Abstract
Abstract. Earthquakes may disturb the lower ionosphere through various coupling mechanisms during the seismogenic and coseismic periods. The VLF (very low-frequency) signal radiated from ground-based transmitters will be affected when it penetrates the disturbed ionosphere above the epicenter area, and this anomaly can be recorded by low-Earth orbit satellites under certain conditions. In this paper, the temporal and spatial variation of the signal-to-noise ratio (SNR) of the VLF transmitter signal in the ionosphere over the epicenter of 2010 Yushu Ms 7.1 earthquake in China is analyzed using DEMETER (Detection of Electro-Magnetic Emission Transmitted from Earthquake Regions) satellite observation. The results show that SNR over the epicenter of the Yushu earthquake especially in the southwestern region decreased (or dropped) before the main shock, and a GPS–TEC (Global Positioning System; total electron content) anomaly accompanied, which implies that the decrease in SNR might be caused by the enhancement of TEC. A full-wave method is used to study the mechanism of the change in SNR before the earthquake. The simulated results show SNR does not always decrease before an earthquake. When the electron density in the lower ionosphere increases by 3 times, the electric field will decrease about 2 dB, indicating that the disturbed-electric-field decrease of 20 % compared with the original electric field and vice versa. It can be concluded that the variation of electron density before earthquakes may be one of the important factors influencing the variation of SNR.
Highlights
The VLF radio waves radiated by the powerful ground-based VLF transmitters have been used for long-distance communication and submarine navigation because of the efficient reflection within the Earth–ionosphere waveguide
Molchanov et al (2006) have found the signal-tonoise ratio (SNR) of the electric field from VLF transmitters recorded by the DEMETER (Detection of Electro-Magnetic Emission Transmitted from Earthquake Regions) satellite de
The SNR distributions of three frequencies (11.9, 12.6, and 14.9 kHz) within the region of the epicenter ±10◦ are shown in Fig. 2, where the value of SNR is denoted with colored dots with different sizes and the black star represents the epicenter of the Yushu earthquake
Summary
The VLF (very low-frequency) radio waves radiated by the powerful ground-based VLF transmitters have been used for long-distance communication and submarine navigation because of the efficient reflection within the Earth–ionosphere waveguide. The signals from transmitters observed by the LEO (low-Earth orbit) satellites can be used to study the propagation of VLF waves in the Earth–ionosphere waveguide and ionosphere, as well as wave–particle interaction in the radiation belt (Inan et al, 2007; Inan and Helliwell, 1982; Lehtinen and Inan, 2009; Parrot et al, 2007). The amplitude of the VLF signals from the ground-based VLF transmitter observed on the ground and from satellites will change when encounter the disturbed area in the ionosphere (Hayakawa, 2007; Maurya et al, 2016; Molchanov et al, 2006; Píša et al, 2013). The amplitude of the VLF signals from the ground-based VLF transmitter observed on the ground and from satellites will change when encounter the disturbed area in the ionosphere (Hayakawa, 2007; Maurya et al, 2016; Molchanov et al, 2006; Píša et al, 2013). Molchanov et al (2006) have found the signal-tonoise ratio (SNR) of the electric field from VLF transmitters recorded by the DEMETER (Detection of Electro-Magnetic Emission Transmitted from Earthquake Regions) satellite de-
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