Development of the Time Dependence of the Nuclear (E1) HEMP Electric Field

The early time (E1) nuclear high altitude electromagnetic pulse electric field calculated with the code CHAP is compared with the field given by an integral solution of Maxwell's equations, also known as the Jefimenko equation, to aid our current understanding on the factors that affect the time dependence of the electromagnetic pulse (EMP). For a fair comparison the CHAP current density is used as a source in the Jefimenko equation. At first, the comparison is simplified by neglecting the conduction current and replacing the standard atmosphere with a constant density air slab. The simplicity of the resultant current density aids in determining the factors that affect the rise, peak and tail of the EMP electric field versus time. The explicit 3-D approach of the Jefimenko equation confirms the importance, as described in the high-frequency approximation, of sources off the line-of-sight and the time dependence of the time derivative of the current density in shaping the EMP electric field time dependence. This conclusion also holds when the conduction current and the standard atmosphere are properly accounted for. Comparison of the CHAP electric field with the Jefimenko electric field is also another way to validate the equations developed under the high-frequency/outgoing wave approximation.