High-altitude Electro Magnetic Pulse Primer

High Altitude Nuclear Test Multimedia and Imagery

Photographic Essay APA-PE-2012-0102

During the late 1950s and early 1960s the United States conducted a series of high altitude nuclear tests to assess weapons effects in the upper atmosphere and near space. These experiments were the first to display nuclear High-altitude ElectroMagnetic Pulse (HEMP) effects.

These tests were conducted as part of the 1958 Hardtack I series of tests, and the 1962 Fishbowl series of tests. Weapon yi

High-altitude Electro Magnetic Pulse

There are three characteristic components to any HEMP event, these are labelled by the IEC as E1, E2 and E3 components in the IEC 61000-2-9 through -13 Basic Electromagnetic Compatibility (EMC) documents.

The initial E1 component is a fast and short high field strength pulse arising as a result of  gamma photons from the weapon ionising gas molecules in the uppermost layers of the atmosphere. The highly energetic relativistic ~1 - 2 MeV electrons interact with the earth's magnetic field, and produce an electric field. The E1 pulse duration is ~ 1 μsec, and yields ground level field strengths of up to 50 kiloVolts/metre for conventional boosted fission or fusion warheads.

The following E2 component is produced by the neutron flux generated by the warhead, and has a duration of up to a second. It is often compared in behaviour to a lightning induced electric field transient. While the E2 HEMP component is less damaging than the E1 component and in itself might be stopped by extant lightning protection mechanisms, wounding of protective hardware by the E1 component may permit the E2 component to do significant damage.

The final E3 component may have a duration of up to hundreds of seconds, and is often described in older literature as the MHD-EMP (Magneto-Hydro Dynamic EMP) effect. This slow pulse is a result of the earth's magnetic field recovering from the perturbation produced by the nuclear weapon effect. It is similar but more intensive than solar weather generated Geomagnetically Induced Current (GIC) effects, well known for damaging power grids and consumer equipment. The E3 component can often penetrate soils and reach buried cables, and if the cable is long enough, the induced voltages may destroy connected electrical devices. The magnitude of the E3 effect is mitigated by highly electrically conductive soils, and exacerbated by dry or otherwise highly resistive soils.

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