Abstract
To solve the non-proliferation problem of nuclear weapons it is necessary to develop methods to identify and monitor underground nuclear explosions remotely. At present the basic method used to monitor underground nuclear explosions remotely is seismic. Because of the potential for rogue nations to test nuclear weapons of smaller and smaller yields, and because of the existence of methods to decrease the seismic efficacy of explosions, it is necessary to further develop the seismic method as well as new alternative means of detecting and identifying underground nuclear explosions. Ionospheric radio sounding, which can detect the ionospheric effect of weak blast waves above the explosion location, is such a promising alternative. An underground explosion initiates a complex sequence of physical processes. The shock wave produced by the explosion passes through soil and causes vertical motion of the ground surface. An acoustic impulse produced by the ground surface motion propagates upward through the atmosphere, growing as it ascends (due to an atmospheric mass density that decreases with increasing altitude). At ionospheric heights (from 70 to 400 km), the acoustic impulse couples with free electrons (via collisional coupling with ions) to perturb the ambient electron density. The electron density perturbation can be detected by radar waves propagating through the disturbed ionosphere.