|Fig. 1: Peak Electric Field at Ground Zero (Volts/meter) as a function of prompt gamma putput (kilotons) from high altitude EMP tests.  (Source: Wikimedia Commons)|
An electromagnetic pulse (EMP) is a burst of electromagnetic energy created by the rapid acceleration of charged particles. There are a number of different types of electromagnetic pulses, but this report will primarily focus on nuclear electromagnetic pulses resulting from a high-altitude nuclear burst. Three main phenomena take place in the wake of a high-altitude nuclear burst. First, the interaction between gamma rays and atmospheric air molecules produces a prompt EMP field, also referred to as E1. This EMP peaks at tens of kilovolts per meter in a few nanoseconds and lasts for a few hundred nanoseconds. E1 has a broad-band power spectrum of frequency content between 10 to 100 megahertz, which allows it to couple with general electrical and electronic systems, without regard for the length of the systems penetrating cables or antenna lines. An E1 induces currents ranging into the 1000s of amperes and any exposed systems may be disturbed or permanently damaged. 
Second, delayed gamma rays and neutron-induced currents produce the second component of the EMP field, referred to as E2. E2 lasts from microseconds to milliseconds, has a magnitude in hundreds of volts per meter, and spectral characteristics similar to naturally occurring lightning. 
Third, the distortion of the earths magnetic field lines due to the expanding nuclear fireball and the rising of heated and ionized layers of the ionosphere, produce the third component of the EMP field, the late-time EMP, referred to as magnetohydrodynamic (MHD) EMP or E3. Changes to the magnetic field at the Earths surface induce currents of 100s-1000s of amperes in long conducting lines, of a few kilometers or greater, that impair components of the electricity grid and connected systems. The combination of these three phenomena could severely jeopardize the sustainability of society as we know if an EMP attack on the United States were successful. 
In a recent assessment on the risks associated with an EMP, the Electromagnetic Defense Task Force (EDTF) of the United States Air Force noted that that the occurrence of an EMP could have dire consequences for society as a whole and that, as of the report's release, the U.S. government has failed to mitigate the consequences of such an attack.  The U.S.'s concern over an EMP attack traces back to the Cold War, where the U.S. worried that the Soviet Union might look to knock out military communications and national command authority through an EMP attack generated by a high-altitude nuclear weapon, therefore eliminating the U.S. military's ability to respond to a nuclear attack. Though the U.S. military has taken steps to ensure that they would maintain national command authority and be able to operate after an attack, the U.S. government has failed to properly protect the electric grid and other critical infrastructure from the existential threat of an EMP attack. 
A successful EMP attack on the U.S. could lead to a nationwide blackout of the electric power grid and a shutdown of critical infrastructure reliant on the grid, including, but not limited to, communications, transportation, food and water supply, and sanitation. Such a shutdown could last as long as a year, and without such critical infrastructure, a large fraction of the America could die from starvation, disease, or the effects of general societal collapse. Furthermore, in a worst-case scenario, all nuclear reactors in the affected region could be impacted, leading to as many as 60 meltdowns similar to Japan's Fukushima Daichi Nuclear Disaster. Without off-site electricity, these reactors would be reliant on on-site systems to prevent a meltdown, which could also be compromised in the event of an EMP attack. Without both off-site and on-site power, the risk of radioactive contamination to the continental United State drastically increases, further compounding the damage done by the attack. 
Though the consequences of an EMP attack or a naturally occurring EMP event would be severe, there are a number of steps the United States can take to mitigate the risks associated with such an event. In its recent report, the Electromagnetic Defense Task Force (EDTF) of the United States Air Force provided recommendations on how to address vulnerabilities to the U.S. power grid and grid-dependent critical infrastructure. First, the task force recommends the immediate creation of a presidentially appointed executive agent to manage U.S. national infrastructure protection from the threat of an EMP. Furthermore, the EDTF advocate for the establishment of new standards to protect critical national infrastructure.  Finally, EDTF urges the government to take immediate action to implement cybersecurity for the electric grid and to prioritize the resilience of U.S. nuclear power, so that in the event of an EMP attack, the cooling systems of America's nuclear power plants operate unhindered, preventing catastrophic meltdowns. 
© Spencer Rogers. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 E. Savage, J. Gilbert, and W. Radasky, "The Early- Time (E1) High-Altitude Electromagnetic Pulse (HEMP) and Its Impact on the U.S. Power Grid," Metatech Corporation, Meta-R-320, January 2010.
 S. Reyes, "Our Enemies Could Use Nuclear Weapons to Create EMP Attack," The Hill, 25 Jan 2018.
 D. Stuckenberg et al., "Electromagnetic Defense Task Force 2018 Report," Air University, November 2018.
[s4] L. W. Seller, Jr., "A Calculational Model for High Altitude EMP," Air Force Institute of Technology, AD-A009-208, March 1975.