High-altitude electromagnetic pulse

Definition
A High-altitude electromagnetic pulse (HEMP) event is caused by the detonation of a nuclear device above the atmosphere, from about 40 to 400 kilometers (approximately 25 to 250 miles) above the earth's surface.

Overview
A HEMP attack is not intended to cause direct physical impacts at the earth's surface, such as injury or damage directly from heat, blast, or radiation, but instead creates an intense electromagnetic pulse that can disrupt computers and damage electronics and insulators, and could cause significant damage to critical electrical infrastructure, such as transformers.