Modeling of thundercloud VHF/UHF radiation on the lightning preliminary breakdown stage |
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| Institution: | 1. Atmospheric and Lightning Research Laboratory, Centre for Telecommunication Research and Innovation (CeTRI), Fakulti Kejuruteraan Elektronik dan Kejuruteraan Komputer, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, 76100 Durian Tunggal, Melaka, Malaysia;2. Institute of High Voltage and High Current (IVAT), Universiti Teknologi Malaysia (UTM), 81310 Skudai, Johor Bharu, Malaysia;3. Key Laboratory of Middle Atmosphere and Global Environment Observation, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China;4. Ångström Laboratory, Division for Electricity, Department of Engineering Sciences, Uppsala University, Box 534, S-75121, Sweden;5. Massachusetts Institute of Technology (MIT), Cambridge, MA 02139-4307, USA;6. Universiti Tenaga Nasional (UNITEN), Jalan IKRAM-UNITEN, 43000 Kajang, Selangor, Malaysia |
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| Abstract: | It is well known that there are two basic parts of electromagnetic emissions from a thunderstorm cloud. The first one is due to a return stroke and the second is generated by microdischarges on the lightning preliminary stage and between successive return strokes. The purpose of this paper is to consider the second part of electromagnetic emissions from thunderstorm clouds in a frequency range from one to hundreds of MHz. A new approach is developed, which is based on a three-dimensional computer simulation of microdischarge activity in thunderstorm clouds. We suggest that microdischarges on the lightning preliminary stage are connected with the growth of internal electric cell structures in a thunderstorm cloud. The characteristic scale of cells ranges from ten to hundred meters. The source of these cells can be a beam-plasma-like instability in the thunderstorm cloud medium where microdischarges appear as a saturation mechanism for this instability. Interaction of neighboring cells leads to the formation of dynamic chains of microdischarges. Following step-by-step computer simulations, we calculate radio emissions from every microdischarge and sum up the wave amplitudes from all intracloud volume at the reception point. The standard model for a separate microdischarge current is adopted, and the electromagnetic radiation is estimated in the far zone. We obtain the waveforms of electromagnetic field, the temporal development of radiation and the number of electromagnetic pulses. We have found that signal statistics and calculated frequency spectra exhibit a universal power-law (fractal) behavior. The results of simulations are found to be in satisfactory agreement with the experimental data, because the model waveforms demonstrate a close similarity to the observed ones. Also the temporal development with the duration of pulse trains from ten to hundreds of microseconds and the microdischarge number rate up to hundreds of thousands per second are in agreement with the corresponding experimental data. |
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