Even before the Trinity nuclear test in July of 1945, physicists predicted a transient electromagnetic signal would be caused by high-energy photons released from the detonation interacting with the air around the detonation. Predictions of these signals were difficult to make due to the complexity of the physics unleashed by the detonation.

Example timescales for high-altitude electromagnetic pulse (HEMP).

Post-World War II, there was a period of active atmospheric nuclear testing until 1962, during which measurements of these signals were made in order to understand the phenomena that were previously largely viewed as an annoyance and impediment to other instrumentation. The name “electromagnetic pulse” (EMP) began to be used to refer to these signals.

EMP is a complex nonlinear phenomenon. Initially, it was observed that signal strengths grew weaker with increasing burst heights, but this trend did not continue as burst heights went ever higher; rather, a new and faster signal was observed. This new signal was estimated ahead of the high-altitude tests of 1962 by William Karzas and Richard Latter; without an adequate theory for this high-altitude EMP (HEMP) phenomenon, however, experimenters and the instrumentation teams had a difficult time collecting the signal on scale. With the end of atmospheric testing, so ended the acquisition of HEMP experimental data.

HEMP waveform can be notionally decomposed into three major time-scales: early time (E1), intermediate time (E2), and late time (E3). These are depicted in the accompanying figure. The early time component, E1, is caused by the prompt, unscattered gamma rays emitted from the nuclear explosion. The intermediate time component, E2, is decomposed into two different parts for high-altitude detonations: the first part of E2, referred to as E2A, is a continuation of E1. The second part of E2, called E2B, is caused by high-energy neutrons interacting with the atmosphere. Historically, these have been separated due to the different physics models used to predict HEMP in these different time regimes.

The late-time component, E3, is also divided into two subcomponents. The first only appears from detonation altitudes above about 250 km. It is called E3 “blast” and is often labeled E3A. It is caused by the expanding debris from the detonation pushing against the earth’s geomagnetic field. The second subcomponent of E3 is called E3 “heave” and is often labeled E3B. It is caused by x-ray and kinetic energy from the detonation heating and ionizing the upper atmosphere. The heating causes the atmosphere to expand and begin rising buoyantly. The ionization, combined with the buoyant rise, attempts to pull the ions across geomagnetic field lines, creating a “heave.”

The physics of E1 and E2 are dominated by nuclear physics of the interaction of the radiation output of the exploding nuclear weapon with the atmosphere. E3 is dominated by the magnetohydrodynamics (MHD) of energetic plasmas interacting with the earth’s geomagnetic field.

The theory of high-altitude, fast electromagnetic pulse signal starts from the idea that an electromagnetic current from a nuclear detonation is produced due to the mostly-radial outward movement of recoil electrons from Compton scattering. In Compton scattering, a gamma-ray from the nuclear detonation “collides” with an electron. This interaction causes the gamma-ray to transfer energy to the electron and moves the electron in a different direction. The outward-moving Compton recoil electrons are also turned as they cross the geomagnetic field lines. The net motion of the electrons is the outward motion (radial) from the detonation plus transverse components from turning in the geomagnetic field. As they traverse through air, these electrons continue to interact with the air, depositing energy into the air or ionizing air molecules, creating conductivity. The amplitude and waveform shape of the electromagnetic pulse are therefore the result of the competition between the creation of the electrical current, which generates the electromagnetic fields, and the creation of conductivity, which dampens electromagnetic fields.

This work was done by Jonathan Morrow-Jones of Applied Research Associates for the Defense Threat Reduction Agency. DTRA-0010

This Brief includes a Technical Support Package (TSP).
Nominal High-Altitude Electromagnetic Pulse (HEMP) Waveforms

(reference DTRA-0010) is currently available for download from the TSP library.

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This article first appeared in the August, 2019 issue of Aerospace & Defense Technology Magazine.

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