NEAR-GROUND TRANSIENT FIELD OF A HIGH-ALTITUDE ELECTROMAGNETIC PULSE (HEMP) CONSIDERING NONLINEAR AIR CONDUCTIVITY AND GROUND REFLECTION

Abstract

Transient field of a high-altitude electromagnetic pulse (HEMP) induced near ground is simulated, of which the ground reflection can not be neglected. The Jefimenko's equation is applied to compute the incident electric field near the ground, attributed to both the primary and the secondary currents in the source region. The field-dependent air conductivity in the source region is obtained by solving three nonlinear governing equations iteratively, and the reflected field is computed in the frequency domain. The γ rays generated by a high-altitude nuclear explosion strike air molecules in the upper atmosphere, producing Compton electrons which move almost in parallel to the γ rays and constitute a primary current (1, 2). The Compton electrons also ionize air molecules, creating secondary electrons which in turns modify the effective conductivity of the atmosphere (2, 3). The electromagnetic field radiated by the primary current, under the influence of the air conductivity, is conventionally called a high-altitude electromagnetic pulse (HEMP). The Lienard-Wiechert potentials were used to compute HEMPs radiated by the primary current (4), where only the synchrotron radiation was considered, while the radiation attributed to frictional losses (Bremsstrahlung radiation) was neglected. In (5), HEMP was computed by using the Jefimenko's equation as well as the CHAP's code (3). The characteristics of electric fields attributed to primary and secondary currents were also discussed. In (6), the electric field of early-time HEMP was simulated with different heights of burst (HOBs), explosive yields and observation locations. The effective conductivity in the source region is nonlinearly related to the in situ electric field, which can significantly affect the accuracy of simulated HEMP waveform near ground. In this work, we propose an iterative method to compute both the electric field and the air conductivity distributions in the source region. The incident electric field near ground is then computed by applying the Jefimenko's equation to both the primary and the secondary currents. When the near-ground transient field in the first millisecond is concerned, reflected field components are mingled with the incident field components radiated from the portion of source region away from the line-of-sight path. The electric field reflected by the ground is computed by transforming the incident field to the frequency domain and multiplying with the frequency-dependent reflection coefficient, then transforming back to the time domain. This paper is organized as follows: The theory to compute the primary current is briefly reviewed in Section 2, the method of solving for the air conductivity and the secondary current is presented in Section 3, the method to compute the reflected field is presented in Section 4, and simulation results are discussed in Section 5. Finally, some conclusions are drawn in Section 6.