"Electromagnetic pulses (EMPs) are intense pulses of electromagnetic energy resulting from solar-caused effects or man-made nuclear and pulse-power devices. Of these, nuclear EMP has the most ubiquitous effects because of the combination of its broadband nature and large area coverage. Nuclear EMP has the demonstrated potential to disrupt, damage, or destroy a wide variety of electrical and electronic equipment. The strength and area coverage of nuclear EMP environments depends on the warhead type and yield, and the altitude and latitude of the detonation. A nuclear device detonated at altitudes between 30 and 400 kilometers generates an EMP with amplitudes in the tens of kilovolts per meter with a radius of effects from hundreds to thousands of kilometers. This high-altitude EMP (also known as HEMP) effect couples to and can disable electrical and electronic systems in general, but poses the highest risks to long-line networks, including electric power and long-haul communications. Although an EMP is also generated by low altitude or surface bursts (referred to as source region EMP or SREMP), the affected area is localized compared to a HEMP. For this reason, this action plan focuses on larger-scale EMP events produced by high altitude detonations."
U.S. Deoartment of Energy: https://energy.gov/