Abstract
In this paper we analyse temporal variations of the phase of a very low frequency (VLF) signal, used for the lower ionosphere monitoring, in periods around four earthquakes (EQs) with magnitude greater than 4. We provide two analyses in time and frequency domains. First, we analyse time evolution of the phase noise. And second, we examine variations of the frequency spectrum using Fast Fourier Transform (FFT) in order to detect hydrodynamic wave excitations and attenuations. This study follows a previous investigation which indicated the noise amplitude reduction, and excitations and attenuations of the hydrodynamic waves less than one hour before the considered EQ events as a new potential ionospheric precursors of earthquakes. We analyse the phase of the ICV VLF transmitter signal emitted in Italy recorded in Serbia in time periods around four earthquakes occurred on 3, 4 and 9 November 2010 which are the most intensive earthquakes analysed in the previous study. The obtained results indicate very similar changes in the noise of phase and amplitude, and show an agreement in recorded acoustic wave excitations. However, properties in the obtained wave attenuation characteristics are different for these two signal parameters.
Highlights
In addition to periodical ionospheric changes, which can be predicted and estimated by different models, sudden events can induce significant ionospheric disturbances and affect many contemporary technologies based on satellite and ground-based electromagnetic (EM) signal propagation [3]
Increases or decreases of the signal amplitude and/or phase are recorded in many studies focused on research of ionospheric disturbances induced by earthquakes [9], solar activity [14,15,16,17,18], tropical cyclones [19,20], solar eclipse [21,22] etc
They relate to disturbances induced by the solar terminator [28], geomagnetic storms [29,30], tropical cyclones [19,31,32] earthquakes [33], solar eclipse [34], and they are based on analyses of the very low frequency (VLF)/LF signals
Summary
In addition to periodical ionospheric changes, which can be predicted and estimated by different models (see, for example, [1,2] and references therein), sudden events can induce significant ionospheric disturbances and affect many contemporary technologies based on satellite and ground-based electromagnetic (EM) signal propagation [3]. Increases or decreases of the signal amplitude and/or phase are recorded in many studies focused on research of ionospheric disturbances induced by earthquakes [9], solar activity [14,15,16,17,18], tropical cyclones [19,20], solar eclipse [21,22] etc. Research of AGW in the lower ionosphere is presented in several studies They relate to disturbances induced by the solar terminator [28], geomagnetic storms [29,30], tropical cyclones [19,31,32] earthquakes [33], solar eclipse [34], and they are based on analyses of the VLF/LF signals
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