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Distances. The study provides a theoretical foundation for further research on nuclear explosion detection and offers scientific support for formulating relevant protective measures. 2. Mechanism of GenerationA NEMP is a strong electromagnetic wave generated instantaneously during a nuclear explosion. The large amount of gamma rays released by the explosion interacts with the surrounding atmosphere, primarily involving the electron–positron pair production, photoelectric effect, and Compton scattering. The average energy of the photons (1.5 MeV) and the atomic numbers of the atmospheric constituents (such as nitrogen and oxygen) determine that the Compton effect is the primary mechanism exciting the NEMP, as illustrated in Figure 1. This paper mainly considers the physical process whereby radiation interacts with surrounding air molecules to produce Compton currents, thereby exciting electromagnetic pulses [20].When a nuclear weapon detonates in a low-altitude environment, the photons radiated downward are absorbed by the upper layers of the ground’s geological medium. In contrast, radiation emitted outward and upward ionizes the air, causing charge separation and resulting in the generation of a significant number of Compton electrons. These electrons move outward, creating a Compton current, which manifests as a net vertical flow of electrons directed upward. Consequently, the source region is excited and radiates electromagnetic energy outward. The electromagnetic pulse generated by low-altitude nuclear explosion is shown in Figure 2. 3. Introduction to Numerical Simulation MethodsA NEMP primarily originates from the Compton currents generated by the interaction of prompt gamma rays produced by explosions with air (or ground media). The ionization of air

Study on electromagnetic pulse of air nuclear explosion. In Proceedings of the 2005 China Association for Science and Technology Annual Conference, Urumqi, China, 20–22 August 2005. [Google Scholar] Figure 1. Comparison of the relative occurrence rates of three processes as a function of photon energy and atomic number the medium. Figure 1. Comparison of the relative occurrence rates of three processes as a function of photon energy and atomic number the medium. Figure 2. Schematic diagram of low–altitude NEMP generation. Figure 2. Schematic diagram of low–altitude NEMP generation. Figure 3. The geometric schematic diagram of the spherical coordinate system ( r , θ , ϕ ) and the rotating ellipsoid–hyperbolic orthogonal coordinate system ( ξ , ζ , ϕ ) at the field point P. Click here to enlarge figure --> Figure 3. The geometric schematic diagram of the spherical coordinate system ( r , θ , ϕ ) and the rotating ellipsoid–hyperbolic orthogonal coordinate system ( ξ , ζ , ϕ ) at the field point P. Figure 4. Comparison of results between the study and existing literature. Figure 4. Comparison of results between the study and existing literature. Figure 5. Distribution map of source area under specific conditions. Figure 5. Distribution map of source area under specific conditions. Figure 6. Schematic diagram of nuclear explosion points and observation points from different angles at the same distance. Figure 6. Schematic diagram of nuclear explosion points and observation points from different angles at the same distance. Figure 7. Comparison of

Results AnalysisThe gamma waveform used in the low-altitude NEMP was a double-pulse waveform, where the two pulses represented the primary and secondary nuclear reactions of the nuclear device, respectively. The time interval between these two pulses corresponded to the action time interval between the primary and secondary stages of a thermonuclear bomb. The two pulses reached their peak values at 0.1 μ s and 1.6 μ s , with a peak ratio of 33.5:1. 4.1. Algorithm VerificationTo verify the correctness of the algorithm presented in this paper, we set the input parameters, such as ground parameters, explosion height, and explosive yield, to be consistent with those in the literature [21]. We selected an explosive yield of 100 kilotons and an explosion height of 10 km, and simulated E θ time-domain waveforms at the monitoring point r = 11.3 km, θ = 49.9 ° . The results were then compared with those from the literature.As shown in Figure 4, the scatter points and the line represent the computed results of this study and those from the literature, respectively. It can be observed that the results of both are in good agreement.Using single-thread calculation, the time step consisted of 7200 steps, and a single calculation took 208 min. However, when OpenMP parallel computing was employed with 6 threads, only requiring 45 min, the acceleration ratio reached 4.62 times, significantly improving computational efficiency. 4.2. Coverage Area of the Source RegionA low-altitude NEMP source region formed by gamma radiation is a function of the

1. IntroductionNuclear explosions produce not only shockwaves, thermal radiation, and radioactive contaminations but also a significant destructive effect known as the nuclear electromagnetic pulse (NEMP) [1,2]. A NEMP is a strong electromagnetic wave released at the moment of a nuclear explosion, posing a severe threat to various electronic devices and systems [3,4,5,6]. To fully understand the information contained within NEMPs, it is essential to have a clear understanding of the source region. Compared to high-altitude nuclear explosions, due to the relatively high air density in a low-altitude environment, the average free path of gamma rays is shorter, so low-altitude nuclear detonations have a smaller source region; however, the energy is concentrated in a smaller area, resulting in greater destructive power in nearby regions. Therefore, in-depth research on the electromagnetic field distribution characteristics of NEMPs from low-altitude nuclear explosions is crucial in enhancing nuclear explosion detection technology and improving systems’ resistance to nuclear radiation.Gilinsky Victor et al. [7] proposed a dipole model for low-altitude NEMPs. Longley H Jerry et al. [8] employed a two-dimensional time-domain finite-difference method based on a spherical–cylindrical coordinate system to calculate the source region fields of near-surface nuclear explosions. The Northwest Nuclear Technology Research Institute was the first to develop a two-dimensional differential calculation program for simulating low-altitude NEMPs [9]. Wang Taichun et al. [10] derived approximate analytical formulas for the current and electric field generated by instantaneous gamma photons in the source region under the conditions of a low-altitude nuclear explosion. Liang Rui et al. [11]

Explosion center is characterized by low impedance, with magnetic induction intensity B ϕ being significantly stronger than E θ ; conversely, the source region above the explosion center exhibits characteristics of a high impedance field, with magnetic induction intensity B ϕ being much weaker than E θ . Overall, the strong electromagnetic field in the source region poses a considerable threat to electronic systems operating within that area. 4.3.2. Comparison of Time-Domain Waveforms of TE WaveThe TE wave field components were B r , B θ , E φ . Since the B r field does not radiate outward, this paper focused on E φ and B θ , and the time-domain waveform comparisons at four observation points are shown in Figure 8.From Figure 8, it can be observed that: (1)The pulse width of TE wave is narrower compared to TM wave. This is attributed to the fact that J ϕ ′ serves as the primary current source for exciting TE wave (as derived from Equations (4) and (7), which indicate that TM wave are excited by J ϕ ′ ). Since gamma rays propagate forward at the speed of light c, the resulting Compton electrons also travel slightly below the speed of light. From the monitoring point’s perspective, the contributions of the current sources along the line connecting the explosion center to the monitoring point to the NEMP are nearly simultaneous, leading to a narrower pulse width for TE wave.(2)In the TE wave, E ϕ and B θ opposite