Influences and variations of wave impedance of electromagnetic fields generated by lightning return strokes

The domains of lightning electromagnetic pulse (LEMP) fields are classified as "near-field," "intermediate-field," and "far-field" based on qualitative descriptions. However, the systematic quantitative classification of LEMP fields is yet to be reported. This study characterized the wave impedance of lightning electromagnetic pulse (WILEMP) to solve this problem. The components and field domain distribution ranges of LEMP were analyzed by varying the WILEMP and the distance between the observation point and radiation source (r). The changes in the WILEMP with r under the influence of different factors were calculated using the finite-difference time-domain (FDTD) method. The results indicated that the LEMP field has high impedance. the WILEMP decreased rapidly with the increase in r when r <= 10 km, consistent with the characteristics of the electrostatic and induction fields (near field). The WILEMP gradually decreased and stabilized with the increase in r when r >= 20 km, and the value was roughly the same as the impedance of free space (377 Omega) when r = 100 km, consistent with the characteristics of radiation field (far field). The decay rate of the WILEMP was lower than that when r <= 10 km but higher than that when r >= 20 km. Finally, when 10 km < r < 20 km, the apparent transitivity was consistent with intermediate-field characteristics.