Abstract
The unusualness in the critical frequency of different layers of earth’s ionosphere is commensurate to be associated with seismic events. We present study of critical frequency of F2 layer (denoted as f0F2) during some major earthquakes in South American region. We use the semi-empirical Barbier’s theorem of air-glow and define a parameter using the critical frequency and virtual height of F2 layer and named it as “F Parameter”. To investigate the variation of this parameter, we consider five large earthquakes in the junction of Nazca plate and South American plate having magnitude greater than M > 6.5 and study the temporal variation of F parameter during these earthquakes. The F Parameter is measured using the ionograms as recorded from the Ionosonde in Jicamarca Radio Observatory (lat. 11.95°S, long 76.87°W) in Chile which lies within the earthquake preparation zones of these five earthquakes. We examine the F Parameter within a span of ±15 days during earthquakes and observed significant change in the evaluated F Parameter in 12 to 3 days prior to the earthquakes. The increment is over +3σ from the normal variation. We also observe significant changes during aftershock events. The solar geomagnetic indices were found to be low which ensures that these anomalies in F Parameter are due to seismic events.
Highlights
The Lithosphere-Atmosphere-Ionosphere Coupling (LAIC) mechanism relates properties of apparently distant and distinct components of Earth system science ranging from lithosphere, lower ionospheric D-layer to upper F2 layer
Starting from the tropospheric thermal anomalies, lower ionospheric electron density variation to upper ionospheric critical frequency modulation, LAIC takes into account a wide range of geochemical and geo-physical phenomena which could be affected simultaneously
We focused our study on the ionospheric F2 layer where we observe the critical frequency variation for a span of three weeks during some strong seismic events in South America region
Summary
Though the path is bent gradually, one can conceive that the incident and reflected rays, when extrapolated, meet at a point This height could be called the virtual reflection height. The well-known Barbier formula establishes a direct relationship between the airglow intensity and the critical frequency of the F2 layer. The variable part of this formula deals with the critical frequency and the virtual reflection height of F layer. The critical frequency is associated with the charge density profile of the ionospheric layers. Can be used as a tool to study the variabilities of the charge density profiles. LAIC mechanism predicts anomalous changes of charge density profile before seismic events. We compute a variable parameter from Barbier theorem using the (f0F2) and virtual height profile and study this parameter for a duration of ±15 days around those five earthquakes.
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