Abstract
ABSTRACTWe present the results of the statistical analysis of very low frequency (VLF) signal transmitted from NWC transmitter (latitude 21.8° S, longitude 114.15° E) at 19.8 kHz received at Ionospheric and Earthquake Research Centre (IERC) (latitude 22.50° N, longitude 87.79° E). We analyse the phase of VLF signal for the whole year 2011 and compute the sunrise and sunset terminator time from it. We observe unusual shift of these sunrise and sunset terminator time during seismic events for which the earthquake epicentres are in India and its subcontinent region. We calculate the total energy accumulation by all those earthquakes for a single day and compute the effective magnitude of all the earthquakes for that day which behaves as a single quake. We compute a cross correlation between fluctuation in terminator time shifts from the normal value with effective magnitude of earthquake. We found that the unusual fluctuations in terminator time are well correlated with earthquake magnitude and the fluctuation is maximum on two days prior to earthquake events.
Highlights
Immense research has been carried out from the last two and a half decades to understand the physics behind the electromagnetic phenomena associated with lower ionosphere during seismic events
Several attempts and techniques have been used to understand what physical parameter is responsible for the ionospheric perturbation which can modify the radio signal characteristics propagated through earth-ionosphere waveguide
To know the correlation between anomalous Very low frequency signal (VLF) phase-day-length with seismic events, we must have the knowledge of the signal variation with a quiet solar condition as solar events can changes the signal characteristics significantly
Summary
Immense research has been carried out from the last two and a half decades to understand the physics behind the electromagnetic phenomena associated with lower ionosphere during seismic events. The lowermost region of the ionosphere namely the D-region (» 50–90 km) has the most dynamic physical and chemical properties as this layer exists during the presence of solar illumination. In this region, the ionization is due to the photoionization process of nitric oxide (NO) by Hydrogen Lyman series-alpha radiation (wavelength 121.5 nanometre) during daytime (Nicolet & Aikin 1960; Lean & Skumanich 1983). During the daytime when solar X-ray and UV become stronger, the ionization process generates free electrons and positive ions. The free electrons in the D-layer oscillate due to the interactions with the propagated wave. During sunrise, when the D-layer is about to form, due to
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