Propagation of Lightning Electromagnetic Pulse (LEMP) through Ionosphere

摘要

The electrically conducting region of the upper atmosphere of earth's surface, the ionosphere, plays a major role in all sorts of radio communication. In 1901 when Marconi was carrying out the experiment, surprisingly the signal was found to be received on the area behind the Atlantic Ocean. The presence of signal could not be justified considering only ground wave propagation. Ground reflection and diffraction effect would be insufficient to permit such a long distance transmission and reception of signal around the earth's curvature. This finally led to the discovery of ionosphere which surrounds the earth. Later details study of ionosphere leads to the discovery of different layers (D, E, F1 and F2) and variation of characteristics from day to night and vice-versa. Lightning is considered to be the strongest natural source on the ground so far. Lightning discharges radiate the bulk of their electromagnetic energy in the very low frequency and extremely low frequency ranges. Radio wave in ELF and VLF are either penetrate through or reflected back from these ionized layers. If the lightning burst takes place not far from ionosphere it will modify the electron-ion density of ionosphere thereby modifying its characteristics.