June 1970 | Technical Memorandum ERL TM 236

The effect of X-ray ionization from a high-altitude nuclear detonation on nearby VLF paths

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James H. Crary


High altitude nuclear detonations yield X-rays which produce intense ionization in about 1 μs at heights much lower than those of the normal D region. The ionization begins to decay immediately. This leads to a very complicated profile of ionization densities along a VLF path within the range of the X-rays, since the apparent profile of ionization along the path can change on a time scale less than the propagation time of the V LF wave from the transmitter to the receiver. This results in very complicated effects upon the received phase and amplitude of the VLF signal. Calculations of V LF phase and amplitude were performed, using simple assumptions, including a homogeneous slab model of the ionosphere whose height varies according to the location and the time after the detonations. The calculation did not compare well with the experimental record up to about 10 ms, or during the time in which the effect of the EM pulse is present and the effect of the X-rays is propagating along the path. The calculations do, however, give a reasonably good indication of the behavior of the phase of the signal during the long recovery from the detonation effect.

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