Electromagnetic Pulse (EMP)
Despite what you may read elsewhere, the fact that an electromagnetic
pulse is produced by a nuclear explosion was known in the earliest days of
nuclear weapons testing. In fact conventional explosions also cause an
electromagnetic pulse, although it is much smaller. The magnitude of the
effects were not known, however. During the
Trinity
test on 16 July 1945, electronic equipment was shielded due to
Enrico Fermi's expectation of the electromagnetic pulse. The official
technical history for that first nuclear test states:
"All signal
lines were completely shielded, in many cases doubly shielded. In spite
of this many records were lost because of spurious pickup at the time of
the explosion that paralyzed the recording equipment." During
British nuclear testing in 1952-1953 instrumentation failures were
thought to be due to
"radioflash", which was their term for EMP.
The first openly reported observation of the unique aspects of
high-altitude nuclear EMP occurred following the high altitude (85,000ft -
26.21km) Yucca nuclear test of the
Hardtack
I series on 28 April 1958. The test was of a small (1.7 kiloton)
weapon and EMP measuring instruments went off their scales. The Yucca EMP
was initially positive-going whereas low-altitude bursts were negative
pulses and the polarization of the EMP signal was horizontal, whereas
low-altitude nuclear EMP was vertically polarized. Later tests confirmed
these findings. EMP is dangerous because it has far-reaching effects at
distances where other nuclear environments are either nonexistent or
inconsequential and because of its high level of broad spectral energy.
Significant EMP levels can occur at the Earth's surface out to the tangent
radius, at low frequencies (below 100kHz) this range is considerably
greater. The tangent radius is where the line of sight from the burst is
tangent with the Earth's surface. If one assumes a spherical Earth of
radius of 6371 kilometres, a height of burst (HOB) of 100 kilometres gives
a tangent radius of 1120 kilometres, and a HOB of 500 kilometres
corresponds to a tangent radius of 2450 kilometres. A detonation at 100
kilometres would affect the whole of Europe, even a moderately sized
weapon, say 100 kT would have a significant effect over the whole of the
area.
The causes of EMP
Prompt gamma rays following the nuclear detonation are the principal cause
of EMP. This gamma radiation causes bursts of electron flow by the Compton
effect, a photoelectric effect, and a "pair production" effect. The
collision of gamma rays with atoms and molecules in the atmosphere knocks
electrons free of the air molecules and causes the electrons to move
rapidly away from the center of the explosion and from the now positively
charged parent air molecules. At high altitudes, above 30 kilometers, the
atmosphere is thin and this allows gamma radiation from the nuclear burst
to travel out radially for large distances, 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 detonation. When the downward directed
rays encounter the upper regions oi 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. At the same timer positive
ions are formed. The separation of electrons and positive ions produces an
electric field. The flow of electrons constitutes a current which radiates
electromagnetic energy, providing some asymmetry exists. The energy
contained in EMP is similar to that in EM waves generated by a lightning
strike, but the high frequency energy content in EMP is a much larger
fraction of the total pulse energy.
EMP generation
The relative importance of all nuclear weapons effects, including EMP,
depends on weapon characteristics, burst point, and position of the system
of interest. Emphasis in this page will be the EMF fields generated by a
high altitude burst and some discussion about surface bursts.
Deposition zone or 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-hurst (HOB) and weapon size and type. A
representative thickness is from 20 kilometres to 40 kilometres, but a
deposition region may be as thick as 70 kilometres for a 300 kilometre HOB
and a 10 Mt weapon. 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 surface of the Earth's surface, as if it
were contained in the same spherical shell as that termed by the original
gamma ray shell.
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 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.
Source Region
For both the high altitude and surface bursts, intense fields appear in
what is called the source region. The source region for a surface burst is
limited to about a two to ten kilometers diameter around the burst. For a
high altitude burst, the source region can be on the order of 3000
kilometers in diameter. This source region extends from about 20 to 40
kilometers in altitude. In the source region, weapon effects other than
EMF must also be considered. The size of the source region is severely
confined by the atmosphere for an atmospheric burst.
Beyond the Source Region
Somewhat less intense fields exist beyond the source regions. Additional
mechanisms radiate the energy of the source fields well beyond the source
regions. In the case of a near surface burst, the net charge separation
caused by asymmetry of the source region contributes to the more distant
radiated fields. In a detonation outside the atmosphere burst, the earth's
magnetic field bends the scattered electron current moving away from the
burst point. This bending produces an efficient conversion of the energy
of the moving electrons into a radiated electromagnetic pulse in the radio
spectrum. This radiation is propagated from the source region onto the
surface of the earth. In the case of a high-altitude burst, a significant
overpressure pulse does not exist near the surface of the earth. Almost
all of the other prompt weapon effects are diminished by the atmosphere,
so that the most significant prompt weapon effect is the EMP. As noted
previously, the source region can be quite large, in the order of 1,000
miles in diameter. As a consequence, the radiated fields from this source
region can cover a substantial fraction of the earth's surface. A single
detonation could affect the whole of Europe, or North America.
Characteristics of the Nuclear Electromagnetic Pulse (NEMP)
Nuclear EMP produces a characteristic multi-pulse, usually described in
terms of three components, as defined by the International
Electrotechnical Commission (IEC). The three components are defined by the
IEC, as "E1", "E2" and "E3". Only the first two components of the
EMP are used in the detection of nuclear detonations by such systems as
AWDREY. Lightning EMP produces radio frequencies which are narrower
in bandwidth then nuclear EMP. The pulse is also evident as a double pulse
in the visible light spectrum.
E1
The E1 pulse is a very fast component of nuclear EMP. E1 is a brief but
intense electromagnetic field that induces high voltages in electrical
conductors. The E1 pulse can destroy computers and communications
equipment and it changes too quickly for ordinary surge protection
precautions to be effective, although there are special fast-acting surge
protectors that will block the E1 pulse. The pulse reaches its maximum in
about 5 nanoseconds, and decays to half the peak value in 200 nanoseconds
and decays to zero by 1,000 nanoseconds
E1 is produced when gamma radiation from the nuclear detonation ionizes
(strips electrons from) atoms in the upper atmosphere. This makes
the electrons radiate EMP over a massive area. Because of the curvature
and downward tilt of Earth's magnetic field over the USA, the maximum EMP
occurs south of the detonation and the minimum occurs to the north. This
is known as the Compton effect and the resulting current is called the
"Compton current". The electrons travel in a generally downward direction
at relativistic speeds (more than 90 percent of the speed of light). In
the absence of a magnetic field, this would produce a large, vertical
pulse of electric current over the entire affected area. The Earth's
magnetic field deflects the electron flow at a right angle to the field.
This interaction produces a very large, but very brief, electromagnetic
pulse over the affected area.
The EMP pulse peaks at about 50,000 volts per me padding:5px 5px 5px
5px;tre art ground level, with a peak power density of 6.6 megawatts per
square metre.
E2
The E2 component is generated by scattered gamma rays and inelastic gammas
produced by neutrons. This E2 component is an "intermediate time" pulse
that, by the IEC definition, lasts from about 1 microsecond to 1 second
after the explosion. E2 has many similarities to lightning, although
lightning-induced E2 may be considerably larger than a nuclear E2. Because
of the similarities and the widespread use of lightning protection
technology, E2 is generally considered to be the easiest to protect
against.
Only the first two components of the EMP are used in the detection of
nuclear detonations by such systems as AWDREY. Lightning EMP produces
radio frequencies which are narrower in bandwidth then nuclear EMP. The
pulse is also evident as a double pulse in the visible light spectrum.
E3
The E3 component is very different from E1 and E2. E3 is a very slow
pulse, lasting tens to hundreds of seconds. It is caused by the nuclear
detonation's temporary distortion of the Earth's magnetic field. The E3
component has similarities to a geomagnetic storm caused by a solar flare.
Like a geomagnetic storm, E3 can produce geomagnetically induced currents
in long electrical conductors, such as telephone lines, and power
distribution line, damaging components such as transformers.
Because of the similarity between solar-induced geomagnetic storms and
nuclear E3, it has become common to refer to solar-induced geomagnetic
storms as "solar EMP." "Solar EMP", however, does not include an E1 or E2
component.
Effects of EMP
The magnitude of the EMP can be of the order of 100kV/metre, a thousand
times more than that of a typical radar beam, known to cause sterility and
blindness in humans, but the EMP is of extremely short duration, of the
order of nanoseconds to seconds, however the effects may last for a
protracted period, even up to years.
Other forms of EMP include magnetohydrodynamic (MHD-EMP) which can induce
near-D,C, currents in very long conducting structures, such as telephone
lines, overhead power cables and the like. Low altitude detonations can
produce intense effects over distances of several kilometres. These latter
are generally not of great significance except for command, control and
communications (C3).
Nuclear EMP induces electric currents in all metallic objects. which by
accident or design act as antennas. Aerial and buried power and
telecommunication networks in particular can collect considerable amounts
of energy in the form of electromotive force (emf). Even short radio
antennas and other electrical lines may experience unusual induced
currents and voltages. The collected EMF energy could disturb, breakdown,
or burn out susceptible electrical and electronic components. Modern
solid-state electronics are far more susceptible to stray emf than older
valve (tube) based technology, but even they are not immune. In fact
systems that are purely electrical, rather than electronic are susceptible
to high energy emf pulses. ln 1958 and 1962, high-altitude nuclear tests
were carried out by the United States over the Pacific Ocean. During
these, some electrical and electronic systems suffered functional damage
or operational malfunctions, even hundreds of kilometers from the test
sites. It is unlikely that EMP would incapacitate all of the exposed
communication systems, power networks, and electronic equipment. However,
a small number of failures distributed through a large and complex system
can disrupt the entire system, or degrade its stability and performance.
EMP could, in fact it is likely, create confusion and isolation at
precisely the time when critical decisions would have to be made regarding
the use of nuclear weapons. Communications among diplomats, political
leaders, and military commanders could he disrupted. EMP could also
degrade sophisticated military command, control, communication and
intelligence (C3I) systems within minutes of the first detonations. Such
effects could hinder a military response and might encourage looser
control over nuclear weapons in the field. Because telecommunications would
play an important role in national and international crisis management,
any major disruption of communication networks could affect the course of
a nuclear conflict.
One of the paradoxes of the problems caused by EMP is that modern
electronics are far more at risk than older valve (tube) based equipment,
due to the physical differences between solid-state components and
thermionic devices.
EMP Conspiracy Theories
Most of the literature on EMP is either still classified or not readily
available to the public. A good deal of what is available, chiefly from
the Internet varies considerably in veracity, and much has major
inaccuracies. Some of what you will find uses the right words, but with
inaccurate interpretations. Based upon a
2010
technical report produced by the Oak Ridge National Laboratory, the
following are the most common myths.
