█ LARRY GILMAN
Any nuclear explosion 25 miles (40 km) or higher above the ground produces a high-altitude electromagnetic pulse (HEMP), a short-lived, overlapping series of intense radio waves that blanket a large swath of ground. These radio waves can induce electrical currents in metallic objects and so cause damage to electrical and electronic equipment, including electrical power grids, telephone networks, radios, and computers. The HEMP produced by a single large (i.e., multi-megaton) nuclear weapon detonated 125 miles (200 km) above the center of the continental United States would affect more than half the country; a weapon detonated at 250 miles (400 km) would affect the entire country, though at lower pulse intensities. Military electronics are often "hardened" against HEMP by enclosures of metal foil and by specialized surge protectors. Civilian systems are not hardened against HEMP.
A typical HEMP consists of a series of overlapping radio pulses, each produced by a different physical aspect of the nuclear explosion. The first, briefest, and most intense component of a HEMP is the prompt gamma signal, which is produced as follows: When a nuclear weapon detonates, large numbers of gamma rays (highenergy photons with wavelengths less than .1 nm) range radiate outward from the burst point. Many of these collide with atoms in the Earth's atmosphere, knocking electrons free. These free electrons are created almost simultaneously in a large volume of the atmosphere surrounding the explosion, and travel rapidly away from the burst point in all directions. Because any charged particle crossing magnetic field lines experiences a force at right angles to its direction of motion, the Earth's magnetic field forces these electrons to follow curved paths, and because charged particles following curved paths emit electromagnetic waves (synchrotron radiation), the explosion-liberated electrons spiraling through the Earth's magnetic field emit a strong radio pulse, namely, the prompt gamma component of the HEMP. Additional pulses, of longer duration but lower magnitude, arrive soon afterward. These are caused by scattered neutrons and gamma rays (radiation that has made one or more bounces, rather than following a straight radial path from the burst point) and by the expansion and ascent of the ionized nuclear fireball through the Earth's magnetic field. The electromagnetic pulse caused by the latter effect, termed the magnetohydrodynamic EMP or HD-EMP, is of low intensity but long duration, and is thought to be a particular threat to power transmission lines.
Although the first nuclear weapon was exploded in 1945, HEMP was unknown to U.S. scientists until July 8, 1962, when a high-altitude nuclear test code-named Starfish was conducted by the U.S. approximately 250 miles (400 km) above the Pacific Ocean, some 800 miles (1280 km) from the Hawaiian island of Oahu. Unexpectedly, some 30 strings of streetlights failed in the island's main town simultaneously with the Starfish explosion. Investigation showed that certain of the lines, randomly oriented so as to pick up the HEMP from Starfish like radio antennae, had absorbed enough energy to blow their fuses. Soviet scientists were probably already aware of HEMP, because the Soviet Union had already conducted high-altitude tests like Starfish. HEMP subsequently became a central component in strategic nuclear war-simulations; many speculative scenarios for a Soviet first strike on the U.S. began with an EMP "lay-down" created by simultaneously exploding a relatively small number of nuclear weapons at high altitude over the United States. The goal would have been to cause widespread damage to civilian and military electrical and electronics systems at relatively low cost, to be followed by a more devastating ground attack. More recently, some U.S. officials considered a smaller-scale EMP laydown attack on Iraq as a prelude to the Gulf War of 1990. (The attack was not carried out.)
Although some planners have worried that a nation or terrorist group possessing only a few nuclear weapons might use one of them to blanket the U.S. with a damaging HEMP, this is thought by most experts to be unlikely. To create a significant HEMP attack, a weapon must be small enough to be lofted on a ballistic missile, and few countries have the know-how either to make powerful nuclear weapons of such small size or to build ballistic missiles. In any case, it is unlikely that an adversary seeking to cause maximal harm and willing to risk using nuclear weapons against a nuclear-armed adversary such as the U.S. would make a HEMP attack. Any nuclear weapon would cause far more destruction by direct blast (if detonated over or in a city) than by HEMP (if detonated at high altitude).
Besides HEMP, two other forms of electromagnetic pulse may be caused by nuclear explosions. The first is generated inside electronic devices by the passage of ionizing radiation (e.g., neutrons and gamma rays) directly into metallic cases, circuit boards, semiconductor chips, and other components, where it can cause brief electrical currents to flow by knocking electrons loose from atoms. This effect is termed systems-generated electromagnetic pulse (SGEMP). The other form of EMP— source-region EMP or SREMP—occurs when a nuclear weapon explodes at low altitude. In this situation, a highly asymmetric electric field is produced in the vicinity of the burst (e.g., within a radius of 3–8 km) having intensities that are much greater than those produced by HEMP. Since the region affected by SREMP corresponds to that effected by the nuclear blast itself, SREMP is relevant only to the defense of hardened targets such as buried missile silos, which are intended to remain functional even in the aftermath of a near-surface nuclear blast.
Carbon-graphite coils capable of generating an electromagnetic pulse used to destroy electronics equipment— especially communications equipment—can be fitted to cruise missiles. Carbon-graphite equipped cruise missiles were used by U.S.-led forces in raids on Baghdad, Iraq in 1991 and in 2003.
Scientists at Lawrence Livermore National Laboratory reportedly developed an HPM weapon for the Department of Justice: aimed at a moving vehicle, the HPM could shut off the electronic ignition, thus bringing a high-speed car chase to an abrupt end.
█ FURTHER READING:
"Electromagnetic Pulse Threats to U.S. Military and Civilian Infrastructure." Hearing Before the Military Research and Development Subcommittee of the Committee on Armed Services, U.S. House of Representatives, Oct. 7, 1999 (H.A.S.C. No. 106–31). Washington, DC: U.S. Government Printing Office, 2000.
Kruse, V. J., et al. "Impacts of a Nominal Nuclear Electromagnetic Pulse on Electric Power Systems: A Probabilistic Approach." IEEE Transactions on Power Delivery. (Vol. 6, No. 3, July 1991): 1251–1263.