M. Indira Devi

A study of VLF wave propagation characteristics in the earth-ionosphere waveguide

In the light of newly found applications for very low frequency (VLF) (3–30 kHz) measurements in the prediction of earthquakes and the detection of lightning discharges and gamma ray bursts, there has been a revival of interest in the study of VLF propagation in the earth-ionosphere waveguide. The propagation characteristics of VLF radiowaves in the earth-ionosphere waveguide critically depend upon the lower ionospheric ionization, which determines the conductivity profile of the upper boundary of the waveguide. In this context, it is potentially worthwhile to revisit the waveguide mode analysis to compute propagation parameters of long-distance VLF transmissions while taking into account different approximations to the ionospheric conductivity. We have carried out a waveguide mode analysis of 16-kHz VLF waves traveling great distances, assuming three different models for conductivity. The computational results show that at heights of less than 70 km, the change in the phase of the VLF waves due to changes in phase velocity is smaller when the ionosphere is sharply bound and assumed to have finite conductivity rather than infinite conductivity. The effect of a non-sharp or diffuse boundary at the top of the earth-ionosphere waveguide is found to cause a lowering of the height of reflection.

A theoretical study of diurnal shift in reflection height of VLF waves using IRI electron density model

Electron density profiles for the International Reference Ionosphere (IRI) 2001 and 2007 models have been utilized in evaluating the D-region conductivity parameter in earth ionosphere wave guide calculations. The day to night shift in reflection height of very low frequency (VLF) waves has been calculated using D-region conductivities derived from IRI models and the results are compared with those obtained from phase variation measurements of VLF transmissions from Rugby (England) made at Visakhapatnam (India). The values derived from the models are found to be much lower than those obtained from the experimental measurements. The values derived from the IRI models are in good agreement with those obtained from exponential conductivity model.

Validity of IRI electron density profiles in relation to vertical incidence absorption measurements

Abstract Discrepancies have been observed between measured radio wave obsorption in the ionosphere and numerical estimates of the same using IRI or rocket-borne determination of N(h) profiles. These are attributed to the assumption that the momentum transfer electron-neutral collision frequency is proportional to the electron thermal energy. Assuming a linear dependence of the collision frequency on the electron thermal speed leads to good agreement between measured and theoretically deduced values of absorption.