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The preparation phase of earthquakes (EQs) has been investigated by making full use of multi-parameter and multi-layer observations of EQ precursors, in order to better understand the lithosphere–atmosphere–ionosphere coupling (LAIC) process. For this purpose, we chose a specific target EQ, the huge EQ of Fukushima-ken-oki EQ on 13 February 2021 (magnitude Mj = 7.3). We initially reported on EQ precursors in different physical parameters not only of the lithosphere, but also of the atmosphere and ionosphere (Hayakawa et al. followed by Akhoondzadeh et al. and Draz et al., both based on satellite observations). Our first two papers dealt with seven electromagnetic precursors in the three layers (with emphasis on our own ground-based observations in the atmosphere and lower ionosphere), while the second paper dealt with Swarm satellite observations of magnetic field, electron density, and GPS TEC in the ionosphere, and the third paper dealt only with climatological parameters on and above the Earth's surface (together with GPS TEC). We have extensively reviewed all of these results, and have coordinated the temporal evolutions of various physical parameters relevant to the LAIC system; we have sought to understand which hypothesis is more plausible in explaining the LAIC process. Then, we came to a conclusion that two possible LAIC channels seem to exist simultaneously for this EQ: a fast channel (nearly simultaneous responses on the ground and ionosphere), and a slow channel (or diffusion-type), with a time delay of a few to several days, in which the agent effects in the lithosphere and lowest atmosphere seem to propagate up to the ionosphere with a definite time delay. Finally, we have suggested some research directions for the future elucidation of LAIC channels, and also made some comments on an early EQ warning system. [ABSTRACT FROM AUTHOR]

Copyright of Journal of National University of Defense Technology / Guofang Keji Daxue Xuebao is the property of NUDT Press and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Comprehensive simulation of a number of main processes induced in all geospheres by the fall and explosion of the Kyiv meteoroid on April 19, 2023, have been conducted. Magnetic, electrical, electromagnetic, ionospheric, and seismic effects and the effects of acoustic gravity waves have been assessed. The magnetic effect of the ionospheric currents and the current in the wake of the meteoroid could be considerably large (approximately 0.4–0.7 nT). Owing to the capture of electrons in the atmospheric gravity wave field, the magnetic effect can reach the levels of 0.6–6 nT. Under the influence of an external electric field, a transient current pulse with a current strength of up to approximately 102–103 A can arise. The electrostatic effect can be accompanied by the accumulation of an electric charge of approximately 1–6 mC, which produces the electric field strength of approximately 10 MV/m. The flow of the electric current in the meteoroid wake can give rise to generation of an electromagnetic pulse in the 10–100 kHz band with an electric field strength in the range of 1–10 V/m. The electromagnetic effect of infrasound could be substantial (approximately 0.6–6 V/m and approximately 2–20 nT). The absorption of the shock wave at the ionospheric dynamo region altitudes (approximately 100–150 km) can generate secondary atmospheric gravity waves with a relative amplitude of approximately 0.1%. The fall of the meteoroid produced a plasma wake not only in the lower atmosphere but also in the upper atmosphere at altitudes of not less than 1000 km. The possibility of the appearance of an electrophonic effect is unlikely. The possibilities of generating the ion and magnetic sound by infrasound and generating gradient drift and drift dissipative instabilities seem to be unlikely as well. The magnetic, electrical, and electromagnetic effects dealt with in this study partially fill up gaps in the theory of physical effects produced by meteoroids in the Earth–atmosphere–ionosphere–magnetosphere system. The magnitudes of magnetic, electrical, electromagnetic, ionospheric, and acoustic effects are significant. The magnitude of the earthquake caused by the meteoroid explosion did not exceed one. The mean rate of recurrence of the fall of celestial bodies similar to the Kyiv meteoroid equals 32.3 yr–1, i.e., one event in 11 days. [ABSTRACT FROM AUTHOR]

Multi-temporal Synthetic Aperture Radar Interferometry (MT-InSAR) is the only geodetic technique allowing to measure ground deformation down to mm/yr over continuous areas. Vegetation cover in equatorial regions favors the use of L-band SAR data to improve interferometric coherence. However, the electron content of ionosphere, affecting the propagation of the SAR signal, shows particularly strong spatio-temporal variations near the equator, while the dispersive nature of the ionosphere makes its effect stronger on low-frequencies, such as L-band signals. To tackle this problem, range split-spectrum method can be implemented to compensate the ionospheric phase contribution. Here, we apply this technique for time-series of ALOS-PALSAR data, and propose optimizations for low-coherence areas. To evaluate the efficiency of this method to retrieve subtle deformation rates in equatorial regions, we compute time-series using four ALOS-PALSAR datasets in contexts of low to medium coherence, showing slow deformation rates (mm/yr to cm/yr). The processed tracks are located in Ecuador, Trinidad and Sumatra, and feature 15 to 19 acquisitions including very high, dominating ionospheric noise, corresponding to equivalent displacements of up to 2 m. The correction method performs well and allows to reduce drastically the noise level due to ionosphere, with significant improvement compared with a simple plane fitting method. This is due to frequent highly non-linear patterns of perturbation, characterizing equatorial TEC distribution. We use semivariograms to quantify the uncertainty of the corrected time-series, highlighting its dependence on spatial distance. Thus, using ALOS-PALSAR-like archive, one can expect a detection threshold on the Line-of-Sight velocity ranging between 3 and 6 mm/yr, depending on the spatial wavelength of the signal to be observed. These values are consistent with the accuracy derived from the comparison of velocities between two tracks in their overlapping area. In the case studies that we processed, the time-series corrected from ionosphere allows to retrieve accurately fault creep and volcanic signal but it is still too noisy for retrieving tiny long-wavelength signals such as slow (mm/yr) interseismic strain accumulation.

The preparation phase of earthquakes (EQs) has been investigated by making full use of multi-parameter and multi-layer observations of EQ precursors, in order to better understand the lithosphere–atmosphere–ionosphere coupling (LAIC) process. For this purpose, we chose a specific target EQ, the huge EQ of Fukushima-ken-oki EQ on 13 February 2021 (magnitude Mj = 7.3). We initially reported on EQ precursors in different physical parameters not only of the lithosphere, but also of the atmosphere and ionosphere (Hayakawa et al. followed by Akhoondzadeh et al. and Draz et al., both based on satellite observations). Our first two papers dealt with seven electromagnetic precursors in the three layers (with emphasis on our own ground-based observations in the atmosphere and lower ionosphere), while the second paper dealt with Swarm satellite observations of magnetic field, electron density, and GPS TEC in the ionosphere, and the third paper dealt only with climatological parameters on and above the Earth’s surface (together with GPS TEC). We have extensively reviewed all of these results, and have coordinated the temporal evolutions of various physical parameters relevant to the LAIC system; we have sought to understand which hypothesis is more plausible in explaining the LAIC process. Then, we came to a conclusion that two possible LAIC channels seem to exist simultaneously for this EQ: a fast channel (nearly simultaneous responses on the ground and ionosphere), and a slow channel (or diffusion-type), with a time delay of a few to several days, in which the agent effects in the lithosphere and lowest atmosphere seem to propagate up to the ionosphere with a definite time delay. Finally, we have suggested some research directions for the future elucidation of LAIC channels, and also made some comments on an early EQ warning system.

The L-band geosynchronous synthetic aperture radar (GEO-SAR) has been widely praised for its advantages of short revisit time, wide coverage and stable backscattering information acquisition. However, due to the ultra-long integrated time, the echo will be affected by the time-variant background ionosphere, leading in particular to defocusing in the azimuth direction. Existing compensation methods suitable for low Earth orbit SAR (LEO-SAR) are based on the SAR image or the semi-focused image at the ionospheric phase screen, assuming that the ionosphere is time-frozen for a short integrated period; thus, accurate reconstruction of the time-variant characteristics for the ionosphere in GEO-SAR cannot be achieved. In this paper, a compensation method of background ionospheric effects on L-band GEO-SAR with fully polarimetric data is proposed. Considering the continuous variation of the ionosphere within the synthetic aperture, a decompression processing is proposed to reconstruct the echo by recovering the temporal sampling according to the imaging geometry. By virtue of the Faraday rotation angle, the time-variant total electron content (TEC) is accurately estimated with the reconstructed echo. Based on the established error model, the ionospheric effects are well compensated with the estimated TEC. Simulations with the real SAR data from ALOS-2 and the measured time-variant TEC from USTEC validate the effectiveness and performance of the proposed method. The impacts from thermal noise and polarimetric calibration error are also quantitatively analyzed. From this, the error thresholds are given to guarantee compensation accuracy, namely 18.96 dB for SNR, −15.63 dB for crosstalk and −1.02 dB to 0.31 dB for the amplitude of the channel imbalance, and the argument of the channel imbalance is suggested to be maintained as close to zero as possible. [ABSTRACT FROM AUTHOR]

An increasing interest in the use of low frequency Synthetic Aperture Radar (SAR) systems, e.g., L- and P-bands, makes the research of the ionospheric effects on SAR interferograms become urgent and significant. As the most pronounced signature in interferograms, the ionosphere-induced azimuth streak was thoroughly investigated in this study through processing of the 19 L-band Advanced Land-Observing Satellite (ALOS) Phased Array type L-band Synthetic Aperture Radar (PALSAR)images over the Chongqing City, China. The investigations show that the visible ionosphere-induced stripe-shape azimuth shifts with the invariable direction of 26°E, 113°N are observed in some interferometric pairs. Relating these anomalous azimuth shifts to the International GNSS Service (IGS) final ionospheric products shows that the detected ionosphere-contaminated SAR images display the relatively large ionospheric variation with time during SAR satellite travelled through the study area, indicating a somewhat correlation between them. After detecting the ionosphere-contaminated interferograms, we estimated the Ionospheric Phase Streak (IPS) based on an approximate linear relationship between IPS and azimuth shift, and then removed them from the original interferograms. The corrected results show that ionospheric phase patterns are largely removed from the ionosphere-contaminated interferograms. The investigation indicates that the direction of the IPS keeps approximately constant in space and time, which provides the potential chance to develop methods to correct the ionospheric effect. Furthermore, this study once more proves that the ionospheric effect on SAR interferogram can be detected, estimated and corrected from azimuth shifts.

Moon-based synthetic aperture radar (SAR) offers unprecedented temporal and spatial coverage. Its repeat-pass interferometry is expected to play a substantial role in earth science because of its large-scale, long-term, near 24.8 h revisit period, and stable earth observation ability. However, it faces a greater challenge when the signal passes through the ionosphere compared with the low Earth orbit (LEO) satellite. In this study, we constructed a total electron content (hereafter referred to as TEC) calculation model for the propagation path of a moon-based SAR signal based on International reference ionosphere model and moon-based SAR interferometry (InSAR) geometry. Subsequently, the ionospheric delay at various carrier frequencies was quantitatively evaluated under different time baseline types. The results show that the interferometric phase space gradient caused by ionospheric can reach ten times that of LEO SAR satellites with superposition of diurnal and seasonal ionospheric variations. In addition, this problem in the observation area with a large incident angle is more severe, which limits the effective swath width of moon-based repeat-pass InSAR. The ionospheric delay effects can be avoided to some extent by selecting the interference combination at nighttime or approaching the solar altitude angle. To highlight the large-scale ground deformation information in most cases and give full play to the long-term and stable observation advantages of the moon-based platform, accurate ionospheric correction or compensation must be considered.

The L-band geosynchronous synthetic aperture radar (GEO-SAR) has been widely praised for its advantages of short revisit time, wide coverage and stable backscattering information acquisition. However, due to the ultra-long integrated time, the echo will be affected by the time-variant background ionosphere, leading in particular to defocusing in the azimuth direction. Existing compensation methods suitable for low Earth orbit SAR (LEO-SAR) are based on the SAR image or the semi-focused image at the ionospheric phase screen, assuming that the ionosphere is time-frozen for a short integrated period; thus, accurate reconstruction of the time-variant characteristics for the ionosphere in GEO-SAR cannot be achieved. In this paper, a compensation method of background ionospheric effects on L-band GEO-SAR with fully polarimetric data is proposed. Considering the continuous variation of the ionosphere within the synthetic aperture, a decompression processing is proposed to reconstruct the echo by recovering the temporal sampling according to the imaging geometry. By virtue of the Faraday rotation angle, the time-variant total electron content (TEC) is accurately estimated with the reconstructed echo. Based on the established error model, the ionospheric effects are well compensated with the estimated TEC. Simulations with the real SAR data from ALOS-2 and the measured time-variant TEC from USTEC validate the effectiveness and performance of the proposed method. The impacts from thermal noise and polarimetric calibration error are also quantitatively analyzed. From this, the error thresholds are given to guarantee compensation accuracy, namely 18.96 dB for SNR, −15.63 dB for crosstalk and −1.02 dB to 0.31 dB for the amplitude of the channel imbalance, and the argument of the channel imbalance is suggested to be maintained as close to zero as possible.

An increasing interest in the use of low frequency Synthetic Aperture Radar (SAR) systems, e.g., L- and P -bands, makes the research of the ionospheric effects on SAR interferograms become urgent and significant. As the most pronounced signature in interferograms, the ionosphere-induced azimuth streak was thoroughly investigated in this study through processing of the 19 L-band Advanced Land-Observing Satellite ( ALOS) Phased Array type L-band Synthetic Aperture Radar ( PALSAR) images over the Chongqing City, China. The investigations show that the visible ionosphere-induced stripe-shape azimuth shifts with the invariable direction of 26°E, l 13°N are observed in some interferometric pairs. Relating these anomalous azimuth shifts to the International GNSS Service ( IGS) final ionospheric products shows that the detected ionosphere -contaminated SAR images display the relatively large ionospheric variation with time during SAR satellite travelled through the study area, indicating a somewhat correlation between them. After detecting the ionosphere-contaminated interferograms, we estimated the Ionospheric Phase Streak ( IPS) based on an approximate linear relationship between IPS and azimuth shift, and then removed them from the original interferograms. The corrected results show that ionospheric phase patterns are largely removed from the ionosphere - contaminated interferograms. The investigation indicates that the direction of the IPS keeps approximately constant in space and time, which provides the potential chance to develop methods to correct the ionospheric effect. Furthermore, this study once more proves that the ionospheric effect on SAR interferogram can be detected, estimated and corrected from azimuth shifts. [ABSTRACT FROM AUTHOR]

The solar flares and geomagnetic storms are considered a potential threat for the upcoming Indian Regional Navigation Satellite System or Navigation with Indian Constellation (NavIC). Therefore, we investigated in detail the occurrence of tracking failure and effects on the positional accuracy of the NavIC L5-band and S-band system under direct exposure of X2.2 and X9.3 solar flare on September 6, 2017, X9.3 is the strongest event since the NavIC signals are in the service. The regional NavIC data (5–6 September 2017) from the equatorial region Trivandrum station and the Equatorial Ionization Anomaly area in India (Surat, Bombay, Hyderabad and Gandhinagar stations), are collected using the accord NavIC dual-frequency (L5-band and S-band) receivers. We have shown that the 1G NavIC signals remain in loss of lock condition for 1 h on September 6, 2017 (X2.2 and X9.3 solar flares), and as a result, positioning accuracy deviated on September 6, 2017. Hence, the solar flare event considered a potential threat for future NavIC based autonomous navigation technologies. [ABSTRACT FROM AUTHOR]

The Medium-Earth-Orbit SAR (MEOSAR) is one of the potential spaceborne SAR of next-generation owing to its excellent performance. Ionospheric effects analysis is one of the critical techniques for the development of MEOSAR. The spatio-temporal variability of the ionosphere is analyzed using the measured ionosphere data. The factors and the mechanism of ionospheric effects on MEOSAR are studied based on the spatio-temporal variability of the ionosphere and the characteristics of MEOSAR such as long synthetic aperture time, wide swath, and high orbit. The results reveal that there are many differences in the ionospheric effects between MEOSAR and LEOSAR.

The Terrain Observation by Progressive Scans (TOPS) acquisition mode of the Sentinel-1 mission provides a wide coverage per acquisition with resolutions of 5 m in range and 20 m in azimuth, which makes this acquisition mode attractive for glacier velocity monitoring. Here, we retrieve surface velocities from the southern Ellesmere Island ice caps (Canadian Arctic) using both offset tracking and Differential Interferometric Synthetic Aperture Radar (D-InSAR) techniques and combining ascending and descending passes. We optimise the offset tracking technique by omitting the azimuth offsets. By doing so, we are able to improve the final resolution of the velocity product, as Sentinel-1 shows a lower resolution in the azimuth direction. Simultaneously, we avoid the undesired ionospheric effect manifested in the data as azimuth streaks. The D-InSAR technique shows its merits when applied to slow-moving areas, while offset tracking is more suitable for fast-moving areas. This research shows that the methods used here are complementary and the use of both to determine glacier velocities is better than only using one or the other. We observe glacier surface velocities of up to 1200 m year-1 for the fastest tidewater glaciers. The land-terminating glaciers show typical velocities between 12 and 33 m year-1, though with peaks up to 150 m year-1 in narrowing zones of the confining valleys. [ABSTRACT FROM AUTHOR]

Copyright of China Sciencepaper is the property of China Sciencepaper and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The ionosphere is a significant source of errors affecting L-Band (1-2 GHz) signal propagation using Global Position System (GPS). The propagated GPS signals that passes through the Earth's atmosphere where affected by ionosphere and troposphere irregularities and caused the signal propagation delay. However, the velocity of signals that propagate within the atmosphere deviates from vacuum line of sight (LOS), while the refractivity of ionosphere and troposphere influence the signal propagation delay causing atmospheric effects due to high electrons density. Both affect the signal in different ways due to their particular refractive properties. The review focuses of some of the previous research studies conducted on empirical modeling of the ionospheric effect in low latitude region. The tabulated findings of such models presented in this article. [ABSTRACT FROM AUTHOR]