Nuclear Weapon EMP Effects
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�?00 km over Kansas would affect all of CONUS. The signal from such an event extends to the visual horizon as seen from the burst point.
The EMP produced by the Compton electrons typically lasts for about 1 microsecond, and this signal is called HEMP. In addition to the prompt EMP, scattered gammas and inelastic gammas produced by weapon neutrons produce an “intermediate time�?signal from about 1 microsecond to 1 second. The energetic debris entering the ionosphere produces ionization and heating of the E-region. In turn, this causes the geomagnetic field to “heave,�?producing a “late-time�?magnetohydrodynamic (MHD) EMP generally called a heave signal.
Initially, the plasma from the weapon is slightly conducting; the geomagnetic field cannot penetrate this volume and is displaced as a result. This impulsive distortion of the geomagnetic field was observed worldwide in the case of the STARFISH test. To be sure, the size of the signal from this process is not large, but systems connected to long lines (e.g., power lines, telephone wires, and tracking wire antennas) are at risk because of the large size of the induced current. The additive effects of the MHD-EMP can cause damage to unprotected civilian and military systems that depend on or use long-line cables. Small, isolated, systems tend to be unaffected.
Source Region Electro-magnetic Pulse [SREMP] is produced by low-altitude nuclear bursts. An effective net vertical electron current is formed by the asymmetric deposition of electrons in the atmosphere and the ground, and the formation and decay of this current emits a pulse of electromagnetic radiation in directions perpendicular to the current. The asymmetry from a low-altitude explosion occurs because some electrons emitted downward are trapped in the upper millimeter of the Earth’s surface while others, moving upward and outward, can travel long distances in the atmosphere, producing ionization and charge separation. A weaker asymmetry can exist for higher altitude explosions due to the density gradient of the atmosphere.
Within the source region, peak electric fields greater than 10 5 V/m and peak magnetic fields greater than 4,000 A/m can exist. These are much larger than those from HEMP and pose a considerable threat to military or civilian systems in the affected region. The ground is also a conductor of electricity and provides a return path for electrons at the outer part of the deposition region toward the burst point. Positive ions, which travel shorter distances than electrons and at lower velocities, remain behind and recombine with the electrons returning through the ground. Thus, strong magnetic fields are produced in the region of ground zero. When the nuclear detonation occurs near to the ground, the SREMP target may not be located in the electromagnetic far field but may instead lie within the electro-magnetic induction region. In this regime the electric and magnetic fields of the radiation are no longer perpendicular to one another, and many of the analytic tools with which we understand EM coupling in the simple plane-wave case no longer apply. The radiated EM field falls off rapidly with increasing distance from the deposition region (near to the currents the EMP does not appear to come from a point source).
As a result, the region where the greatest damage can be produced is from about 3 to 8 km from ground zero. In this same region structures housing electrical equipment are also likely to be severely damaged by blast and shock. According to the third edition of The Effects of Nuclear Weapons, by S. Glasstone and P. Dolan, “the threat to electrical and electronic systems from a surface-burst EMP may extend as far as the distance at which the peak overpressure from a 1-megaton burst is 2 pounds per square inch.�?
"Society has entered the information age and is more dependent on electronic systems that work with components that are very susceptible to excessive electric currents and voltages." Many systems needed are controlled by a semiconductor in some way. Failure of semi-conductive chips could destroy industrial processes, railway networks, power and phone systems, and access to water supplies. Semiconductor devices fail when they encounter an EMP because of the local heating that occurs. When a semi-conductive device absorbs the EMP energy, it displaces the resulting heat that is produced relatively slowly when compared to the time scale of the EMP. Because the heat is not dissipated quickly, the semiconductor can quickly heat up to temperatures near the melting point of the material. Soon the device will short and fail. This type of failure is call thermal second-breakdown failure.
"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. "Most of the energy is distributed throughout the lower frequencies between 3 Hz and 30 kHz.
The Electro-Magnetic Pulse will electrify all sorts of metallic structures that are not normally electrified except by the occasional short circuit or lightening strike. This will be a lot like the whole country getting struck by lightening all at the same time.
As computer chips make better and better use of "real estate", using more and more delicate electronic circuits, the more tightly-packed transistors, capacitors, diodes and resistors become more and more vulnerable to the EMP which will be carried into the chips via the connecting wires. The Electro-Magnetic Pulse is one of the reasons above-ground testing was stopped.
EMP is unlikely to harm most people, Pacemakers, for example, may stop working because of the "hit" from the EMP. It will be quite something to see people in a thousand mile radius of the epicenter of the blast (or further) who are using pacemakers, suddenly drop dead, and all the computers permanently go down and all the lights go out, all at the same time. And commercial and private aircraft will drop out of the sky, since their sensitive electronics and fly-by-wire systems are not very well shielded from the EMP. These planes will then not be available for evacuation purposes, nor will they be available to air-drop food, water, morphine and cyanide, all of which will be in great demand throughout the area.
Home plumbing systems and most other plumbing systems are good examples of large metallic structures that will suddenly become electrified, destroying the motors, gauges, electronics, etc. which are attached to the plumbing systems. More and more pumping equipment is computer controlled nowadays for efficiency. Imbedded controllers are becoming prevalent but as they do, the potential damage from the Electro-Magnetic Pulse increases dramatically.
In plain language, it is an electromagnetic "shock wave" that is released from nuclear detonations that can seriously damage any electrical components in its path. EMP can travel either through air or through conductive pathways such as electrical or phone lines. It can affect electronic equipment regardless of whether the equipment is switched on or off.
Adapted from - Nuclear Weapons Effects Technology Militarily Critical Technologies List (MCTL) Part II: Weapons of Mass Destruction Technologies
Makoff, Greg and Kosta Tsipis, "The Nuclear Electromagnetic Pulse," Report #19. Program in Science and Technology for International Security, Cambridge, MA, March 1988, p.3.
Glasstone, Samuel and Phillip J. Dolan, The Effects of Nuclear Weapons. Department of the Army, Washington D. C., 1977, pp. 353-54
High Altitude Electromagnetic Pulse Protection for Ground Based Facilities. Naval Facilities Engineering Command, Alexandria, VA, 1986, p. 12.02-4.
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