An electromagnetic pulse (EMP), also sometimes called a transient electromagnetic disturbance, is a short burst of electromagnetic energy. Such a pulse’s origination may be a natural occurrence or man-made and can occur as a radiated, electric or magnetic field or a conducted electric current, depending on the source. The term “electromagnetic pulse” is commonly abbreviated EMP (which is pronounced by saying the letters separately, “E-M-P”).
EMP interference is generally disruptive or damaging to electronic equipment, and at higher energy levels a powerful EMP event such as a lightning strike can damage physical objects such as buildings and aircraft structures. The management of EMP effects is an important branch of electromagnetic compatibility (EMC) engineering.
The damaging effects of high-energy EMP have been used to create EMP weapons. These are typically divided into nuclear and non-nuclear devices. Such weapons, both real and fictional, have become known to the public by means of popular culture.
An electromagnetic pulse is a short burst of electromagnetic energy. Its short duration means that it will be spread over a range of frequencies. Pulses are typically characterised by:
- The type of energy (radiated, electric, magnetic or conducted).
- The range or spectrum of frequencies present.
- Pulse waveform: shape, duration and amplitude.
The last two of these, the frequency spectrum and the pulse waveform, are interrelated via the Fourier transform and may be seen as two different ways of describing the same pulse.
Types of energy
As with any electromagnetic signal, EMP energy may be transferred in any of four forms:
In general, only radiation acts over long distances, with the others acting only over short distances. There are a few exceptions, such as a solar magnetic flare.
A pulse of electromagnetic energy typically comprises many frequencies from DC (zero Hz) to some upper limit depending on the source. The range defined as EMP, sometimes referred to as “DC to daylight”, excludes the highest frequencies comprising the optical (infrared, visible, ultraviolet) and ionizing (X and gamma rays) ranges.
Some types of EMP events can leave an optical trail, such as lightning and sparks, but these are side effects of the current flow through the air and are not part of the EMP itself.
The waveform of a pulse describes how its instantaneous amplitude (field strength or current) changes over time. Real pulses tend to be quite complicated, so simplified models are often used. Such a model is typically shown either as a diagram or as a mathematical equation.
Double exponential pulse
Damped sinewave pulse
Most pulses have a very sharp leading edge, building up quickly to their maximum level. The classic model is a double-exponential curve which climbs steeply, quickly reaches a peak and then decays more slowly. However, pulses from a controlled switching circuit often take the form of a rectangular or “square” pulse.
In a pulse train, such as from a digital clock circuit, the waveform is repeated at regular intervals.
EMP events usually induce a corresponding signal in the victim equipment, due to coupling between the source and victim. Coupling usually occurs most strongly over a relatively narrow frequency band, leading to a characteristic damped sine wave signal in the victim. Visually it is shown as a high frequency sine wave growing and decaying within the longer-lived envelope of the double-exponential curve. A damped sinewave typically has much lower energy and a narrower frequency spread than the original pulse, due to the transfer characteristic of the coupling mode. In practice, EMP test equipment often injects these damped sinewaves directly rather than attempting to recreate the high-energy threat pulses.
Minor EMP events, and especially pulse trains, cause low levels of electrical noise or interference which can affect the operation of susceptible devices. For example, a common problem in the mid-twentieth century was interference emitted by the ignition systems of gasoline engines, which caused radio sets to crackle and TV sets to show stripes on the screen. Laws had to be introduced to make vehicle manufacturers fit interference suppressors.
At a high voltage level an EMP can induce a spark, for example from an electrostatic discharge when fuelling a gasoline-engined vehicle. Such sparks have been known to cause fuel-air explosions and precautions must be taken to prevent them.
A large and energetic EMP can induce high currents and voltages in the victim, damaging electrical equipment or disrupting its function.
A very large EMP event such as a lightning strike is also capable of damaging objects such as trees, buildings and aircraft directly, either through heating effects or the disruptive effects of the very large magnetic field generated by the current. An indirect effect can be electrical fires caused by heating. Most engineered structures and systems require some form of protection against lightning to be designed in.
The damaging effects of EMP have led to the introduction of EMP weapons, from tactical missiles with a small radius of effect to nuclear bombs tailored for maximum EMP effect over a wide area.
Types of EMP
An EMP arises where the source emits a short-duration pulse of energy. The energy is usually broadband by nature, although it often excites a relatively narrow-band damped sine wave response in the victim. Some types are generated as repetitive and regular pulse trains.
Different types of EMP arise from natural, man-made and weapons effects.
Types of natural EMP event include:
- Lightning electromagnetic pulse (LEMP). The discharge is typically an initial huge current flow, at least mega-amps, followed by a train of pulses of decreasing energy.
- Electrostatic discharge (ESD), as a result of two charged objects coming into close proximity or even contact.
- Meteoric EMP. The discharge of electromagnetic energy resulting from either the impact of a meteoroid with a spacecraft or the explosive breakup of a meteoroid passing through the Earth’s atmosphere.
- Coronal Mass Ejection (CME). A massive burst of gas and magnetic field arising from the solar corona and being released into the solar wind sometimes referred to as a Solar EMP.
Types of (civil) man-made EMP event include:
- Switching action of electrical circuitry, whether isolated or repetitive (as a pulse train).
- Electric motors can create a train of pulses as the internal electrical contacts make and break connections as the armature rotates.
- Gasoline engine ignition systems can create a train of pulses as the spark plugs are energized or fired.
- Continual switching actions of digital electronic circuitry.
- Power line surges. These can be up to several kilovolts, enough to damage electronic equipment that is insufficiently protected.
Types of military EMP include:
- Nuclear electromagnetic pulse (NEMP), as a result of a nuclear explosion. A variant of this is the high altitude nuclear EMP (HEMP), which produces a pulse of a much larger amplitude and different characteristics due to particle interactions with the Earth’s atmosphere and subsequently the Earth’s magnetic fields driving an oscillation in electric current after the original pulse from the particle and ray interactions on the atmosphere.
- Non-nuclear electromagnetic pulse (NNEMP) weapons.