Letter Re: More About Understanding EMP and HEMP

Your discussion about EMP effects from ground blast or a low altitude nuclear explosion [posted on April 23rd] was excellent. Apart from electromagnetic coupling to conductors, which would extend the destructive horizon, atmospheric nuclear explosion EMP effects are limited in range. This is due to several factors, first by the rapid absorption of gamma rays by molecules in the atmosphere (small absorption layer or boundary effect), and second by the line of sight radiation from the decay of the short lived Compton electrons (limited horizon effect). You correctly discount the likelihood of a high altitude EMP (HEMP) as an unlikely terrorist tactic since it would effectively require an intercontinental ballistic missile to position from outside the continental US (CONUS) or a clandestine ballistic missile launch from North America . There exist terrorist states (and likely terrorist organizations) with the capability to vertically launch a missile to the effective HEMP altitude of 300-450 km.

A little discussed effect of HEMP is that the EMP effect is “mirrored” by the earth’s magnetic lines of force to the opposite hemisphere, known as the magnetic conjugate (note Fig. 3 in the link). Thus a HEMP attack on the CONUS would also produce EMP effects in the southern hemisphere. It would be possible for a terrorist organization or state to launch a relatively unsophisticated nuclear armed missile straight up from a ship positioned in the southeast Pacific Ocean off the coast of Chile . Properly positioned, this could theoretically blanket the entire CONUS with the “mirrored” EMP blast from the southern hemisphere.

Given your expertise and experience in military communications, do you know how efficient the magnetic conjugate coupling is, and whether this is a tactic we may need to consider? – NC Bluedog

JWR Replies: The Beyond Line of Sight (BLOS) coupling factor is a great unknown, since there have beem no US atmospheric nuclear tests since the early 1960s. There is therefore a dearth of useful data. My advice is to just assume the worst case, and plan/design your countermeasures accordingly.