- From a paper by Carey Sublette Federation
of American Scientists (www.fas.org) From their nuclear weapon section
at http://www.fas.org/nuke/hew/index.html 5-20-99
- Jon Bell's question about EMP and its
- "As I understand them, EMP weapons
have a radius of coverage of about 25 to 50 miles from their detonation
point (still quite a problem). Does anyone know if Lukin's statement that
the EMP from a nuclear weapon detonated at high altitude could affect the
entire U.S. is true? "
- An "EMP weapon" is not the
same as a large nuclear explosion that is denotated at high altitude with
the damage range over 1000 km. (http://www.fas.org/nuke/hew/Library/Damage)
- My source of information comes from a
paper by Carey Sublette of the Federation of American Scientists (www.fas.org)and
their nuclear weapon section at http://www.fas.org/nuke/hew/index.html
- Allow this gentleman to explain the science:
- (Begin excerpt....)
- 5.5 Electromagnetic Effects (of nuclear
weapons) The high temperatures and energetic radiation produced by nuclear
explosions also produce large amounts of ionized (electrically charged)
matter which is present immediately after the explosion. Under the right
conditions, intense currents and electromagnetic fields can be produced,
generically called EMP (Electromagnetic Pulse), that are felt at long distances.
Living organisms are impervious to these effects, but electrical and electronic
equipment can be temporarily or permanently disabled by them. Ionized gases
can also block short wavelength radio and radar signals (fireball blackout)
for extended periods.
- The occurrence of EMP is strongly dependent
on the altitude of burst. It can be significant for surface or low altitude
bursts (below 4,000 m); it is very significant for high altitude bursts
(above 30,000 m); but it is not significant for altitudes between these
extremes. This is because EMP is generated by the asymmetric absorption
of instantaneous gamma rays produced by the explosion. At intermediate
altitudes the air absorbs these rays fairly uniformly and does not generate
long range electromagnetic disturbances.
- The formation EMP begins with the very
intense, but very short burst of gamma rays caused by the nuclear reactions
in the bomb. About 0.3% of the bomb's energy is in this pulse, but it lasts
for only 10 nanoseconds or so. These gamma rays collide with electrons
in air molecules, and eject the electrons at high energies through a process
called Compton scattering. These energetic electrons in turn knock other
electrons loose, and create a cascade effect that produces some 30,000
electrons for every original gamma ray.
- In low altitude explosions the electrons,
being very light, move much more quickly than the ionized atoms they are
removed from and diffuse away from the region where they are formed. This
creates a very strong electric field which peaks in intensity at 10 nanoseconds.
The gamma rays emitted downward however are absorbed by the ground which
prevents charge separation from occurring. This creates a very strong vertical
electric current which generates intense electromagnetic emissions over
a wide frequency range (up to 100 MHZ) that emanate mostly horizontally.
At the same time, the earth acts as a conductor allowing the electrons
to flow back toward the burst point where the positive ions are concentrated.
This produces a strong magnetic field along the ground. Although only about
3x10^-10 of the total explosion energy is radiated as EMP in a ground burst
(10^6 joules for 1 Mt bomb), it is concentrated in a very short pulse.
The charge separation persists for only a few tens of microseconds, making
the emission power some 100 gigawatts. The field strengths for ground bursts
are high only in the immediate vicinity of the explosion. For smaller bombs
they aren't very important because they are strong only where the destruction
is intense anyway. With increasing yields, they reach farther from the
zone of intense destruction. With a 1 Mt bomb, they remain significant
out to the 2 psi overpressure zone (5 miles).
- High altitude explosions produce EMPs
that are dramatically more destructive. About 3x10^-5 of the bomb's total
energy goes into EMP in this case, 10^11 joules for a 1 Mt bomb. EMP is
formed in high altitude explosions when the downwardly directed gamma rays
encounter denser layers of air below. A pancake shaped ionization region
is formed below the bomb. The zone can extend all the way to the horizon,
to 2500 km for an explosion at an altitude of 500 km. The ionization zone
is up to 80 km thick at the center. The Earth's magnetic field causes the
electrons in this layer to spiral as they travel, creating a powerful downward
directed electromagnetic pulse lasting a few microseconds. A strong vertical
electrical field (20-50 KV/m) is also generated between the Earth's surface
and the ionized layer, this field lasts for several minutes until the electrons
are recaptured by the air. Although the peak EMP field strengths from high
altitude bursts are only 1-10% as intense as the peak ground burst fields,
they are nearly constant over the entire Earth's surface under the ionized
- The effects of these field on electronics
is difficult to predict, but can be profound. Enormous induced electric
currents are generated in wires, antennas, and metal objects (like missiles,
airplanes, and building frames). Commercial electrical grids are immense
EMP antennas and would be subjected to voltage surges far exceeding those
created by lightning, and over vastly greater areas. Modern VLSI chips
are extremely sensitive to voltage surges, and would be burned out by even
small leakage currents. Military equipment is generally designed to be
resistant to EMP, but realistic tests are very difficult to perform and
EMP protection rests on attention to detail. Minor changes in design, incorrect
maintenance procedures, poorly fitting parts, loose debris, moisture, and
ordinary dirt can all cause elaborate EMP protections to be totally circumvented.
It can be expected that a single high yield, high altitude explosion over
an industrialized area would cause massive disruption for an indeterminable
period, and would cause huge economic damages (all those damaged chips
- A separate effect is the ability of the
ionized fireball to block radio and radar signals. Like EMP, this effect
becomes important with high altitude bursts. Fireball blackout can cause
radar to be blocked for tens of seconds to minutes over an area tens of
kilometers across. High frequency radio can be disrupted over hundreds
to thousands of kilometers for minutes to hours depending on exact conditions.