On the EMP of High Altitude Nuclear Explosions

A high altitude nuclear explosion (height 20 - 30 km) creates an intense electromagnetic signal in a wide area on the earth's surface. The high frequency portion of this signal (W. Karzas and R. Latter, 1965) consists of a pulse with a rise time near 10 nsec, an electric field amplitude near 30 kV/m and with a duration near 100 nsec. The area of impact has a surface of the order of 106 km2. Since the amplitude of the low frequency part (10 kHz-range) is believed to be down by at least one or two orders of magnitude, only the high frequency pulse generation will be discussed. In addition to the basic mechanism for pulse generation, (i.e. coherent radiation from relativistic electrons, produced by gamma rays and gyrating about the geomagnetic field) its saturation will also be investigated. The two main saturation mechanisms are absorption of the radiated signal by slow secondary elecrons produced by the fast electrons and interaction of the fast electrons with the field they create. Some numerical results for different bomb models are also presented.