Threat Posed by Electromagnetic Pulse (EMP) to U.S. Military Systems and Civil Infrastructure
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 will experience EMP effects. A burst on the order of 500 kilometers altitude would cover the entire continental United States.
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 nucdet is 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. A localized lightning strike, by comparison, 10 meters away, has a higher peak amplitude by about an order of magnitude, but rises more slowly than an EMP peak, and is simpler to protect against.
Much infrastructure we depend on today would be susceptible to EMP effects, both military and civilian. 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 housing computers networks 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.
All unhardened satellites in low Earth orbit traversing enhanced belts resulting from a high-altitude burst can be expected to demise from the total ionizing radiation dose in a matter of days to weeks following the nudet. A knowledgeable adversary, such as Iran or North Korea, armed with a few nuclear weapons, but having a missile delivery system could seek to exploit any such perceived vulnerability, thereby severely degrading the significant U.S. technological advantage built on a dependence upon sophisticated electronic systems.
A particularly good news story is that EMP protection is quite affordable. If EMP hardening is designed in from the start, the cost of EMP hardening the smart grid is a relatively small fraction of the overall system's cost, approximately 1 to 5 percent. Protecting an existing unprotected infrastructure is significantly more expensive.
We consider a terrorist acquiring a nuclear weapon and positioning it at the high altitude necessary for the generation of an EMP burst that would debilitate our infrastructures to be a remote possibility. Consequently, we are not considering any special measures to counter such a threat, though a high-altitude EMP attack would devastate telecommunications and other critical infrastructures.
The present likelihood of a terrorist obtaining a nuclear weapon is uncertain. Generating a sufficient high-altitude nuclear detonation required to produce a devastating EMP attack would be extremely challenging. There are many easier, less costly, and more dramatic ways for terrorists to use nuclear weapons than delivery to a high altitude. Such an event is so unlikely and difficult to achieve that I do not believe it warrants serious concern at this time. The administration's policy is to prevent proliferation and unauthorized access.
The EMP effects of a more likely improvissed ground burst would be limited mostly to the area effected by the Moderate Damage Radius, with extensions propogated by long conductors such as power lines, railroad tracks tracks and pipelines.
