By Stuart Slade (imported from the old board).
When people in The Business start to discuss the effects of nuclear initiations with outsiders, sooner or later, the subject of Electro-Magnetic Pulse or EMP is raised. This is usually in the connection of some vastly powerful, unstoppable force that can devastate the electrical equipment of whole continents, leave radars blinded over vast areas and do other hideous things. While these are chilling descriptions and suggest dreadful possibilities, there is one small difficulty. EMP as it is described in such accounts simply does not exist. It is a creation of science fiction authors and politically-inspired pundits who seized upon a menacing-sounding term, took a few out-of-context comments and used them to build a chimera. We'll return to this imaginary "Scifi-EMP" later. In reality, the electronic effects of a nuclear initiation represent a complex web of interacting phenomena that have wildly different characteristics. Their effects are also widely differentiated and they exert those effects over durations that range from microseconds to years.
The existence of the electronic effects of nuclear initiations have been known since the 1940's when nuclear weapons were being developed and tested. At these early tests, a number of effects were noted including landline howls and interference with radio and radar transmissions. However, these effects were not considered to present any serious threat since they only reached dangerous proportions in areas where the more obvious effects of a nuclear initiation were such that there were unlikely to be any survivors to suffer from the electronic problems. As the testing moved to handle larger and larger devices, the problems caused by electronic effects actually diminished. They were only significant within a small radius of the point of initiation, and , as the size of the device initiated, the radius of the fireball grew faster than the radius over which the electronic effects were dangerous. While the fireball radius never actually overtook the electronic effects radius (the crossover point was estimated to be an initiation of 1,250 megatons), the obvious electronic effects of the initiation was not considered to be worth investigating further.
In retrospect, that was a mistake; something was going on and people should have looked at it a bit more closely. They didn't, not until 1962 when the US conducted a series of high-altitude atmospheric tests, code named "Fishbowl." The nuclear explosion, "Starfish Prime," which was detonated in the Pacific Ocean 800 miles from Hawaii, caused an effect that disrupted radio stations and electrical equipment throughout Hawaii. The disruption caused substantial alarm and consternation that prompted an investigation into the electronic effects of nuclear weapons. This quickly revealed that what had been dismissed as a few insignificant quirk occurrences were the visible manifestations of an entirely unexpected range of phenomena.
High-Altitude EMP (HEMP)
Electro-magnetic pulse is a broadband, high-intensity, short-duration burst of electromagnetic energy, most commonly resulting from a high-altitude nuclear initiation. The electromagnetic pulse consists of a continuous frequency spectrum from below one hertz to one gigahertz. with most of the energy distributed through the 3 Hz and 30 kHz bands. Peak electric fields can reach tens of thousands of volts per meter. These effects cover an area defined by the line of sight from the detonation to the earth's horizon. In other words, if the initiation can be seen from a given location, that location will experience some level of EMP. The area that can be effected by a HEMP pulse is enormous. For example, a suitable large nuclear device initiated 400 kilometers (250 miles) above Kansas will generate an electromagnetic pulse that will cover nearly the entire contiguous 48 states. The pulse itself is relatively harmless. The danger comes when it is picked up by metallic conductors such as wires or power cables. These act as antennas to conduct the energy shockwave into the electronic systems of cars, airplanes, and communications equipment. There, they can produce damaging currents and voltage surges.
So, what is this HEMP of which so many speak? HEMP is actually a child of a thing called the Compton Effect. Gamma radiation from a nuclear initiation isn't really that much of a problem; its absorbed by the atmosphere before it can do any harm to anything that hasn't already been comprehensively destroyed by the other effects of a nuclear device. However, once we start initiating devices at higher and higher altitudes, the atmosphere gets thinner and thinner and the distance traveled by the gamma radiation gets greater and greater. When we initiate a device in a vacuum, there is nothing to absorb the gamma radiation. It drops in intensity with distance due to the cube-law effect (for a sphere, surface area is proportional to the cube of the radius; thus doubling the radius form the point of initiation reduces the intensity of the gamma pulse by a factor of eight, tripling the radius by a factor of 27). Note the importance of atmospheric density here; if it's too great, the gamma gets absorbed before it can do anything. In real terms, the lower limit for a HEMP-generating initiation is 40 kilometers, any lower than that and HEMP just doesn't happen.
When we initiate a nuclear device, the formation of a HEMP begins with the very intense, but very short burst of gamma rays caused by the nuclear reactions within the device. About 0.3% of the device's yield is contained within the pulse which lasts for only 10 nanoseconds at most. This is a very important thing to note. Electromagnetic pulse is just that - a pulse. It lasts for 10 nanoseconds. It does not hang around for hours. It is, quite literally, in the same order of duration as a lightning flash.
So, the gamma rays generated by the initiation propagate outwards as a sphere from the point of initiation. The only ones of any interest are the ones that go downwards, towards the earth. Initially, the atmosphere is thin and the gamma rays penetrate it. As they do so, a thing called the Compton Recoil takes place. The gamma collides with electrons in air molecules, and eject the electrons at high energies by way of a process called Compton scattering. (Compton did pretty well, he has a lot of things named after him). These energetic electrons in turn knock other electrons loose, and create a cascade effect that produces some 30,000 electrons for every original gamma ray. Eventually, the air gets too thick for this to continue and it peters out. That happens at around 20 kilometers. So we now have a defined layer of atmosphere in which HEMP is generated 20 to 40 kilometers altitude. Please note this is quite independent of device initiation altitude or the size of the device. These electrons released in the upper atmosphere by the incident Gamma rays encounter the earths magnetic field. This causes them to spiral round the field, thus producing a huge line of current loops and creating a powerful downward directed electromagnetic pulse lasting a few microseconds.
So we have an EMP gneration defined. To occur, the device must be initiated above a critical altitude and the EMP is generated within the atmosphere below that device. The strength of the HEMP generated is directly proportional to the intensity of the gamma ray burst that kicked it off. Thus, the higher the initiation is above the critical atmosphere band that generates EMP, the weaker is the gamma ray pulse that hits the band. Eventually, if the device is high enough, the strength of the EMP pulse generated is inconsequential.
The pulse generated is pretty much a bell curve. This is very important; there is a period when the electrical surge induced in wiring and antennas by the HEMP is detectable but not dangerous. This can be used to detect the presence of a pulse coming and undertake counter-actions. Contrary to mythology, comprehensive protection against HEMP surges is technically very easy; it adds around five to ten percent of the cost of a piece of military electronics (proportionally the cost of protection drops as the cost of equipment rises). Pretty much all military electronics is EMP-protected. To put the HEMP surge into context, its at least an order of magnitude less powerful that that resulting from a lightning strike on a telephone line.
The simplest countermeasure of all is also the most effective. Switch the kit off. That's what normally happens. The surge is detected , the kit switches off, 10 nanoseconds later the surge has gone and the kit switches back on again. For, taking an example, a radar system, that 10 nanosecond gap is inconsequential; the operators wouldn't even notice it.
Adrian Carton de Wiart, VC wrote:
Frankly I had enjoyed the war...and why do people want peace if the war is so much fun?