An electromagnetic pulse (EMP), also called a transient electromagnetic disturbance (TED), is a short burst of electromagnetic energy. EMPs can come from natural or human-made sources and may appear as an electromagnetic field, a magnetic field, or an electric current. The interference from an EMP can stop communications and harm electronic devices. For example, an EMP caused by lightning can physically damage structures like buildings and airplanes. Managing the effects of EMPs is part of a field called electromagnetic compatibility (EMC) engineering.
The first known damage caused by an EMP happened during a solar storm in August 1859, called the Carrington Event.
In modern warfare, weapons that create high-energy EMPs are designed to stop communication devices, computers used to operate advanced airplanes, or even disable a country’s entire electrical system. Nuclear weapons can also generate nuclear electromagnetic pulses.
General characteristics
An electromagnetic pulse is a brief burst of electromagnetic energy. Because it lasts only a short time, its energy is spread across many different frequencies. Pulses are usually described by:
The frequency range and the shape of the pulse are connected through a mathematical method called the Fourier transform. This method explains how different wave patterns can combine to form the observed frequency range.
EMP energy can be transferred in four different ways:
According to Maxwell's equations, a pulse of electric energy is always paired with a pulse of magnetic energy. In most cases, either the electric or magnetic part is stronger. It can be shown that the complex Maxwell's equations can create solutions where electromagnetic waves form sudden changes in both electric and magnetic fields.
Generally, only radiation can travel long distances, while electric and magnetic fields act over short distances. There are some exceptions, such as a solar magnetic flare.
An electromagnetic pulse usually includes many frequencies, from very low to a higher limit, depending on the source. The range called EMP, sometimes called "DC to daylight," does not include the highest frequencies found in light (infrared, visible, ultraviolet) and ionizing radiation (X-rays and gamma rays).
Some EMP events may create visible effects, such as lightning or sparks, but these are caused by electricity moving through the air and are not part of the EMP itself.
The shape of a pulse shows how its strength (electric field or current) changes over time. Real pulses are often complex, so simpler models are used for study. These models are usually shown in diagrams or described with equations.
Most electromagnetic pulses start very quickly, reaching their strongest point rapidly. A common model is a double-exponential curve, which rises sharply, peaks, and then declines slowly. However, pulses from controlled circuits often resemble a rectangular or "square" shape.
EMP events usually create a matching signal in nearby materials or the environment. This connection is strongest at specific frequencies, forming a damped sine wave. This wave appears as a high-frequency sine wave that grows and fades within the longer-lasting envelope of the double-exponential curve. A damped sine wave usually has less energy and covers a narrower range of frequencies than the original pulse, due to how energy is transferred. In practice, EMP testing equipment often uses these damped sine waves instead of trying to recreate high-energy pulses.
In a pulse train, such as from a digital clock circuit, the same waveform repeats regularly. A single complete pulse is enough to describe such a regular, repeating pattern.
Types
An EMP occurs when a source releases a short burst of energy. This energy usually covers a wide range of frequencies, though it often causes a specific, narrow frequency pattern in the environment. Some EMPs happen as repeated, regular pulses.
EMP events can come from natural causes, human activities, or weapons.
Natural EMP events include:
– Lightning
– Other natural phenomena
Man-made EMP events include:
– Electrical equipment
– Human activities
Military EMP events include:
– Weapons designed to create EMPs
Lightning is unique because it often begins with a small, low-energy discharge before the main pulse. This main pulse may be followed by smaller bursts later.
ESD events involve very high voltages, sometimes thousands of volts, but the current is usually small. These events can cause visible sparks. ESD is typically a small, local occurrence, though lightning is a large ESD event. ESD can also be caused by human-made sources, such as shocks from a Van de Graaff generator.
ESD can harm electronic devices by sending a high-voltage pulse into them. It can also create sparks that might start fires or cause fuel-vapor explosions. For safety, fuel nozzles are connected to aircraft before refueling to discharge static electricity.
Switching electricity in a circuit causes a sudden change in current flow. This sudden change is a type of EMP.
Simple electrical sources, like relays, solenoids, and motor parts, can send pulses through wires and radiate energy. These pulses are usually weak and may be treated as "noise." When a circuit is turned off, the sudden stop in current can create a large pulse, causing sparks and damage. Design features are often added to reduce these effects.
Electronic devices like transistors and diodes can switch on and off quickly, creating similar issues. One-time pulses may come from occasional devices, but computers with millions of transistors can create continuous interference by switching rapidly.
A nuclear electromagnetic pulse (EMP) is a sudden burst of electromagnetic radiation from a nuclear explosion. This radiation creates rapidly changing electric and magnetic fields that can damage electronic systems by causing voltage and current surges.
The intense gamma radiation from a nuclear explosion can also ionize air, creating a secondary EMP as air atoms lose and regain electrons.
NEMP weapons are designed to maximize EMP effects as their main way of causing damage. These weapons can destroy electronic equipment over large areas.
A high-altitude EMP (HEMP) weapon is a type of NEMP designed to explode far above Earth. The gamma rays from the explosion ionize the air in the stratosphere, and the charged particles interact with Earth’s magnetic field to create a stronger EMP than at lower altitudes.
Non-nuclear EMP (NNEMP) is an EMP created without nuclear technology. Devices like large capacitors, microwave generators, or explosively pumped flux compression generators can produce NNEMP. Wave-shaping circuits or microwave generators are added to shape the pulse for better coupling with targets. Vircators, which are vacuum tubes, are well-suited for converting high-energy pulses into microwaves.
NNEMP generators can be carried by bombs, missiles, or drones. They have fewer mechanical, thermal, or radiation effects than nuclear weapons. However, NNEMP weapons have a shorter range than nuclear EMPs. They rely on chemical explosives for energy, producing only one-millionth the energy of nuclear weapons of similar weight. Unlike nuclear EMPs, which create EMPs as a side effect, NNEMP weapons generate EMPs directly from the device itself. This limits their range but allows for more precise targeting. Small NNEMP devices, like e-bombs, have been used in some military or terrorist operations to destroy electronic systems in vehicles and aircraft.
The idea of using explosively pumped flux compression generators to create non-nuclear EMPs was first proposed in 1951 by Andrei Sakharov in the Soviet Union. Work on non-nuclear EMPs remained classified until similar ideas were developed in other countries.
Effects
Small EMP events, including repeated short bursts of energy, can create small amounts of electrical noise or interference that may disrupt the operation of sensitive devices. For example, in the middle of the 20th century, electrical interference from the ignition systems of gasoline engines caused old radios to make crackling sounds and televisions to display horizontal lines on their screens. To address this, CISPR 25 was created to establish rules that vehicles must follow to limit the electromagnetic interference (EMI) they produce.
At very high voltage levels, an EMP can cause sparks, such as when static electricity builds up while fueling a gasoline-powered vehicle. These sparks can lead to dangerous fuel-air explosions, and steps must be taken to avoid them.
A large and strong EMP can produce high levels of electricity and voltage in affected devices, temporarily stopping them from working or causing permanent damage.
A very powerful EMP can also directly harm magnetic materials and erase data stored on magnetic media, such as magnetic tape or computer hard drives. Hard drives are often protected by thick metal cases. Some companies that recycle electronics use a controlled EMP to erase data from magnetic storage devices.
A very large EMP event, like a lightning strike or a nuclear weapon exploding in the air, can directly damage objects such as trees, buildings, and airplanes. This can happen because of the heat produced or the strong magnetic fields created by the electrical current. Indirectly, these events can cause electrical fires due to heat. Most buildings and systems are designed with some type of protection against lightning. A common method of protection is a Faraday shield, which helps prevent damage to certain items.
Control
Electromagnetic interference, including EMP threats, can be managed using control measures. These measures apply to both natural and human-made sources of EMP.
Most control methods aim to reduce how easily equipment is affected by EMP and to protect it from damage. Human-made sources, except for weapons, are also controlled to limit the amount of energy in the pulse they produce.
The study of ensuring that equipment functions correctly despite EMP and other radio frequency threats is called electromagnetic compatibility (EMC).
To test how EMP affects systems and equipment, an EMP simulator can be used.
Induced pulses have much less energy than actual threat pulses, making them easier to create but harder to predict. A common testing method involves using a current clamp in reverse to send a range of damped sine wave signals into a cable connected to the equipment being tested. These signals mimic the types of induced pulses that might occur.
Sometimes, the actual threat pulse is simulated in a controlled and repeatable way. This can be done at low energy to study how equipment responds before testing with damped sine waves, or at high energy to recreate real threat conditions. Small EMP simulators, like those used for electrostatic discharge, can be handheld. Larger simulators, such as those used for bench or room testing, vary in design based on the type and strength of the threat they are meant to replicate.
At the highest level, large outdoor test facilities with high-energy EMP simulators have been built by several countries. These facilities can test entire vehicles, such as ships and aircraft, for their ability to withstand EMP. Most of these large simulators use a special type of device called a Marx generator. For example, the ATLAS-I simulator (also known as TRESTLE) at Sandia National Labs in New Mexico was once the largest EMP simulator in the world. Information about these and other large EMP simulators used by the United States during the Cold War, along with general details about electromagnetic pulses, is now managed by the SUMMA Foundation, which is based at the University of New Mexico. The U.S. Navy also operates a large facility called the Electro Magnetic Pulse Radiation Environmental Simulator for Ships I (EMPRESS I).
Safety
Strong EMP signals can be dangerous to people. If someone touches a live electrical conductor, like a Van de Graaff generator or another charged object, they should avoid direct contact. If contact happens, the person should let go of the object and then discharge their body through something with high resistance to prevent a harmful shock when moving away.
Very strong electric fields can cause air to break down and create a dangerous electrical current, similar to lightning. However, electric fields up to 200 kV/m are considered safe for people.
A 2019 report by the Electric Power Research Institute, which is supported by utility companies, found that a large EMP attack might cause power outages in certain areas but not a complete failure of the national power grid. Recovery times would likely be similar to those of other major power disruptions. It is unclear how long these outages might last or how much damage could occur across the country. It is also possible that neighboring countries of the U.S. could be affected, depending on the location of the attack.
An article by Naureen Malik stated that due to North Korea's progress in missile and warhead testing, the U.S. Congress renewed funding for the Commission to Assess the Threat to the U.S. from Electromagnetic Pulse Attack as part of the National Defense Authorization Act.
A 2016 article by Yoshida Reiji from the Tokyo-based nonprofit Center for Information and Security Trade Control mentioned that Onizuka warned a high-altitude EMP attack could damage or destroy Japan’s power, communication, and transportation systems. It could also disable banks, hospitals, and nuclear power plants.
In popular culture
By 1981, many articles about electromagnetic pulse (EMP) in magazines and newspapers helped people learn about EMP. This led to EMP being used in many stories and other forms of media. However, movies, books, and other media often show EMP effects in the wrong way, which can cause confusion for people and experts. In the United States, official programs have been created to fix these misunderstandings.