Abstract:
Lightning discharges during thunderstorm are the significant natural source of
electromagnetic waves. They generate electromagnetic pulses, which vary from few Hz to tens
of MHz, but the maximum radiated energy is confined in extremely low (ELF: 3-3000Hz) and
very low (VLF: 3-30 KHz) frequency band. These pulsed signals with frequency dispersion are
known as radio atmospherics or tweeks. These waves propagate through the process of multiple
reflections in the earth-ionosphere waveguide over long distances with very low attenuation (2-
3 dB/1000km). Since these waves are reflected by lower boundary of ionosphere, these are
used extensively for probing the D-region ionosphere. D-region is important to the space
weather, as well as the submarine communication and navigational aid. In this perspective the
measurement of electron density profiles of the D-region is undoubtedly of great interest to
both the development of reliable models and radio wave propagation. Earlier work on the
tweeks is mainly focused to the theoretical considerations related to polarization, waveform
analysis, and occurrence time and propagation mechanism. In this study we investigate tweeks
to determine the equivalent night time electron densities at reflection height of the D-region.
Distance traveled by the VLF waves from the causative lightning discharges to the receiving
station has also been calculated. Tweeks recorded at a low latitude ground station of Allahabad
(Geomag. lat. 16.05°N) during the night of 23 March 2007 have been used in the present
analysis. Based on the analysis of the fundamental cut-off frequency of tweeks, the estimated
equivalent electron density of the D-region has been found to be in the range of ~20 to 25
el/cm³ at ionospheric reflection height of ~80 to 95 km respectively. Propagation distance in
Earth-Ionosphere wave guide (EIWG) from causative lightning source to experimental site
varies from ~1500 to 8000 km.
