NUCLEAR EMP EFFECTS
10.1 INTRODUCTION. In addition to the blast, thermal effects, and radioactive fallout, a nuclear detonation
produces an intense electromagnetic effect. Under the proper circumstances, a nuclear detonation generates a
high-intensity electromagnetic pulse (EMP) whose frequency spectrum may extend from below 1 Hz to above
300 MHz. This high-intensity EMP can disrupt or damage critical electronic facilities over an area as large as
the continental United States, unless protective measures are taken in the facilities. The development of such
protective measures involves grounding, bonding, and shielding and requires an understanding of the EMP itself.
10.2 EMP GENERATION.
10.2.1 High-Altitude EMP (HEMP). The high-altitude EMP (HEMP) produced by an exoatmospheric nuclear
explosion is the form of EMP commonly of most interest because of the large area covered by a single bomb.
The HEMP is also the form for which interaction and protection are most advanced. The standard HEMP
waveforms to be used for tests and analyses of hardened systems are given in DoD-STD-2169 (SECRET-RD). A
brief description of the three parts of the standard waveform is given below.
10.2.1.1 Early-Time HEMP.
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 where the mean free path of the gamma photons is
large, these photons travel great distances before they interact with another particle. As illustrated in Figure
10-1, gamma rays directed toward the earth encounter dense atmosphere where they interact with air
molecules to produce Compton recoil electrons and positive ions. The Compton recoil electrons also travel
radially away from the burst center initially, but these moving charged particles are acted upon by the Earth's
magnetic field, which causes them to turn about the magnetic field lines (10-1).
The Earth's magnetic field accelerating the Compton recoil electrons causes them to radiate an electrodynamic
field. Thus, the early-time HEMP is produced by this charge acceleration (electron turning) phenomenon that
occurs in the atmosphere in a region about 20 km thick and 30 km above the Earth's surface (sea level). This
source region covers the Earth -within the solid angle subtended by rays from the burst point that are tangent to
the surface of the Earth, as illustrated in Figure 10-2. To an observer on the ground, the incoming wave
appears to be a plane wave propagating toward him from the burst point. The amplitude, duration, and
polarization of the wave depend on the positions of the burst and the observer, relative to the Earth's magnetic
field lines. Peak electric field strengths of over 50 kV/m with risetimes of a few nanoseconds and decay times
of less than 1 are typical for this early-time portion of the HEMP (10-2).