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Name: Brent M.
Status: student
Age: 15
Location: N/A 
Country: N/A
Date: 12/10/2004

Can you explain how electromagnetic impulses can be made with a nuclear explosion?

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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