EMP is the abbreviation for electromagnetic pulse. Another term that is sometimes used to refer to an EMP is transient electromagnetic disturbance. Both terms refer to electromagnetic energy that happens in a short, intense burst, resulting in disruption or damage to electrical equipment.
The energy that is transferred can take several forms: electric or magnetic field, electromagnetic radiation, or electrical conduction. Pulses can have different wave forms, from rectangular to exponential to sinewave. An EMP can be any of these, with the mildest a damped sinewave.
EMPs can happen naturally. Lightning strikes are a type of very localized EMP that can damage electronic equipment and can also damage other physical objects due to its magnitude. Because of the potential damage, many homes have surge protectors that aid in mitigating the possible damage from a lightning bolt. In the case of a severe thunderstorm, however, it is safer to unplug important electronics.
However, there are also man-made deliverers of electromagnetic pulses, including weapons and sources of nuclear energy. A hydrogen bomb, for instance, detonated at a high altitude, can cause an EMP that covers a large area of land below it. This can result in damage to phone lines, utility cables, and wireless antennas, which would cause a serious breakdown in communications.
In a disaster scenario, the expected EMP is much larger than the natural type of pulse. When discussing potential apocalypse-style scenarios, the EMPs that are considered to be the most likely are one resulting from a nuclear explosion or one resulting from a solar flare.
Nuclear Explosion
Nuclear explosions cause three types of pulses. One of these (called E1) is a microsecond pulse that is extremely intense, which spreads gamma rays, which collide with molecules of air and release electrons that rain down and affect smaller electronics – circuits, sensors, computers, communication systems like phones and intercoms, and protective systems like alarms. The second (E2) is slower and spreads neutrons which shoot in all directions at high speed that affects tower antennas, long conductive lines, and the antennas that trail some aircraft. The third (E3) is based on the size of the nuclear explosion, but is always the slowest. It can last up to days and will knock out power lines and underground and undersea cables. Because of the way these spread, it is generally pretty much safe directly under the blast, but since these blasts will knock out power and communications, even those people would be effectively cut off.
Tests done with these in the 1960s found that some of the pulses can travel up to 900 miles and cause some interruption in electrical and telecommunications equipment. Other tests on vehicles found that permanent damage was unlikely, though some electrical systems could be affected, requiring the vehicle to be restarted and limiting dash displays.
Ground-based nuclear explosions are more likely to result in physical damage, but it is unlikely to cause any widespread EMP damage.
However, it is still unknown if it is possible for a high altitude nuclear explosion to actually cause widespread disabling of a nation – or even a portion of a nation – for any length of time.
Solar Flare
A solar flare (or coronal mass ejection – CME) may not be generally considered an EMP, per se, but the effects of the high-intensity radiation that can burst from the sun can be very similar. Solar flares happen regularly, but they are rarely strong enough to cause noticeable damage. One exception to that happened in 1859, when a surprisingly strong flare cause telegraph disruption around the world, requiring repair or replacement to many telegraph devices.
EMP Weapons
High power microwave weapons (HPMWs) and e-bombs are possible ways for a person to cause EMP damage. Because this type of weapon is military-grade, there is little known about the actual products.
However, individual terrorists have been known to put together flux compression generator bombs, which are a sort of e-bomb created in the 1950s. Like concussion bombs, it consists of a cylinder of explosive and electrical components that are meant to create a magnetic field of high intensity. An explosion short-circuits the mechanism and generates an electromagnetic eruption. A weapon of this type has a much smaller area of effect than most.
Protection
The best protection for electrical equipment is a Faraday cage – which is a surround made from metal and grounded, so any electrical or electromagnetic pulses are redirected to the ground. A metal roof, if well grounded, can potentially protect an entire house. It is also possible to obtain individual packaging to protect items from EMPs.