The large quantities of ionizing radiation produced by a high-altitude, high-yield nuclear detonation can severely change the environment of the upper atmosphere, producing heavily ionized regions which can disrupt electromagnetic waves passing through those zones. These disturbed regions can easily be the size of North America and can persist for tens of hours. The trapping mechanism for these high-energy electrons may be similar to that which produces the Van Allen radiation belts.
The actual degree of communications interruption is dependent upon the scenario and includes weapon yield and HOB, time of day, cloud cover, latitude and longitude of the burst, the specific communications path, and the time after the detonation. Other systems which may be affected by nuclear weapons effects on electromagnetic wave propagation include sensors in the IR, visible, and UV regions, and laser communications which may be affected by the background IR. A very hot (but transparent) region of the atmosphere can act as a lens to refract a laser communications beam off of its intended receiver.
Radar beams are both attenuated and refracted when passing through a nuclear fireball at altitudes below 25 km. At these altitudes the mean free path is small, and it is reasonable to speak of the fireball as being in local thermal equilibrium. Under these circumstances it is difficult to track incoming reentry vehicles (RV). Optical systems will suffer increased noise levels both because of ionized regions and from blackbody radiation from the fireball, and long-wave infrared (LWIR) systems may be unable to see through the fireball to an RV in the distance and may not be able to see an RV nearer to the sensor than the fireball because of the background.
No high-altitude nuclear tests have been carried out by the United States since the ratification of the 1963 Limited Test Ban Treaty (LTBT). Apparently, few IR data were obtained from the CHECKMATE, KINGFISH, ORANGE, and STARFISH high-altitude tests, so the visual information from those tests has been extrapolated to the IR regime. The main sources of high-altitude IR which would produce clutter include plasma emission, molecular and atomic emission from excited states, and emission from uranium oxide. All of these are functions of electron density.
At frequencies above about 300 MHz (UHF, SHF, and EHF), signals may be disrupted by scintillation, primarily characterized by intermittent fading and multipath transmission. These effects may persist for long periods and can degrade and distort a signal almost beyond recognition (for example, the plasma clouds are dispersive so that the speed of all frequencies of electromagnetic radiation are not equal in the cloud). Temporal and frequency coherence can both be destroyed.The vast majority of information relating to the propagation of electromagnetic radiation in a nuclear environment is pure science, primarily ionospheric and auroral physics including such phenomena as whistlers between northern and southern hemisphere locations. It requires no protection, but information on the mitigation of the effects may be classified because of considerations applicable to specific systems. All of the declared nuclear weapon states may have some capability to determine the effects of nuclear environments on electromagnetic signal propagation. All have access to and/or have contributed to the unclassified literature on RF propagation through structured media. The United States and the UK have provided models for calculating line-of-sight communications effects; the status of similar models in the other three nations is unknown.