Abstract
Taking the 2018 Ms6.9 Indonesia earthquake as a case study, the ionospheric perturbations in very low frequency (VLF) transmitters recorded by China Seismo-Electromagnetic Satellite (CSES) were mainly investigated, as well as the multi parameters of the plasma and electromagnetic field. The characteristics of electron density (Ne), GPS TEC, ULF electric field, ion drift velocity, and ionosphere height were extracted and compared with the features of the signal-noise ratio (SNR) from VLF transmitters of NWC at the southern hemisphere and JJI at the northern hemisphere. Most disturbances in VLF radio waves occurred along the orbits near the epicenter within 10 days before the earthquake. Along these orbits, we observed simultaneous modulations in the Ne and ULF electric field, as well as the changed ion drifting directions. There was also high spatial correspondence between both SNR and ionospheric height anomalies over the epicentral and its magnetic conjugate regions. Combined with the multi observations, these results suggest that the genesis of perturbations in signals emitted by VLF transmitters on satellite was more likely related to the overlapped electric field in the preparation area of the earthquake.
Highlights
Since last century, a lot of papers have been published concerning the seismic application research using very low frequency (VLF)/LF transmitters detected both on ground-based stations and satellites [1,2]
Eight scientific payloads have been installed onboard the satellite, including High Precision Magnetometer (HPM), Electric Field Detector (EFD) and Search Coil Magnetometer (SCM) for electromagnetic field detection; Langmuir Probe (LAP) and Plasma Analyzer Package (PAP) for plasma parameters; High Energetic Particle Package (HEPP) and High Energetic Particle Detector (HEPD) together for energetic particles; and GNSS Occultation Receiver (GOR) and Tri-Band Beacon (TBB) for electron density profiles [31]
The atmospheric gravity waves (AGWs) induced on the surface propagate into the atmosphere, causing an ozone content increase and ionospheric height decrease, disturbing the electrons and ions to move upwards or downwards to construct an overlapped electric field emission in ULF band, and influence the penetration of VLF waves to lead to lower values of signal-noise ratio (SNR) detected at the satellite height
Summary
A lot of papers have been published concerning the seismic application research using VLF/LF transmitters detected both on ground-based stations and satellites [1,2]. Zhang presented a quick signal drop, occurring a few days before EQ, in the electric field amplitude and SNR from an LF transmitter at 162 kHz in Mongolia and detected by the DEMETER/ICE experiment [19] All these results show that the VLF transmitter radio wave is an important ionospheric sounding tool and is becoming more and more widely used in the earthquake research community despite the existence of some controversial points due to the complex characteristics of ionospheric precursor generations and wave propagation processes. Three main parameters have been employed in the VLF radio wave propagation research in relation with seismic activities, which are TT (the meaning of “TT” is stated in the previous paragraph) related to the phase of signals, the average amplitude connected with the strength of signals, and the fluctuations associated with the overlapped ULF waves To explain these abnormal disturbances occurring before earthquakes, some coupling mechanism and theoretical models have been suggested and developed. We investigate anomalies recorded in the VLF transmitter signals above the seismic region, and provide further suggestions in the understanding of the relationship between the seismic VLF signals disturbances and the physical processes
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