A Simulation of the Nuclear High-Altitude Electromagnetic Pulse (HEMP) Produced by the X-Ray in the Ionosphere

Since the 1960s, scientists have been studying the nuclear high-altitude electromagnetic pulse (HEMP) produced by gamma-rays. However, the HEMP produced by X-rays in the ionosphere has rarely been studied by previous studies. In this study, we investigate this issue via a 1-D particle-in-cell/Monte Carlo (PIC/MC) simulation model. We find that the amplitude of the electrostatic component quickly increases to the peak, and then decreases gradually. The amplitude of the electromagnetic field component gradually increases and exceeds the amplitude of the electrostatic field. This phenomenon, which contradicts previous hypotheses, adds to our understanding of the X-ray-produced HEMP. The evolution of secondary electrons formed by photoelectrons through ionization is very intriguing: the "newly born" secondary electrons deviate from the drifting Maxwellian distribution; thus, they cannot be described with the "swarm theory" which is based on the assumption that electrons satisfy a drifting Maxwellian distribution. This result is consistent with the recent experiment. In addition, our research indicates that assuming that all photoelectrons go forward considerably increases the amplitude of electric fields, making secondary electrons take longer to reach equilibrium. This widely used assumption in previous studies of the gamma-ray-produced HEMP is invalid to study the X-ray-produced HEMP.