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In this thesis, we theoretically predict the normal characteristics of VeryLow Frequency (3~30 kHz) radio wave propagation through Earth-ionospherewaveguide corresponding to normal behavior of the D-region ionosphere. We tookthe VLF narrow band data from the receivers of Indian Centre for Space Physics(ICSP) to validate our models. Detection of sudden ionospheric disturbances(SIDs) are common to all the measurements. We apply our theoretical models toinfer the D-region characteristics and to reproduce the observed VLF signalbehavior corresponding to such SIDs.  We develop a code based on ray theory to simulate the diurnal behavior of VLFsignals over short propagation paths (2000~3000 km). The diurnal variation fromthis code are comparable to the variation obtained from a more general LongWave Propagation Capability (LWPC) code which is based on mode theory approach.We simulate the observational results obtained during the Total Solar Eclipseof July 22, 2009 in India. We also report and simulate a historic event,namely, the lunar occultation of a solar flare during the annular solar eclipseof 15th January, 2010 and find the effects on the D-region electron densityprofiles.

Numerical modelling of VLF radio wave propagation through earth-ionosphere waveguide and its application to sudden ionospheric disturbances