SOME EXPERIMENTAL STUDIES ON LIGHTNING AND VLF ATMOSPHERICS

Abstract

Lightning is a transient natural electrical phenomena taking place mainly between, the lower charge centre of a cloud, to ground, known as cloud to ground (CG) discharge,; between the two opposite charge centres of a same cloud known as intracloud discharge or between the two opposite charge centre of two different clouds, anpwn as intercloud discharge. The later two types of discharges can not be distinguished easily and are commonly known as intracloud (IC) discharge. A lightning discharge emits electromagnetic radiation in the frequency range from a few hertz to beyond the visible region, produces thunder and the electrostatic field change on the ground. The scientific investigations on the discharge processes started with photography of the visible portion and later the measurements of electrostatic field changes, thunder and radiation fields followed. As the intracloud discharges and the cloud portions of ground discharges remain mostly hidden inside the huge volumes of the cloud, visual and photographic observations of these processes are very rare. The main point of dispute about an intracloud discharge is the initial streamer, whether an intracloud discharge is initiated by a positive streamer moving downward or by a negative streamer moving upward. Further, due to the intense electric field and turbulence inside the cloud, in-situ observations do not give fruitful results and the study is restricted to the measurements of thunder, electrostatic and radiation fields only. The very low frequency impulsive part of the electromagnetic radiation emitted by the lightning discharges is known as atmospherics. Due. to the good transmission characteristics of the terrestrial waveguide between earth and ionosphere for VLF waves, the atmospherics propagate to great distances before its electromagnetic field decays below the noise level. As the properties of the earth surface do not change appreciably with time, the propagation characteristics of the atmospherics is mainly dependent on the ionospheric properties. Moreover, since the atmospherics are always present in the earth ionosphere cavity (about 100/sec over the entire globe), these can be used to survey the thunderstorm/atmospherics activity and to study the propagation characteristics for VLF waves, in the earth-ionosphere waveguide. The studies in this thesis have been divided, into five chapters. e Chapter 1 reviews the present state of knowledge on lightning discharges and VLF atmospherics. The efforts made to-date to study the discharge processes and the results obtained have been briefly surveyed. The civil and ,commercial importance of lightning has also been briefly mentioned. The chapter has been concluded with a description of the benefits of lightning research. Chapter II deals with the description of the instruments used for the measurements of electro-static field changes, thunder and radiation fields. The instruments have been described in detail. The moving film camera designed for the present study has also been described. The procedure of recording data and analysis has been described in this chapter. The principles of measurement of various parameters associated with the discharges have been discussed with mathematical equations and geometry of the discharge charnel. The measurement errors and their effects on the conclusions drawn, have been mentioned at the end of the chapter. In Chapter III the lightning activity of eight overhead thunderstorms observed during the summer and winter of 1985 have been. analysed. The observations have been made for each thunderstorm from its initial stage of lightning activity to the final stage. The electrostatic and radiation fields for these storms were recorded continuously. From the data a complete CW) history of lightning from a thunderstorm has' been made. It has been found that the lightning activity in a thunderstorm is always initiated by the IC discharges, which last for 3-4 minutes and produce 6-12 discharges. The ground discharges mostly occur during the maximum activity and decaying stages of the thunderstorms. Two thunderstorms showed double activity peaks and IC discharge channel lengths in these storms were found to increase with time. The channel lengths of ground discharges remained almost constant.