A nuclear weapon detonated at high altitude releases some of its energy in the form of gamma rays. These gamma rays collide with air molecules and produce what are called Compton electrons. The Compton electrons, in turn, interact with the earth’s magnetic field, producing an intense electromagnetic pulse that propagates downward to the earth’s surface. The initial gamma rays and resultant EMP move with the speed of light. The effects encompass an area along the line of sight from the detonation to the earth’s horizon. Any system within view of the detonation will experience some level of EMP. For example, if a high-yield weapon were to be detonated 400 kilometers (250 miles) above the United States, nearly the entire contiguous 48 states would be within the line-of-sight. The frequency range of the pulse is enormously wide — from below one hertz to one gigahertz. Peak electric fields can reach tens of thousands of volts per meter. All types of modern electronics are potentially at risk, from Boston to Los Angeles; from Chicago to New Orleans.
One of our earliest experiences with HEMP dates back to the resumption of atmospheric nuclear testing in 1962 following a three year testing moratorium. Starfish Prime, a 1.4 megaton device, was detonated at an altitude of 400 kilometers over Johnston Island. Failures of electronic systems resulted in Hawaii, 1,300 kilometers away from the detonation. Street lights and fuzes failed on Oahu and telephone service was disrupted on the island of Kauai. Subsequent tests with lower yield devices produced electronic upsets on an instrumentation aircraft that was approximately 300 kilometers away from the detonations.
What this means is that a single well-placed nuclear detonation in the upper atmosphere could disrupt electrical service, electronic devices, pretty much our entire technological way of life for days or weeks.
Add to this, the sudden loss of unhardened satellites — navigational, meteorological, etc. — with the resulting risks to safety and commerce, and this looks like a serious threat.
When various news sources reported that North Korea had a missile program capable of reaching the west coast of the US, the threat was interpreted as being to the cities. But while destroying Los Angeles or Seattle would be a conventional tactic, that seems a remote possibility. Consider the idea of an initial HEMP blast that disrupts communications and negates a retaliatory strike or at the best limits its effectiveness while completely disrupting rescue and humanitarian efforts in the target zones.
Given that North Korea is rumored to only have one or two plutonium weapons, a HEMP blast gives them the best “bang for the buck.”
Let’s hope that while we settle old scores with Iraq, someone is keeping an eye on the other members of the “Axis of Evil.”