Nuclear blackout

Nuclear blackout, also known as fireball blackout or radar blackout, is an effect caused by explosions of nuclear weapons that disturbs radio communications and causes radar systems to be blacked out or heavily refracted so they can no longer be used for accurate tracking and guidance. Within the atmosphere, the effect is caused by the large volume of ionized air created by the energy of the explosion, while above the atmosphere it is due to the action of high-energy beta particles released from the decaying bomb debris. At high altitudes, the effect can spread over large areas, hundreds of kilometers. The effect slowly fades as the fireball dissipates.
The effect was known from the earliest days of nuclear testing when radar systems were used to track the nuclear mushroom clouds at very long distances. Its extended effects when exploded outside the atmosphere were first noticed in 1958 as part of the Hardtack and Argus nuclear tests, which caused widespread radio interference extending over thousands of kilometers. The effect was so disconcerting that both the Soviets and US broke the informal testing moratorium that had been in place since late 1958 to run series of tests to gather further information on the various high-altitude effects like blackout and electromagnetic pulse.
Blackout is a particular concern for anti-ballistic missile systems. By exploding a warhead in the upper atmosphere just beyond the range of defensive missiles, an attacker can blanket a wide area of the sky beyond which additional approaching warheads cannot be seen. When those warheads emerge from the blackout area there may not be enough time for the defensive system to develop tracking information and attack them. This was a serious concern for the LIM-49 Nike Zeus program of the late 1950s, and one of the reasons it was ultimately canceled. A key discovery revealed in testing was that the effect cleared more quickly for higher frequencies. Later missile defense designs used radars operating at higher frequencies in the UHF and microwave region to mitigate the effect.

Bomb effects

Within the atmosphere

When a nuclear bomb is exploded near ground level, the dense atmosphere interacts with many of the subatomic particles being released. This normally takes place within a short distance, on the order of meters. This energy heats the air, promptly ionizing it to incandescence and causing a roughly spherical fireball to form within microseconds.
Proceeding at a slower speed is the actual explosion, which creates a powerful shock wave moving outward. The energy released by the shock wave is enough to compression heat the air into incandescence, creating a second fireball. This second fireball continues to expand, passing the radiative one. As it expands, the amount of energy in the shock wave drops according to the inverse-square law, while additional energy is lost through direct radiation in the visible and ultraviolet spectrum. Eventually the shock wave loses so much energy that it no longer heats the air enough to cause it to glow. At that point, known as breakaway, the shock front becomes transparent, and the fireball stops growing.
The diameter of the fireball for a bomb exploded clear of the ground can be estimated using the formula:
kilometers
Where is the yield in megatons, and is the ratio of the sea level air density to the air density at altitude. So, a bomb exploded at a burst altitude around will expand to about. The ratio can be calculated over a wide range by assuming an exponential relationship:
where is the altitude of the burst in feet. So the same burst at will be at a pressure of about 0.1 atmospheres, resulting in a fireball on the order of in diameter, about twice the size of one near the ground. For a high altitude burst, say, the fireball will expand to about in diameter.

Outside the atmosphere

When the bomb is exploded outside the atmosphere, generally any altitude above about, the lack of interaction with the air changes the nature of the fireball formation. In this case, the various subatomic particles can travel arbitrary distances, and continue to outpace the expanding bomb debris. The lack of atmosphere also means that no shockwave forms, and it is only the glowing bomb debris themselves that forms the fireball. In these sorts of explosions, the fireball itself is not a significant radar issue, but the particles' interactions with the atmosphere below them causes a number of secondary effects that are just as effective at blocking radar as a fireball at low altitude.
For simple geometric reasons, about half of the particles released by the explosion will be traveling towards the Earth and interact with the upper layers of the atmosphere, while the other half travels upwards into space. The particles penetrate the atmosphere to a depth depending on their energy:

Particles	Energy	Altitude
fission debris		150 kilometers
X-rays	4 keV	80 kilometers
beta particles	1 MeV	60 kilometers
gamma rays	3 MeV	30 kilometers
neutrons	1 MeV	30 kilometers

Two of these effects are particularly notable. The first is due to the gammas, which arrive as a burst directly below the explosion and promptly ionize the air, causing a huge pulse of downward moving electrons. The neutrons, arriving slightly later and stretched out in time, cause similar effects but less intense and over a slightly longer time. These gammas and neutrons are the source of the nuclear electromagnetic pulse, or EMP, which can damage electronics that are not shielded from its effects.
The second important effect is caused by the high energy beta particles. These are constantly being created by the radioactive decay of the uranium tamper that surrounds the fusion core, so the magnitude of this effect is largely a function of the size of the bomb and its physical dispersal in space. Since betas are both lightweight and electrically charged, they follow the Earth's magnetic field. This returns upward moving betas back to the Earth, although perhaps not at the same location.
Unlike the gammas, which ionize only the atoms they strike, a rapidly moving beta induces enormous magnetic fields in atoms they pass nearby, causing them to ionize while slowing the beta down. Each beta can thus cause multiple ionizations, as well as being a free electron on its own. This causes a much larger but spread out current pulse of lower energy electrons released from these air molecules. Since the reaction takes place between 50 and 60 km, the result is a disk of ionized air about 10 km thick and several hundred kilometers across.
Additionally, betas that are traveling roughly parallel to the Earth's magnetic fields will be trapped and cause similar effects where the magnetic field intersects the atmosphere. At any given longitude there are two locations where this occurs, north and south of the equator, and the effect is maximized by exploding the bomb within one of these locations in order to create as strong a signal as possible at the magnetic conjugate area. Known as the Christofilos effect, this was the subject of serious research in the late 1950s, but the effect was less powerful than expected.

Blackout effects

When bound to atoms and molecules, quantum mechanics causes electrons to naturally assume a set of distinct energy levels. Some of these correspond to photons of different energies, including radio frequencies. A passing photon can cause the electron to move between two of the atom's energy levels, if the photon is close to the energy difference in those levels. In metals, the energy levels are so closely spaced that the electrons in them will respond to almost any radio frequency photon, which is why metals are excellent materials for use in antennas. The same is true for free electrons, but in this case, there are no inherent energy levels at all and the electrons will react to almost any photon.

In fireballs

Within a nuclear fireball, the air is ionized, consisting of a mixture of nuclei and free electrons. The latter so strongly refract radio waves that it forms a mirror-like surface when the electron density is above a critical value. As the fireball radiates away energy and cools, the ions and electrons re-form back into atoms and the effect slowly fades over a period of seconds or minutes. Even as it cools the cloud attenuates signals, perhaps to the point to make it useless for radar use.
Total reflection from the fireball occurs when the radio frequency is less than the plasma frequency:
Hz
where is the number of free electrons per cubic centimeter. For a 1 m wavelength signal this occurs when the density is 10⁹ free electrons per cubic centimeter. Even at very low densities the ionization will refract radio energy. Attenuation occurs via electron collisions with neutrals according to:
× decibels/km
where 8.686 is the conversion factor to a decibel power ratio from a neper amplitude ratio, is the speed of light in kilometers per second, is the plasma frequency as above, is the frequency of the radio signal, and is the electron-neutral collision frequency. The latter is a function of the density, and thus the altitude:
× Hz
where is the air density at the explosion altitude, and is the density at sea level. Since the fireball can expand to hundreds of kilometers at high altitude, this means that a typical attenuation of 1 dB per kilometer through a fireball at mid to high-altitudes which expands to 10 km will completely attenuate the signal, making tracking objects on the far side impossible.

Outside the atmosphere

The effects of the exoatmospheric beta release are more difficult to assess because much depends on the geometry of the burst. However, it is possible to determine the density of the fission products, and thus relationship between the size of the ionization disk and its strength, by considering the yield of products for an explosion of in megatons:
tons/unit area
where is the diameter of the disk for a given explosion.