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An excerpt from here:

http://commdocs.house.gov/committees/security/has197010.000/has197010_1.HTM

Figure 1, which my colleague, Mr. Ron Wiltsie, is illustrating, and which is also on page 2 of your copy, shows the basic phenomenology of an EMP event. The detonation of a nuclear weapon produces high energy gamma radiation that travels radially away from the burst center. When the detonation occurs at high altitudes, greater than about 40 kilometers, the gamma rays directed toward the Earth encounter the atmosphere, where they interact with air molecules to produce positive ions and recoil electrons called Compton electrons, after the man who discovered the effect.

The gamma radiation, interacting with the air molecules, produces charge separation as the Compton recoil electrons are ejected and leave behind the more massive positive ions. The Earth's magnetic field interacts with the Compton recoil electrons and causes charge acceleration, which further radiates electromagnetic energy. EMP is produced by these charge separation and charge acceleration phenomena, which occur in the atmosphere in a layer about 20 kilometers thick and about 30 kilometers above the Earth's surface.

The area of the Earth's surface directly illuminated by EMP is determined entirely by the height of burst. All points on the Earth's surface within the horizon, as seen from the burst point, will experience EMP effects as depicted in figure 2, which is on page 3 of your handout. Note that a burst on the order of 500 kilometers in altitude can cover the entire continental United States.

Mr. WELDON. What strength burst would that be?

Dr. SMITH. It is not terribly burst-strength dependent; almost any burst will produce that kind of radiation. The strength of the field will change at the various radii from the burst point, but it will cover the same area regardless of the strength of the burst.

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The amplitude, duration and polarization of the wave depend on the location of the burst, the type of weapon, the yield, and the relative position of the observer. The electric field resulting from a high-altitude nuclear detonation can be on the order of 50 kilovolts per meter with a rise time on the order of 10 nanoseconds and a decay time to half maximum of about 200 nanoseconds. It is very fast.

A localized lightning strike, by comparison, 10 meters away, has a higher peak amplitude by about an order of magnitude, but it rises more slowly than the EMP peak, and therefore it may be simpler to protect against.

It is important to point out, however, that the peak amplitude, signal rise rate, and duration of the EMP wave are not uniform over the illuminated area; the largest peak intensities of the EMP signal occur in that region of the illuminated area where the line of sight to the burst is perpendicular to the Earth's magnetic field. At the edge of the illuminated area, that is, farthest towards the horizon as seen from the burst, the peak field intensity will be about half of the maximum levels, and the EMP fields will be somewhat longer lasting than in the areas where the peak intensities are the largest.

The EMP threat is unique in two respects. First, its peak field amplitude and rise rate are high. These quantities depend upon the rate of rise and the energy of the gamma ray output of the weapon. These features of EMP will induce potentially damaging voltages and currents in unprotected electronic circuits and components.

Second, the area covered by an EMP signal can be immense. As a consequence, large portions of extended power and communications networks, for example, can simultaneously be put at risk. Such far-reaching effects are peculiar to EMP. Neither natural phenomena nor any other nuclear weapon effects are so widespread.

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Much of what we depend on today would be susceptible to EMP effects, both in the military and civilian infrastructure. An electromagnetic field interacts with metallic conductors by inducing currents to flow through them. A television antenna, for example, is a collection of metal conductors arranged to facilitate the induced current flow in the frequency range allocated for television broadcasting and to transfer the signal to the receiver.

Other conducting structures, such as aircraft, ships, automobiles, railroad tracks, power lines, and communication lines connected to ground facilities, also effectively serve as receiving antennas for EMP coupling. If the resulting induced currents and voltages, which can be large, are allowed to interact with sensitive electronic circuit and components, they can induce an upset in digital logic circuits or cause damage to the components themselves.

Ground facilities, for example, those housing the large computers central to the functioning of our financial systems, are typically nodes in a larger network and are connected to overhead or buried cables for power and communication. They are also connected to buried pipes for water supply and waste disposal and are typically equipped with communication antennas and distributed security systems of various types. All of these features can direct EMP energy into the facility.

Analyses and simulated EMP testing have shown that currents carried to a facility by long overhead or buried conductors can reach thousands of amperes. Shorter penetrating conductors can carry hundreds of amperes into facilities. Direct EMP penetration through the walls and windows of an unshielded building can induce currents of tens of amperes on illuminated interior conductors.

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When EMP energy enters the interior of a potentially vulnerable system, it can cause a variety of adverse effects. These effects include transients, resettable or permanent upset of digital logic circuits, and performance degradation or burnout of electronic components. The collected EMP energy itself can cause malfunction or device failure directly, or it can trigger the system's internal power sources in unintended ways, causing damage by the power sources within the system itself.

In summary, EMP introduces two collectively unique features to the overall picture of system susceptibility to nuclear effects. These features, taken together, distinguish EMP from all other forms, both natural and man-made, of electrical stress and response. First, stresses induced by EMP can significantly exceed those ordinarily encountered in system circuits and components and can thereby increase the probability of upset and burnout occurring in electrical and electronic systems. Second, EMP can cause this increase to occur nearly simultaneously over a large area, about one million square kilometers for a high-altitude burst.

These unique features, together with the lack of occurrence of EMP-like phenomena in the normal day-to-day environment, cause great difficulty in attempting to deal with EMP as a normal engineering problem. In particular, EMP can induce multiple, simultaneous upsets and failures over this wide area.

The coverage and levels that would ensue from an EMP attack are well understood. However, the overall effects on specific terrestrial systems are not as well understood. How much of the telecommunications systems would fail and for how long, how much of the power grid would be disrupted and for how long, how many cars would stop and/or would not start are things that are extremely difficult to predict.