Thread by @sovietologist

Folks are asking about the electromagnetic pulse (EMP) effects from the battlefield use of nuclear weapons. TL;DR-if you're close enough to a near-surface nuclear burst to worry about its EMP, you're probably going to be killed by something else--but there are exceptions 1/24

While every nuclear explosion produces an electromagnetic pulse of some kind, not every nuclear EMP is a militarily significant effect. 2/24

There's a lot of confusion surrounding EMP. One reason for this is that there are multiple different kinds of nuclear EMP, and these are different phenomena with distinct underlying mechanisms 3/24

The type of EMP that dominates in the public imagination is high-altitude EMP (HEMP). This is the one that can, in theory, damage electronics over huge areas 4/24

But high-altitude EMP is really several distinct phenomena that operate on different timescales ranging from a few nanoseconds to hundreds of seconds 5/24

Moreover, the physics behind high-altitude EMP depend upon the nuclear burst occurring at high altitude. It needs to be at a minimum of 30 kilometers above the surface and generally much higher for a strong effect 6/24 atomicarchive.com/resources/docu…

You *cannot* create a local version of the high-altitude EMP by lowering the burst height below 30 km! Players in wargames often want to do this, but down in a thicker atmosphere one gets a different, weaker EMP that doesn't cause strong effects on the surface 7/24

So the Russians couldn't, say, detonate a nuclear weapon at an altitude of 15 or 20km to create a powerful high-altitude EMP that impacted a well-defined area, like a single Ukrainian city. The underlying physics forbids it 8/24

At lower altitudes, the kind of nuclear EMP we need to worry about is source-region EMP (SREMP). 9/24

Unlike HEMP, which we don't understand completely because only a few high-altitude nuclear tests were ever conducted, we have lots of experience with SREMP from hundreds of near-surface atmospheric nuclear tests 10/24

In the 1950s SREMP (then sometimes called "radioflash") was regarded as an annoyance that interfered with test instrumentation rather than something the military had to worry about--hinting at its limited significance for battlefield nuclear use 11/24 en.wikipedia.org/wiki/Radioflash

SREMP stems from the gamma radiation from the nuclear explosion, which knocks electrons out of atoms in the surrounding air. Since those electrons travel away from the center of the burst, they produce an effective current 12/24

The area in which the gamma radiation gets absorbed and these electrons are produced is the "source region" that gives SREMP its name. Electric field strengths inside the source region are immense--much higher than one would expect on the surface from HEMP 13/24

But anything in the source region is getting cooked by the same gamma radiation driving the EMP effect, and will probably be destroyed by other effects such as blast 14/24

As a consequence, analysts during the Cold War mostly thought of SREMP as a concern for hardened facilities, such as ICBM silos, that were designed to survive the other effects of a nearby nuclear explosion 15/24

But SREMP can cause some effects outside the source region. If the burst occurs near the ground, the source region is asymmetrical, and this results in a radiated signal as well as powerful ground currents 16/24

The radiated signal from SREMP in a near-surface burst can be significant, but it falls off rapidly with distance from the source region, so it's still a localized effect 17/24

Another way that SREMP can cause damage outside of the source region is if something conducts the induced currents out--like wires or cables 18/24

Since the SREMP effect occurs very quickly, faster than the blastwave propagates, power lines or other conductors can convey the induced currents out of the source region before those conductors get destroyed by other effects of the nuclear blast 19/24

These currents could then damage equipment connected to those conductors at appreciable distances from the nuclear burst (10s of miles) 20/24

Even accounting for these possibilities, the likely consequences of SREMP in places that won't be devastated by other nuclear weapons effects are relatively modest--as this 2021 study from Sandia National Lab attests 21/24 osti.gov/servlets/purl/…

It finds that the range at which SREMP might "cause permanent failure to some electronics" is smaller than the radius devastated by blast for nuclear explosions of most yields. Hardly the apocalyptic effects sometimes envisioned for EMP 22/24

The exception is the smallest weapons (less than a kiloton). This is because of how radiation and blast effects scale differently with yield. Even so, the expected SREMP effects outside the blast radius are still pretty modest 23/24

To sum up, nuclear EMP effects are likely to be a secondary concern in the event of battlefield nuclear use compared to other effects such as blast and radiation. We can't dismiss EMP altogether, but we have good reason to expect it'll be one of our lesser problems. 24/24