SOME STUDIES ON LIGHTNING AND SPRITES

Abstract

Lightning discharge is one of the most dangerous and largest impulsive energy sources among other natural cataclysm within the Earth’s atmosphere. Its magnificent power has attracted the human beings since aeons. Huge amount of energy liberated from the lightning affects our atmosphere in different ways. The most interesting and spectacular structures of lightning have been discovered during last two decades. The first direct visual lightning effects in mesospheric region have been documented by scientists from University of Minnesota in July 6, 1989. Latter, the optical signatures of these events were named ‘red sprites’ by the researchers at the University of Alaska. Experimental and theoretical studies have shown that red sprites are associated to strong cloud-to-ground (CG) lightning specially positive CG lightning discharges and last from about 1 ms to several tens of ms. The altitudes of occurrence of red sprites range from 40 km to 90 km. Some other events associated to thunderstorms/lightning activity have also been observed which are known as ‘blue starters’, ‘blue jets’, ‘gigantic jets’ and ‘elves’. Blue starters initiate at the top of the thunderstorms and propagate up to 25 km of altitudes. Blue jets and gigantic jets also initiate at the top of the thunderstorms but they propagate to the higher altitudes, sometime up to the ionospheric levels and last for several hundreds of ms. Elves are dim, flattened, expanding glow around 400 km in diameter and lasts for typically 1 ms at ionospheric altitudes (90 km). Combined all these electrical activities, generated from thundercloud top to the ionosphere are known as ‘High Altitude Lightning Discharges’. Red sprites are the most frequent candidates among them. The present thesis reports some theoretical studies on tropospheric and high altitude lightning discharges in the light of previous experimental works. First we formulated the velocity and current expressions for red sprites and calculated its various electrical parameters. A comparative study of conventional return stroke-lateral corona currents associated to CG lightning and red sprites has been done. Further, we calculated the contribution of red sprites as compared to the return stroke-lateral corona system to the global phenomena known as Schuman resonances. At last, we calculated the energy loss in CG and high altitude lightning discharges. The whole work of the thesis is presented through in six Chapters which has been briefly described below iii Chapter-1 introduces the subject of the thesis and presents a brief review of the recent experimental and theoretical works in the area of tropospheric and upper atmospheric lightning discharges, specially red sprites. The thundercloud formation, mechanism of cloud-to-ground lightning, and various mechanisms of upper atmospheric discharges specially red sprites have been presented. The electrodynamic coupling between thunderstorms and upper atmosphere has also been presented. Recent experimental and theoretical works which may affect our atmosphere are reviewed. Chapter-2 presents the study of electrical parameters of red sprites like their velocity of propagation, current within their body, current moment, charge moment change, radiated electric field and energy loss in their channel. First, we formulated the velocity and current expressions for red sprites. The velocity and current expressions come out to be Gaussian and double exponential respectively. Using these, the calculations of various electrical parameters of red sprites have been done. Chapter-3 described the comparative study of CG lightning discharge and red sprite. In this chapter, we calculated the electric and magnetic fields due to both return stroke-lateral corona system associated with cloud-to-ground lightning discharge and red sprits at a point on ground surface in time as well as in frequency domains. In these calculations, ground and ionosphere are assumed to be perfectly conducting. In case of return stroke-lateral corona system, both electric and magnetic fields peak at around 5 μs. Similarly, in case of red sprites, the electric and magnetic fields in time domain peak at around 4 ms. Their peaks shift according to the parameters of current and velocity. The basic difference between the two atmospheric discharges is that the tropospheric CG lightning discharge is the source of very low frequency (VLF) whereas the red sprite is the great source of extremely low frequency (ELF) radiation. Our results have been compared with the experimental observations and found to be in conformity with them. Chapter-4 deals with the study of ELF wave propagation in the Earth-ionosphere waveguide. In this calculation, we took the curvature of Earth into account. The electric and magnetic fields due to both return stroke-lateral corona system and red sprites in the Schumann band (3-60 Hz) have been calculated. It has been found that the red sprite contributes strongly to the Schumann resonances. iv Chapter-5 is associated with the study of energy loss in both tropospheric CG lightning and upper atmospheric lightning discharges. The energy loss in the form of heat and radiation due to CG lightning has been estimated. It is found that the energy loss in the form of heat is much larger than the radiation losses. The upper atmospheric lightning generating electric field has also been calculated. This electric field deposits the heat energy in the body of upper atmospheric discharges. For that, an altitude profile of energy loss in the body of upper atmospheric lightning discharges has been obtained. It is found that the electric field deposits the maximum heat energy at lower altitudes as compared to the higher altitudes. This study shows that lower altitude discharges are much luminous as compared to the higher altitude discharges as reported by previous workers. The last Chapter-6 gives the summary of the entire work presented in the thesis as well as the concluding remarks, comments and suggestions.