ESSAY: ‘E.M.P.’

One of the joys of being a physicist is trying to understand what’s going on in the physical world as one goes through it. I tried to get that joy across to my students. Out of about three hundred in eleven years as a professor, I think I succeeded with two… plus my son, Jason, who is also a physicist. My physics students were mostly pre-meds, taking physics because it was required by medical schools. What can you expect?

One of the things I figured out after I retired to the Colorado mountains was the cause of a lot of the electrical glitches we get. After a snowstorm, especially a wet snowstorm, we get a lot of power surges. They are mostly small. (Big surges are typically caused by squirrels getting across transformer terminals.) At my house, they cause a reset of the clock on our microwave, which is about twenty years old. They don’t affect the clock on our stove, which is much newer. It’s a tossup whether they knock our Internet router off-line.

These small surges are caused by ice accumulating on our power line… Well, that isn’t quite right. Rather, they are caused by the ice that has accumulated on our power line falling off. That happens the day after the snowstorm, when the sun comes out. When the ice falls off, the line between two poles whips up and down, causing and EMP — an Electro-Magnetic Pulse. The EMP is a result of Faraday’s Law of Induction, discovered by Michael Faraday in 1831 and by Joseph Henry is 1832. In 1861 James Clerk Maxwell incorporated Faraday induction into his equations which present the law in a cleaner form, integrated with the other laws of electricity and magnetism.

I admire Michael Faraday, probably because he was a hick like me, or whatever was the British equivalent of a hick at the time. He never learned much math (these days you can’t be a physicist without being pretty good at math), but he had insight; he could see what was going on. He was actually a chemist; physics was a sideline.

But I digress…

When the ice falls off a power line, it whips up and down. You can see it. The power line crossing the Earth’s magnetic field generates a voltage pulse by Faraday induction. It isn’t a very strong pulse; it doesn’t have much effect on electric motors, lights, or heaters, but it can mess up digital electronics. They use electric power in very small increments, comparable to the surges produced by a power line whipping up and down. I’ve seen the line in front of my house whip up and down and rushed inside to see the microwave clock reset button flashing.

Tonight on the news I saw some government guy talking about the danger of North Korea setting off a bomb on a satellite that would cause an EMP that could knock out the US power grid. That got my attention. I had heard about the EMP associated with an H-bomb explosion. I sort of understand that. The expanding plasma bubble pushes the magnetic field, causing Faraday induction out front. The moving magnetic field will induce electric pulses in any power lines it crosses.

OK, H-bombs are bad news any way you look at them. I didn’t worry much about EMP in the context of H-bombs.

The government guy talked about EMP caused by a small bomb that could destroy our power grid. The North Koreans claim to have bombs: fission bombs, not thermonuclear. He said a bomb that focused on EMPs would be fission and smaller. The Korean tests have been small. He said that indicated that they may be EMP bombs…

I thought they were just fizzles…

The gov’t guy then mentioned gamma rays and the Air Force Starfish experiment in 1962, that caused a large EMP. That got my attention. I was at NASA in 1962, and worked on the effects of the Starfish bomb. It created an artificial Van Allen belt, filled with fission electrons. I was assigned the job of showing the radiation would not be a danger to Scott Carpenter’s MA-9 flight. I did, and he flew the mission with no problem. We never considered EMPs.

I Googled EMP and Korea. I found a not quite satisfactory discussion of the effect of electrons produced by gamma rays causing EMPs. The mechanism was not described, but having been a plasma physicist in one of my lives, I saw how the electrons and positrons from pair-production would tend to cancel the Earth’s magnetic field. That’s what charged particles do in a magnetic field. There’s a diamagnetic cavity behind the Moon. (“Behind” relative to the direction of the solar wind.) The flowing plasma is blocked by the Moon; its absence increases the field strength. A friend of mine measured the field increase with a satellite orbiting the Moon fifty years ago.
I’m not convinced a Korean fizzle would produce enough gamma rays to cause much of an EMP. We don’t even know whether there’s a bomb on the two satellites North Korea has up. And the positrons, being antimatter, rather quickly take out an electron each and we are back where we started.

Maybe a better physicist than I am can explain it.

In the meanwhile, I suspect a neglected bureaucrat is seeking attention.

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Jerry Modisette

Jerry Modisette, PhD, was at NASA Houston during the Mercury, Gemini, and Apollo flights and performed the tests selecting the Mercury heat shield materials. He also has extensive experience working in the petroleum industry. He lives in Cabezon Canyon, west of Pagosa Springs.