EMP and EMP attack is evidenced in at least Britain, France, Germany, Israel, Egypt, Taiwan, Sweden, Cuba, India, Pakistan, Iraq under Saddam Hussein, Iran, North Korea, China and Russia.. . .
Many foreign analysts–particularly in Iran, North Korea, China, and Russia–view the United States as a potential aggressor that would be willing to use its entire panoply of weapons, including nuclear weapons, in a first strike. They perceive the United States as having contingency plans to make a nuclear EMP attack, and as being willing to execute those plans under a broad range of circumstances.
Russian and Chinese military scientists in open source writings describe the basic principles of nuclear weapons designed specifically to generate an enhanced-EMP effect, that they term "Super-EMP" weapons. "Super-EMP" weapons, according to these foreign open source writings, can destroy even the best protected U.S. military and civilian electronic systems.
In non-technical writings about nuclear EMP, both in print and on the Internet, some common misconceptions about EMP are nearly always found. These widely-repeated misconceptions have led to a very considerable amount of confusion about the subject. In 2010, a technical report written for the US government's Oak Ridge National Laboratory even included a brief section addressing some of those EMP myths. Here are some further clarifications on common areas of confusion that have already been discussed (with references) in the above sections of this article:
1. Most nuclear weapons effects vary greatly depending upon the altitude of the detonation. This is especially true of nuclear EMP. The standard reference text on nuclear weapon effects published by the U.S. Department of Defense discusses this relationship extensively in the first two chapters, and provides mutually-exclusive definitions for phrases such as "air burst" and "high-altitude burst."  As explained in above sections of this article, nuclear detonations at all altitudes within the Earth's magnetic field will produce an electromagnetic pulse; but the magnitude of the EMP and area that is affected by the EMP are strongly affected by many factors, and is especially strongly dependent upon the altitude of the detonation. (See the discussion above in the "Weapon altitude" and "Weapon distance" sections.) A nuclear explosion in deep space and not in a strong planetary magnetic field would be ineffective at generating EMP.
2. EMP is not a new kind of weapon effect. As stated in the "History" section above, nuclear EMP from a nuclear air burst has been known since 1945. The unique characteristics of high-altitude nuclear EMP have been known since at least 1962. Non-nuclear EMP has been known since at least 1951. Electromagnetic pulse is a prompt secondary effect of a nuclear explosion, and nearly all of the nuclear EMP is produced outside of the weapon. All nuclear weapons can produce EMP as a secondary effect, but the effect can be enhanced by special weapon design.
3. The E3 component of nuclear EMP that produces geomagnetically induced currents in very long electrical conductors is roughly proportional to the total energy yield of the weapon. The other components of nuclear EMP are less likely to be dependent on total energy yield of the weapon. The E1 component, in particular, is proportional to prompt gamma ray output; but EMP levels can be strongly affected if more than one burst of gamma rays occurs in a short time period. Large thermonuclear weapons produce large energy yields through a multi-stage process. This multi-stage process is completed within a small fraction of a second, but it nevertheless requires a finite length of time. The first fission reaction is usually of relatively small yield, and the gamma rays produced by the first stage pre-ionize atmospheric molecules in the stratosphere. This pre-ionization causes the gamma ray emission from the high-energy final stage of the thermonuclear weapon (a fraction of a second later) to be relatively ineffective at producing a large E1 pulse. (See the blue pre-ionization curve in the "Peak Electric Field at Ground Zero" graph above.)4. It has long been known that there are many ways to protect against nuclear EMP (or to quickly begin repairs where protection is not practical); but the United States EMP Commission determined that such protections are almost completely absent in the civilian infrastructure of the United States, and that even large sectors of the United States military services were no longer protected against EMP to the level that they were during the Cold War. The public statements of the physicists and engineers working in the EMP field tend to emphasize the importance of making electronic equipment and electrical components resistant to EMP — and of keeping adequate spare parts on hand, and in the proper location, to enable prompt repairs to be made. The United States EMP Commission did not look at the civilian infrastructures of other nations (Wikipedia, 2012).