The subject of EMP is red-hot this week, as a new novel about America after electronics - 'One Second After.'
Again, I haven't read the book (yet -I will) and I'm no expert on the effects of nuclear weapons. But some amateur math confirms the gut impression that a small (10 - 20Kt) weapon isn't going to have a massive national impact.
I'm bringing forward a post I did back in 2006 below so you can check my math:
OK, I'm looking at the likely effects of EMP and doing the classic blogger thing of dipping into serious issues as a rank amateur. But I may be right, and if not, I'll trigger a darn interesting discussion.
TG works close to the Los Angeles Public Library, and we have a deal where I'll find a book I'm interested in, email her the catalog link, and she'll pick it up and bring it home for me. The Department of Homeland Security is doubtless interested in her borrowing habits...
Today, she brought home Glasstone & Dolan's "The Effects of Nuclear Weapons," Third Edition.
Here's what I learned. To maximize EMP effect, the weapon has to explode at an altitude of over 19 miles - there's a dramatic increase in the amount of gamma converted to electricity at that height. The EMP effect is generally limited to the line-of-sight to the weapon, and does diminish somewhat as the weapon explodes at greater and greater heights - because more of the gamma radiation which is converted to electrical energy by the atmosphere is radiated upward.
The end result of my quick Excel calculations is the energy per square mile would vary between 0.03 joules/mile for a 10KT weapon detonated 15 miles up, with an effective radius of 350 miles and 17,800 joules/mile over an area with a radius of 1500 miles for a 1 megaton blast at 300 miles up.
Now this may sound like a lot, but recall that a lightning bolt has about 109 - 1010 joules.
And a Shahab-1 has a maximum height of about 55 miles.
Here's some math [formatting fixed by Joe]. There are three cases for calculating EMP; ground burst, mid-level air burst, and high-altitude burst.
High-altitude is defined as over 19 miles; there the effect is far greater (more of the gamma radiation from the weapon interacts with the atmosphere, creating a plasma, and thus the burst of electrical energy).
For a high-altitude burst, about 10-2 of the gamma radiation is transferred to EMP. For a mid-altitude burst, it's about 10-7. For a one-megaton weapon, the total energy output is about 4.2 × 1022 ergs. About 3 × 10-3 of that becomes gamma radiation, or 1.26 × 1020 ergs.
At low altitude, this yields about 1.26 × 1013 ergs, at high altitude, about 1.26 × 1018.
In joules, that's about 1.26 × 106 for low altitude, and 1.26 × 1011 for high. It's linear to weapon yield, so a 10 KT weapon would have 1.26 × 104 at low altitude and 1.26 × 109 at high.
But that area is dispersed over a wide area - the total energy matters, but the energy density matters as well (total energy matters more in effects on long conductors, like power lines).
Even at very high altitudes, the EMP effect is limited to the 'tangent radius' of the blast - the height at which it goes below the horizon. So at a 15-mile blast height, the radius looks like 350 miles. At 300 miles, it would be about 1500 miles.
So, as above the energy per square mile would vary between 0.03 joules/mile for a 10KT weapon at 15 miles, with an effective radius of 350 miles; and 17,800 joules/mile over an area with a radius of 1500 miles for a 1 megaton blast at 300 miles.
What's my point?
When Iran or whoever can develop weapons with yields in the megaton range, and the ability to deliver them to a height of 300 miles, we need to worry about EMP. Until then, I'd say we've got other problems.
Corrections and comments welcome...