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EM Impulse and Nuclear Explosion
Name: Brent M.
Status: student
Age: 15
Location: N/A
Country: N/A
Date: 12/10/2004
Question:
Can you explain how electromagnetic impulses can be
made with a nuclear explosion?
Replies:
I believe the typical explanation is:
- the first product emerging from a nuclear explosion is high-energy
photons:
gamma rays from the splitting atoms, and X-rays from the
extreme temperature.
- photons mostly just wiggle or excite things, but if intense they can
push a bit, too.
They are much better at pushing light charged electrons than
they are at pushing heavier nuclei, ions and atoms.
- so a large negative charge gets abruptly pushed outwards by some
distance, while the positive nuclei stay relatively still.
- in some milliseconds most of this separated charge gradually
migrates back together, and the impulse tails off.
- the concentration of electrons moved is very high compared to what
we usually experience of moving charges,
and the volume over which this happens is large,
so the resulting electric field and electro-magnetic wave is more powerful
than what we are accustomed to.
The nano-second abruptness of the start of the impulse makes it "sharp" too.
Having short-wavelength VHF- or UHF-frequency components
which are likely to sneak into metal boxes through long loose seams and
spike the transistors inside.
A nuclear explosion in mid-atmosphere would suddenly have positive charge
in the middle surrounded by a spherical shell of negative charge.
This does not create much "dipole moment", so it does not launch the
strongest waves.
Electromagnetic waves happen when you grab a (+) charge in one hand, (-)
charge in the other, and suddenly pull them farther apart.
The wave emerges not along the axis of motion, the (+) to (-) line, but
perpendicular to that axis.
Imagine a flagpole-shaped loudspeaker broadcasting laterally across a
school yard, 360 degrees around, but not much up or down.
Separation between a (+) and a (-) is called "dipole moment".
So the strongest waves would be launched by an explosion at the top of the
atmosphere,
because there is more air below the explosion than above.
So negative charge driven downwards would be stronger than the negative
charge driven upwards,
and there is a strong vertical dipole moment.
Because the direction of charge motion is vertical, the impulse waves
would radiate horizontally in a growing, fading circle.
This swelling circle would slide around the globe and re-converge on the
point at the other side.
An explosion high over Hawaii might cause some electrical damage in
southern Africa.
The earth would continue to "ring" or echo for several cycles at 7 times
per second, not doing much damage but very easy to detect.
There is also a strong "near-field electric impulse" locally.
It is much like being between two metal plates, when the upper one is
abruptly charged to a very high negative voltage.
The electric field change is equivalent if the upper plate is previously
charged-up positive high voltage, and then instantly discharged via a spark.
I think EMP testing is often done this way.
In the "real" nuclear-event case the lower plate is earth's electrically
conductive ground,
and the upper plate is the top of the atmosphere, the lower boundary of
Earth's ionosphere.
Jim Swenson
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Update: June 2012
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