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 MIL-HDBK-419A
10.2.1.2 Late-Time HEMP (MHDEMP). Much later (0.1 to 100 s), currents are induced in the ground by the
effects of the expanding and rising fireball constituents. These effects are called the magnetohydrodynamic
EMP (MHDEMP). They arise from the motions of the rapidly expanding bomb debris and hot ionized gases in the
Earth's magnetic field. MHDEMP has two phases produced by two principal effects. The first effect is an
ionospheric blast wave that deforms the geomagnetic field lines and produces an early phase of the MHDEMP
that reaches the Earth's surface in 2 to 10 seconds and can be seen worldwide. The second effect is the
"atmospheric heave," in which hot debris and air ions are moved across geomagnetic field lines to cause large
circulating currents in the ionosphere. These currents induce image currents in the ground over a period of 10
to 100 seconds. Although the field strengths produced at the surface by the MHDEMP are small (tens of volts
per kilometer), they occur over long times. Thus, the MHDEMP is a consideration for long power and
communications lines and, because of its duration, for the energy it can deliver to protective devices.
10.2.1.3 Intermediate-Time HEMP. Between the early-time HEMP and the MHDEMP, transitory phenomena
produce what is called intermediate-time HEMP. This HEMP lasts from about 1
to about 0.1 s. The
intermediate-time HEMP observed at the Earth's surface has a peak electric field strength of a few hundred
volts per meter and is predominantly vertically polarized.
10.2.2 Surface-Burst EMP. When a nuclear weapon is detonated at or near the surface of the Earth, neutrons
and gamma rays are ejected radially outward from the burst center. The gamma ray photons emitted by the
bomb, and others produced by neutron inelastic collisions with air, ground, and water, interact with air
molecules to produce Compton recoil electrons. At or near sea level, however, the Compton recoil electrons
quickly collide with air molecules to provide a copious supply of low-energy secondary electrons and ions. Thus,
the Compton recoil electrons account for a large charge separation and, because of the secondary ionization, a
fairly conductive air. As illustrated in Figure 10-3, the charge displacement is asymmetrical because of the
Earth's surface. The initial dipole charge is discharged by current through the ionized air and soil. From a
large distance, the EMP from a surface burst appears to emanate from a dipole source; it is vertically polarized
and attenuated as l/r with distance, r, from the burst point. Thus, the surface-burst EMP is a more localized
source than the HEMP. However, within the source region where the Compton electrons, secondary ionization,
and relaxation currents occur, the fields are large, and long conductors, such as power lines and communication
cables, may have large currents induced on them. These currents may be propagated along the conductors for
great distances from their source. Therefore, this source-region EMP (SREMP) may be important to systems
far outside the source region if they are connected to the source region through wires, cables, or other
conductors.
10-3
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