Electromagnetic pulse

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The EMP Kill Medal given to a player in Halo 3: ODST and in Halo 3 when killing an enemy shortly after their shields are deactivated.

Electromagnetic pulse, or EMP is the electromagnetic radiation from an explosion (especially a nuclear explosion) or an intensely fluctuating magnetic field caused by Compton-recoil electrons and photoelectrons from photons scattered in the materials of the electronic or explosive device or in a surrounding medium. While organic tissue is unaffected, the resulting electric and magnetic fields may couple with electrical/electronic systems to produce damaging current and voltage surges. The effects are usually not noticeable beyond the blast radius unless the device is nuclear or specifically designed to produce an electromagnetic shockwave. An alternate definition is: a broadband, high-intensity, short-duration burst of electromagnetic energy.

The worst of an electromagnetic pulse generally only lasts for a second, but any unprotected electrical equipment will be affected by the pulse.

Notable examples

There have been several noteworthy instances in which an EMP has appeared.

The detonation of a nuclear weapon, such as a Shiva-class nuclear missile, is almost always followed by a massive electromagnetic pulse.
In the aftermath of Operation: UPPER CUT, both SPARTAN-B312 and the satellite Navigation Beacon RA-15 witnessed a purple-tinted EMP cloud emanate from the wreckage of the Covenant supercarrier Long Night of Solace after the improvised slipspace bomb was activated. The cloud disabled all electronics within its vicinity, including the NAV Beacon.
During the Battle of Installation 04, Cortana diverted John-117's energy shields into an EMP to disable Halo's phase pulse generators.^[1]
The deep-space artifact let out an enormous electromagnetic pulse that briefly canceled out all communications in the Sol system. It later let out another pulse upon deactivation.
During the Skirmish over Threshold, John-117 used the EMP emitted by a Plasma Grenade to disable the electronics of a locked door on Ascendant Justice, which allowed him to open the door.
The in-atmosphere jumping of the Solemn Penance to slipspace above New Mombasa created a massive electromagnetic pulse that damaged electrical systems in the city, and fried the navigational and safety systems of nearly five dozen SOEIV pods unfortunate enough to drop into the city at that exact moment, plunging them and the Orbital Drop Shock Troopers inside free-falling to near-certain death.^[2]
The UNSC Port Stanley was equipped with an EMP cannon, for disabling ships not hardened against it, which it used against the Hudal-class auxiliary vessel Piety.^[3]
While trying to stop the Didact, Cortana again modified the Master Chief's shields to give off an EMP to disable a carrier wave generator. She referred to it as "taking a page out of our old playbook."

Gameplay

EMP is also an in-game effect in the Halo games, that disables energy shields and vehicles. It is produced by certain weapons and equipment.

In Halo 2, the Sentinels sometimes emit an EMP when destroyed. This effect disables shields, but vehicles are unaffected.
In Halo 3, the power drain uses EMP to disable shields, vehicles and other power sources.
A charged Plasma Pistol blast will temporarily disable vehicles and energy shields much like the power drain.
In Halo Wars, Professor Anders' vehicle, the Gremlin, uses EMP as main armament to disable enemy non-infantry units.
In Halo Wars, the Vampire's stasis cannon uses EMP as well to drain power from enemy aircraft.
In Halo: Reach, Armor Lock can trigger an EMP after use, which can take out a Spartan's shields. Combined with a quick and accurate melee, this can result in an almost instant kill.
In Halo: Reach the M319 Individual Grenade Launcher has a secondary function that acts like an EMP. It awards a player an EMP Blast Medal when it kills an enemy, being the only other weapon in the game that awards the medal.

Trivia

If you disable a vehicle using an EMP, it will show an animation of some sparks flying around, but if you disable it again before it is drivable then the animation will stop and the vehicle will not work.
Some vehicles disabled by an EMP will still have full functionality of its weapons if it has any. For example, if a Warthog is disabled by an EMP, the turret will still be able to turn around and shoot. Likewise, a Banshee, while grounded, will still be able to boost and perform tricks.

Gallery

An EMP from a Plasma Pistol overcharge hits a Spartan III in the Halo: Reach Beta.
Another devastating EMP charge from the Plasma Pistol in the Beta.
An EMP wave from the Armor Lock ability can disrupt nearby enemy shields.
The Plasma Pistol's AOE secondary charge function emits an EMP burst to deactivate a Covenant Wraith tank for a short period of time.
A Type-26 Banshee GSA when hit by the EMP effect of an overcharged Plasma Pistol in Halo: Reach.