In addition to other effects, a nuclear weapon detonated in or above the earth’s atmosphere or alternatively an E-Bomb (see below) can create an electromagnetic pulse (EMP), a high-density electrical field. EMP acts like a stroke of lightning but is stronger, faster and briefer. 

                      The gamma rays interact with the atoms in air molecules through a process called the Compton effect, wherein electrons are scattered at high energies, thus ionizing the atmosphere and generating a powerful electrical field. The strength of the EMP depends highly on the altitude at which it is released. At altitudes above 30,000m, it is the strongest. It is also significant at surface or low altitude bursts, but is not as effective between the two extremes.

EMP can seriously damage electronic devices connected to power sources or antennas. This include communication systems, computers, electrical appliances, and automobile or aircraft ignition systems. The damage could range from a minor interruption to actual burnout of components. Most electronic equipment within 1,000 miles of a high-altitude nuclear detonation could be affected. 

The electromagnetic radiation from a nuclear explosion caused by Compton-recoil electrons and photoelectrons from photons scattered in the materials of the nuclear device or in a surrounding medium. The resulting electric and magnetic fields may couple with electrical/electronic systems to produce damaging current and voltage surges. May also be caused by nonnuclear means.

A broadband, high-intensity, short-duration burst of electromagnetic energy. Note: In the case of a nuclear detonation, the electromagnetic pulse consists of a continuous frequency spectrum. Most of the energy is distributed throughout the lower frequencies between 3 Hz and 30 kHz.

The three components of nuclear EMP, as defined by the IEC, are called E1, E2 and E3.

The E1 pulse is the very fast component of nuclear EMP. The E1 component is a very brief but intense electromagnetic field that can quickly induce very high voltages in electrical conductors. The E1 component causes most of its damage by causing electrical breakdown voltages to be exceeded. E1 is the component that can destroy computers and communications equipment and is too fast for ordinary lightning protectors.

The E1 component is produced when gamma radiation from the nuclear detonation knocks electrons out of the atoms in the upper atmosphere. The electrons travel in a generally downward direction at relativistic speeds (more than 90 percent of the speed of light). This essentially produces a large pulse of electrical current vertically in the upper atmosphere over the entire affected area. This electrical current is acted upon by the Earth's magnetic field to produce a very large, but very brief, electromagnetic pulse over the affected area. For the E1 waveform, its electromagnetic field is very similar to the field generated close to an electrostatic discharge (ESD). The ESD field can reach ~10 kV/m at a distance of 10 cm from an arc. It also has a rise time of 0.7 ns and a pulse width of 30 ns. For conducted transients that are naturally observed, most electronic equipment is exposed to the electrical fast transient (EFT) waveform that is generated in electrical substations and propagates to factories and homes through the power network. These EFT waveforms typically reach peak levels of ~4 kV and have a 5 ns rise time and a 50 ns decay time at the locations of electronic equipment.

The E2 component of the pulse has many similarities to the electromagnetic pulses produced by lightning. Because of the similarities to lightning-caused pulses and the widespread use of lightning protection technology, the E2 pulse is generally considered to be the easiest to protect against. To compare to the E2 waveform, the electromagnetic field produced by a lightning ground return stroke is similar in waveshape and can reach levels of 100 kV/m very close (~50 meters) to the stroke, but these fields decrease rapidly with distance from the stroke. The pulse widths of these fields extend from 100 microseconds to as long as 1 millisecond for positive lightning strokes. The E2 fields from HEMP are much lower, but do not vary significantly with distance. It is possible that the E2 fields could be a problem for very long power or communication lines.

The E3 component of the pulse is a very slow pulse, lasting tens to hundreds of seconds, that is caused by the nuclear detonation heaving the Earth's magnetic field out of the way, followed by the restoration of the magnetic field to its natural place. The E3 component has similarities to a geomagnetic storm caused by a very severe solar flare. Like a geomagnetic storm, E3 can produce geomagnetically induced currents in long electrical conductors, which can then damage components such as power line transformers. To compare to the E3 HEMP waveform, the fields created by a geomagnetic (solar) storm last from a few to hundreds of seconds [10]. It is known that a large geomagnetic storm can produce electric fields on the order of 1 V/km, and levels such as these have caused a regional power grid blackout as experienced by the Hydro-Quebec Power Company on March 13, 1989.

Note: log-log axes used on this graph

(Source: EMP Environment (MIL-STD-464, "Electromagnet Environmental Effects Requirements For Systems").

MIL-STD-2169, a classified document, apparently provides detailed information about the EMP threat wave forms. For an unclassified version of the EMP threat wave form has been released, and it describes a 50 kV potential which develops in literally just nanoseconds.
 This is important because:
• 50 kV is a very high voltage, more than enough to zap sensitive unprotected electronic devices
• a few nanosecond rise time is so fast that most conventional surge suppressing technologies (aimed at much slower-building pulses, such as lightning), typically wouldn't have time to react

Height of burstApproximate effects radius
40 km
50 km
100 km
200 km
300 km
400 km
712 km
796 km
1,121 km
1,576 km
1,918 km
2,201 km