NEMP, HEMP, SREMP Oh My! (money-saving education for preppers)

 "First you need to hit the barn, next you wait fifty to a hundred shakes.
And then the bomb blows up."

You can all stop worrying about nuclear (or simulated) EMP.  This article will explain why.

1. Understanding NEMP (Nuclear ElectroMagnetic Pulse)

 First of all, everybody gets EMP wrong. Movies get EMP wrong.  Writers get EMP wrong.  Preppers get it wrong, but it's hardly their fault, they're constantly being lied to.  "NASA Engineers" (who just happen to sell EMP protection snake oil) get EMP wrong. Your mother's hairdresser's uncle, who saw an EMP once, gets EMP wrong.  Unless your name (was) Dr. Carl Baum, you've probably been getting NEMP wrong.  I got it wrong for years.

 After this, you will not get EMP wrong and you may go about educating others or just feeling smug about it, as is your wont.

 'EMP' is an acronym for ElectroMagnetic Pulse, which is an intense burst of radio waves created by nuclear explosions.

 The high altitude version has enough range to be a problem; it can easily reach the ground two hundred miles below.  The strength of the signal is so great, it can damage electronics and induce damaging currents - much like the solar wind - into power or telephone lines.  EMP is not harmful to life... unless you wear an external electronic medical device.  One EMP pulse occurs per nuclear detonation.

 There are two flavors of 'real'* (as in, generated by a nuclear detonation) - EMP:

  HEMP (High altitude Electromagnetic Pulse)

 This is the big powerful EMP effect everyone thinks of when they think of EMP, for example when a ground burst happens and everyone's car stops working in The Day After (ludicrous scene - does not happen in real life; read on for why). 

 HEMP occurs when a very high-yield missile warhead - roughly a megaton or higher - is detonated above two hundred miles altitude.  The enormous burst of gamma radiation, and to a lesser (and later) extent neutrons, from the nuclear detonation, violently changes the direction of travel of electrons. This motion, combined with the Earth's magnetic field also interacting with them as they move, causes the electrons to dump energy as radio waves across a big chunk of the electromagnetic (radio) spectrum.
For more on this, please see Section 2 of this DTRA-sponsored report.

This diagram is for a 10MT burst.  There are no 10MT weapons in anyone's inventory.
Important: the effects on the ground are negligible if yield is much below 1MT

 That's the majority of HEMP.  Long after the main pulse, two smaller pulses occur, with completely different mechanisms of creation: first, the heat and energy from the bomb physically striking the ionosphere, causes a small pulse, and after that, the swelling of the ionosphere from being heated rapidly, causes another little bump.  These are called the MHD signal because 'magnetohydrodynamic signal' is too much of a mouthful to say at cocktail parties.

"types" is the wrong word - "parts" would have been better... but it's a nice chart

  But those two signals weren't the part everyone was worried about, as they did not contain enough energy to affect electronics.  They were only of interest to researchers with sensitive equipment. All of these multiple source effects cause the pulse to be stretched out in time, with a long tapering tail after the very fast-rising front edge of the pulse.  The chart above is idealized.

 Note that all HEMP measurement and characterizing ceased when atmospheric testing ended in the 1960s.  The phenomenon requires a lot of space to occur, it cannot be recreated in a lab or an "EMP bomb" (those have been studied, but physics got in the way and they were abandoned as ineffectual)

 The pulse has a funny shaped outline, due to the complex phenomena which create its various parts, but its bandwidth is known; HEMP has significant energy from DC to 4 GHz.

 SREMP (Source Region Electromagnetic Pulse)

 This effect is not known to most people, most preppers, most writers.  And that's fine, because you don't need to know about it.  Why don't you need to know about it?  Okay, so SREMP is a weaker EMP effect created by atmospheric detonations, be they airbursts as is most common, or ground bursts.  And, like HEMP, the effect is negligible with sub-megaton warheads.

 SREMP is lower intensity than HEMP, and it has a much shorter range: a maximum of about two miles.  Your average modern boosted fission weapon (with a fusion stage or not!) has a radius of total destruction at least that large.  If you aren't destroyed by the blast or heat, the SREMP won't reach you, and you can start worrying about fallout.  If you're close enough for SREMP to zorch your electronics, you and your electronics will get zorched by the fireball and mach stem.  Forget about SREMP.

 As for where SREMP come from...

Q: "If EMP requires low gas pressures or vacuum, how can it occur in atmosphere?"
A1:  Smart-assed answer (and wrong): There is a vacuum inside the physics package. :)
A2: The real reason is that jamming those gammas into air does create the same Compton shower
       effect, but the gammas can't penetrate the air very far, so the generated signal is quite small.


 2. Why HEMP Is Not A Danger Either

 Reason #1: HEMP requires two things to happen: altitude of a minimum of two hundred miles, and a very large yield warhead, of at least 1 MT or higher.  The intensity of the effect tapers off quickly with warheads below 750kT yield.

 Since both Russia and the USA dismantled their megaton-class missile warheads in the early 2000s, using the fissionables to make smaller, lighter, more efficient physics packages, the conditions for creating HEMP can no longer be met.  MT-class gravity bombs are obviously of no use here, but those are slowly leaving our arsenal too.

 Reason #2: HEMP was never thought of by anyone as a valid form of attack, because the effects on the ground turned out to be too unpredictable.  The areas of high intensity are crescent shaped, nested with areas of low-to-no intensity, and the direction this occurs from the burst is unpredictable - the effects are impossible to aim, and the effects do not blanket large areas.  To effectively blanket either the USA (don't forget Alaska) or Russia (don't forget northern Siberia) you'd need to throw away a dozen or more MT-class warheads (!!) ...on a long shot.  No general was ever going to waste a single MT class weapon on a lousy bet like that.

3. Conclusion

 It appears extremely unlikely, absent the failure of multiple baroswitches and inertial sensors all at once, that we will ever see another ground-affecting EMP event.  Other nuclear weapon wonks agree with me.

 Every single person, self-proclaimed "NASA engineer", and company selling EMP protection products are charlatans pushing snake oil.  Most of them wouldn't work, also.

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* an example of simulated EMP are pulsed magnetic fields created by "EMP Simulators", such as the famous Trestle at Kirtland AFB. (These do not reproduce the nuclear EMP waveform.  Instead, they typically create an approximate square wave pulse with risetimes and pulse widths corresponding to the NEMP bandwidth of 4 GHz.  This is typically accomplished by firing a Marx generator (a device which makes very short, very high voltage pulses) into an antenna structure, typically something called a TEM cell, which just refers to its shape and how the EM wavefront travels through it.

 Note that the military largely stopped performing EMP susceptibility testing in the 1980s.  If the military stopped worrying about EMP, shouldn't you?

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 I want y'all to know just how difficult this article was to write. Not because of any PTSD thing from Missile Warning, but because most of it was written with a kitten on my lap. 😍💖