HEMP generation. HEMP is caused by a nuclear burst at high altitudes. Prompt gamma rays following the nuclear detonation are the principal source of HEMP. This gamma radiation causes bursts of electron flow from the Compton effect, a photoelectric effect, and a “pair production” effect. Of these three effects, however, the primary source of HEMP is the Compton effect.

Gamma radiation. At high altitudes (above 30 kilometers), the atmosphere is thin and thus allows gamma radiation from the nuclear burst to travel out radially for long distances. Below the center of the burst, however, the atmospheric density increases as the earth’s surface is approached. The prompt gamma rays propagate toward the earth in a thin spherical shell, moving at the speed of light away from the burst...

Compton scattering. When the downward directed rays encounter the upper regions of the atmosphere, they begin to interact with the atoms (or molecules) of the atmosphere at a rate which is a function of atmospheric density and burst conditions. The dominant interaction is Compton scattering, in which the energy of a gamma ray is partially transferred to an electron of an air atom (or molecule). The electron then begins traveling in approximately the same direction as the gamma ray. The other product of collision is a gamma ray of reduced energy. The spherical shell of gamma rays is converted during Compton scattering into a spherical shell of accelerated electrons.

Deposition region. The region in which Compton scattering occurs is called the deposition region. The thickness and surface range of the deposition region is a function of height-of-burst (HOB) and weapon size and type. A representative thickness is from 20 kilometers to 40 kilometers, but a deposition region may be as thick as 70 kilometers (10 kilometer to 80 kilometer altitude) for a 300 kilometer HOB and a 10 megaton weapon.

Radiating magnetic field. In the spherical shell of Compton electrons, the electrons are charged particles that rotate spirally around the earth’s geomagnetic field lines. The electrons thus have a velocity component transverse to the direction of the gamma radiation. These transverse currents give rise to a radiating magnetic field. This field propagates through the atmosphere to the earth’s surface as if it were contained in the same spherical shell as that formed by the original gamma ray shell.

HEMP ground coverage. Significant HEMP levels can occur at the Earth’s surface out to the tangent bounded range of effect (and beyond, for frequencies below 100 kilohertz). The tangent bounded range of effect is where the line of sight from the burst is tangent with the Earth’s surface.

As an example, the HEMP generated by a nuclear explosion at an altitude of 500 kilometers would illuminate the whole continental United States. If high-yield weapons are used, the field strength will not vary much with HOB, so this large geographic area can be covered with little reduction in peak field strength.

Magneto hydrodynamic EMP (MHD-EMP) MHD-EMP is the late time (t > 0.1 second) component of EMP caused by a high altitude nuclear burst. Two distinct physical mechanisms produce different parts of the MHD-EMP signal: an “early phase” from 0.1 to 10 seconds after the detonation, and “late phase” lasting from 0.1 to 1000 seconds. MHD-EMP fields have low amplitudes, large spatial extent, and very low frequency. Such fields can threaten very long landlines, including telephone cables and power lines and submarine cables.

MHD-EMP early phase generation. A nuclear burst at high altitudes gives rise to a rapidly expanding fireball of bomb debris and hot ionized gas. This plasma tends to be diamagnetic in that it acts to exclude the earth’s magnetic field from the inside of the fireball. Thus, as the fireball expands and rises in early stages, it will deform the geomagnetic field lines and thereby set up the early phases of the MHD-EMP, which can propagate worldwide. The region on the ground immediately below the burst is shielded from early time MHD-EMP by a layer of ionized gas (the X-ray patch) produced by X-rays from the nuclear burst.

MHD-EMP late phase generation. Residual ionization and the bomb-heated air under the rising fireball are mainly responsible for the late phase of the MHD-EMP. As the bomb-heated air rises, residual ionization moves across geomagnetic field lines and large current loops form in the ionosphere. The ionosphere current loops then induce earth potentials. The late phase of the MHD-EMP is seen in large sections of the earth’s surface, including regions at the magnetic conjugate points. Though amplitudes are smaller than for HEMP, the low frequency fields can introduce damaging potential differences on long cable systems.

Surface burst EMP (SBEMP). SBEMP is produced by a nuclear burst close (less than 0.2 kilometers) to the earth’s surface. The EMP is generated in the source region, which extends out to a bounded range of effect of 1 to 5 kilometers from the burst. A surface burst also has fields radiating outside the source region, with those field amplitudes significant (greater than 5 kilometers per meter) out to ranges of 10 kilometers and more. In this range, the radiated EMP is a principal threat to systems that respond to very low frequencies or have very large energy collectors such as long lines. Conducted EMP for these systems is such that special attention must be given to surge protection to ensure that the high currents can be dissipated.

Source region. The generation of EMP by a surface burst starts when the gamma rays travel out radially from the burst. These rays scatter Compton electrons radially from the burst, leaving behind relatively immobile positive ions. This charge separation produces radial electric fields (Er) with amplitudes over 100 kilovolts per meter (amplitudes may approach 1 megavolt per meter) and risetimes as short as a few nanoseconds. Since the ground conducts better than the air at early times, the strong radial electric field causes a ground current to flow in a direction opposite to the radial Compton current in the air. The resulting current loops produce azimuthal magnetic fields. Magnetic fields are strongest at the earth’s surface and diffuse both upward and downward from the interface. The discontinuity due to the air-earth interface also generates strong vertical electric fields in the source region. Source region fields depend strongly on factors such as weapon yields (gammas and neutrons), HOB, and distance from the burst. The interaction with a system is very complex: besides EM fields, the system may be exposed to nuclear radiation, in addition to being located in a region of time varying currents and conductivity. In specifying a source region environment for a system, then, the concept of balanced survivability is useful as it is with all EMP environments. If a facility is designed to withstand ionizing radiation and other nuclear effects at a specific range from a given burst, it should also be designed to withstand the EMP effects generated at that range.

Air-burst EMP. Air-burst EMP results from a nuclear explosion at intermediate altitudes – 2 to 20 kilometers. The EMP produced by a burst at heights between 0.2 and 2 kilometers will share characteristics of air and surface bursts, and a burst between 20 and 40 kilometers will cause EMP sharing characteristics of air-burst and high altitude EMP. The source region resembles the surface burst source region in that weapon gammas scatter Compton electrons radially outward. Positive ions are left behind, producing charge separation and radial electric fields. For air burst EMP, there is no return path through the grounds. Due to ionization, however, increased air conductivity enables a conduction current to flow opposite the Compton current in the air. Still, no significant current loops are formed, and the large azimuthally magnetic fields typical of a surface burst do not result.

Environment-to-facility coupling. To analyze how HEMP will affect facilities and electronic equipment, the exterior free field threats must be related to system, sub-system and circuit responses. The functional relationship between external causes and internal effects, is often called a “transfer function”. The analysis involves learning how the system collects energy from the incident HEMP field. The result is usually a matrix of internal fields and transient voltages and currents that may flow in circuits and sub-systems. This is called “determination of the coupling interactions between the external threat and the system”. Generally, HEMP enters shielded enclosures by three different modes: diffusion through the shield; leakage through apertures such as seams, joints and windows; and coupling from intentional or inadvertent antennas.

At Hardened Structures Hardened Shelters LLC, we provide complete shelter, business, government and military CBRE, HEMP and EMP design/build services. Please call us for a free consultation.



607 Lynnhaven Parkway 
Virginia Beach, VA 23452

Toll Free: +1 (877) 486-0084