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A thermospheric O and N2 column density ratio (ΣO/N2) depletion with long-duration (>16 hr) was observed by the Global-scale Observations of the Limb and Disk at the Atlantic longitudes (75W–20W) and middle latitudes (20N–50N) during the recovery phase of the 8 June 2019 geomagnetic storm. The National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) simulations reproduced the ΣO/N2 depletion patterns with a similar magnitude, and indicated that the composition recovery at middle latitudes began several hours after the beginning of the recovery phase of the geomagnetic storm. The TIEGCM simulations enable quantitative analysis of the physical mechanisms driving the middle-latitude composition changes during the storm recovery phase. This analysis indicates that vertical advection and molecular diffusion dominated the initial recovery of composition perturbations at middle latitudes. Horizontal advection was also a main driver in the initial recovery of composition, but its contribution decreased rapidly. In the late recovery phase, the composition recovery was mainly determined by horizontal advection. In comparison, vertical advection and molecular diffusion played a much less important role.

The temporal as well as vertical and horizontal variations of the concentric traveling ionospheric disturbances (CTIDs) caused by hurricane Matthew on 7 October 2016 were reconstructed using 3-dimensional computerized ionospheric tomography (3DCIT) technology, based upon the GNSS data from the dense receiver network over North America. The frequency range of disturbances was determined by spectrum analysis, and a Butterworth band-pass filter was used to de-trend the total electron content (TEC) sequences in order to determine TIDs. A remarkable CTID segment was detected at a distance of 1000 – 1500 km from the hurricane eye at ~5:40 – 6:10 UT on 7 October 2016, moving westward with the horizontal phase velocity of ~153.4 m/s, the period of ~30 min and the horizontal wavelength of ~276.1 km. The positive and negative wavefronts dominated the CITD at different times during the event. From 4:00 to 8:00 UT, the altitudinal variation of the CTIDs in electron density exhibited clear downward phase progression predominately in the range of 150 – 400 km altitudes, however, the percentage electron density disturbances were larger below 250 km. The inverted cone-like geometry of CTID wavefronts was presented. The vertical phase velocities of the CTIDs ~1100 km away from the hurricane eye in the northwest direction near 88°W, 34°N were ~203.7 – 277.8 m/s, and at the same location, the horizontal phase velocities at 300 km altitude were ~149.1 – 181.5 m/s, slightly larger than those at 200 km altitude (~145.1 – 178.5 m/s).

Previously conducted studies have established that mining activities can activate faults, which will cause floor water inrush disasters and cause loss of personnel and property. In order to reduce the possibility of water inrush disasters in mining, it is particularly important to study the dynamic characteristics and rules of floor fault activation under the influence of mining. In this work, firstly, a microseismic monitoring system was established in the working face to analyze the changes of microseismic indexes before and after grouting. It was found that grouting can enhance the strength of a rock mass and play a role in sealing the water channel. Secondly, the quadratic kernel function of microseismic event energy was established. It was found that the accumulation degree of microseismic events and the region of high energy kernel density increased with the decrease of the distance between the working face and the left boundary of the “analysis region”. Combined with a microseismic event index and water inflow, the activation process of the floor fault was divided into five stages. Finally, the plastic failure region of surrounding rock under different excavation steps was analyzed by numerical simulation, and the characteristics of fault activation were further explained. A method of taking measures to prevent water inrush in the “sign stage of fault activation” was proposed.

The electron density structures of the seismo-traveling ionospheric disturbances (STIDs) during the Tohoku earthquake are reconstructed by applying the three-dimensional computerized ionospheric tomography (3DCIT) technique with a 30-s time resolution for the first time. The vertical distribution of 3DCIT results is consistent with the constellation observing system for meteorology, ionosphere and climate (COSMIC) observations. The horizontal speeds of STIDs at different altitudes are estimated, and the three types of STIDs related to Rayleigh waves, acoustic waves and gravity waves are identified by their propagation characters. The magnitude of STIDs related to Rayleigh waves decreased with altitude, and there was no significant difference between the speeds (~ 2500 m/s) at different altitudes. The STIDs caused by acoustic waves traveled faster at 300 km altitude (~ 666–724 m/s) than at 150 km altitude (~ 500–550 m/s). From 150 to 250 km altitudes, in the STIDs induced by gravity waves, the magnitude of positive and negative wave fronts showed the opposite trend. The speed at 300 km altitude (~ 332 m/s) was slightly larger than at 150 km altitude (~ 310 m/s). The Rayleigh waves related STIDs showed a conic-like geometry, whereas the acoustic waves and gravity waves induced STIDs showed inverted conic-like geometries. The possible propagation mechanisms of different types of STIDs are also discussed.

In order to adapt the International Reference Ionosphere (IRI) model from ionospheric climatological model to near real-time weather predictions, total electron content (TEC) data from Global Ionosphere Maps are ingested into the IRI-2016 model through retrieving the optimal ionospheric index (IG) over Europe on an hourly basis. When the retrieved hourly effective IG indices are used to drive the IRI-2016 model, the resulting ionospheric parameters are externally evaluated with respect to multiple sources, including the COSMIC/ionosonde electron density (Ne) profiles, ionosonde F2 layer critical frequency (foF2), and individual GNSS-derived TEC for both quiet and storm conditions. Results show that: (1) The updated IG indices for different latitudinal zones tend to follow a similar trend under quiet conditions, but vary much more significantly during storm days. (2) The retrieved Ne profiles from the updated IRI-2016 agree better with those from the COSMIC Ne profiles, especially for the F2 layer maximum electron density (NmF2) values. Furthermore, the updated IRI-2016 Ne profiles show improved agreement with ionosonde measurements under quiet conditions, particularly for the bottom-side Ne profiles and NmF2 as well as for the storm-time Ne profiles. (3) Comparing the IRI-updated TEC with the GNSS-derived TEC, IRI-updated TEC improved approximately 19% for both quiet and storm days, and the nighttime TEC improvement is better than that during daytime. When compared to the ionosonde foF2 measurements, the daytime IRI-updated foF2 improvement during quiet time is better than that during storm condition, while the performance for nighttime foF2 drops during quiet time. Discussions about possible reasons for the nighttime foF2 degradation are included.

Global Navigation Satellite System (GNSS) is now widely used for continuous ionospheric observations. Three-dimensional computerized ionospheric tomography (3DCIT) is an important tool for the reconstruction of electron density distributions in the ionosphere through effective use of the GNSS data. More specifically, the 3DCIT technique is able to resolve the three-dimensional electron density distributions over the reconstructed area based on the GNSS slant total electron content (STEC) observations. We present an Improved Constrained Simultaneous Iterative Reconstruction Technique (ICSIRT) algorithm that differs from the traditional ionospheric tomography methods in 3 ways. First, the ICSIRT computes the electron density corrections based on the product of the intercept and electron density within voxels so that the assignment of corrections at different heights becomes more reasonable. Second, an Inverse Distance Weighted (IDW) interpolation is used to restrict the electron density values in the voxels not traversed by GNSS rays, thereby ensuring the smoothness of the reconstructed region. Also, to improve the reconstruction accuracy around the HmF2 (the peak height of the F2 layer) altitude, a multiresolution grid is adopted in the vertical direction, with a 10-km resolution from 200 to 420 km and a 50-km resolution at other altitudes. The new algorithm has been applied to the GNSS data over the European and North American regions in different case studies that involve different seasonal conditions as well as a major storm. In the European region experiment, reconstruction results show that the new ICSIRT algorithm can effectively improve the reconstruction of the GNSS data. The electron density profiles retrieved from ICSIRT are much closer to the ionosonde observations than those from its predecessor, namely, the Constrained Simultaneous Iteration Reconstruction Technique (CSIRT). The reconstruction accuracy is significantly improved. In the North American region experiment, the electron density profiles in ICSIRT results show better agreement with incoherent scatter radar observations than CSIRT, even for the topside profiles.

The Co-Seismic Ionospheric Disturbance of the 2015 Nepal earthquake is analyzed in this paper. GNSS data are used to obtain the Satellite-Station TEC sequences. After removing the de-trended TEC variation, a clear ionospheric disturbance was observed 10 min after the earthquake, while the geomagnetic conditions, solar activity, and weather condition remained calm according to the Kp, Dst, F10.7 indices and meteorological records during the period of interest. Computerized ionosphere tomography (CIT) is then used to present the tridimensional ionosphere variation with a 10-min time resolution. The CIT results indicate that (1) the disturbance of the ionospheric electron density above the epicenter during the 2015 Nepal earthquake is confined at a relatively low altitude (approximately 150–300 km); (2) the ionospheric disturbances on the west side and east sides of the epicenter are precisely opposite. A newly established electric field penetration model of the lithosphere–atmosphere–ionosphere coupling is used to investigate the potential physical mechanism.

Twenty-seven-day variation caused by solar rotation is one of the main periodic effects of solar radiation influence on the ionosphere, and there have been many studies on this periodicity using peak electron density $$\mathrm{N_{m}F_{2}}$$ and solar radio flux index F10.7. In this paper, the global electron content (GEC) and observation of Solar EUV Monitor (SEM) represent the whole ionosphere and solar EUV flux, respectively, to investigate the 27-day variation. The 27-day period components of indices $$(\hbox {GEC}_{27}$$ , $$\hbox {SEM}_{27}$$ , $$\hbox {F10.7}_{27}$$ , $$\hbox {Ap}_{27})$$ are obtained using Chebyshev band-pass filter. The comparison of regression results indicates that the index SEM has higher coherence than F10.7 with 27-day variation of the ionosphere. The regression coefficients of $$\hbox {SEM}_{27 }$$ varied from 0.6 to 1.4 and the coefficients of $$\hbox {Ap}_{27}$$ varied from $${-}$$ 0.6 to 0.3, which suggests that EUV radiation seasonal variations are the primary driver for the 27-day variations of the ionosphere for most periods. TEC map grid points on three meridians where IGS stations are dense are selected for regression, and the results show that the contribution of solar EUV radiation is positive at all geomagnetic latitudes and larger than geomagnetic activity in most latitudes. The contribution of geomagnetic activity is negative at high geomagnetic latitude, increasing with decreasing geomagnetic latitudes, and positive at low geomagnetic latitudes. The global structure of 27-day variation of ionosphere is presented and demonstrates that there are two zonal anomaly regions along with the geomagnetic latitudes lines and two peaks in the north of Southeast Asia and the Middle Pacific where $$\hbox {TEC}_{27}$$ magnitude values are notably larger than elsewhere along zonal anomaly regions.

The global navigation satellite system (GNSS) total electron content (TEC) sequences were used to capture the arrival time and location of the ionosphere disturbances in response to the 2015 Typhoon Dujuan. After removing the de-trended TEC variation, the clear ionosphere disturbances on the typhoon landing day could be distinguished, and these disturbances disappeared from the TEC sequences before and after the typhoon landing day. The foF2 data observed by Xiamen ionosonde station also show ionosphere disturbances. Based on the advantages of GNSS multi-point observations, the disturbances horizontal velocity in the ionosphere were estimated according to the linear theory for a dispersion relation of acoustic gravity waves (AGWs) in an isothermal atmosphere. The average horizontal velocity ( $$\sim $$ 240 m/s) and the radial velocity ( $$\sim $$ 287 m/s) were used in the two-dimensional grid search for the origin point on the Earth’s surface. The origin area was determined to be on the eastern side of Taiwan. Lastly, a possible physical mechanism is discussed in this study. When typhoons land on Taiwan, the severe convective storms and the drag effect from the Central Mountains create an ideal location for development of AGWs. Topographic conditions, like the high lapse rate, contribute to the formation of AGWs, which then propagates into the ionosphere altitude.

A new algorithm for ionosphere tomography using the mapping function is proposed in this paper. First, the new solution splits the integration process into four layers along the observation ray, and then, the single-layer model (SLM) is applied to each integration part using a mapping function. Next, the model parameters are estimated layer by layer with the Kalman filtering method by introducing the scale factor (SF) $$\gamma $$ to solve the ill-posed problem. Finally, the inversed images of different layers are combined into the final CIT image. We utilized simulated data from 23 IGS GPS stations around Europe to verify the estimation accuracy of the new algorithm; the results show that the new CIT model has better accuracy than the SLM in dense data areas and the CIT residuals are more closely grouped. The stability of the new algorithm is discussed by analyzing model accuracy under different error levels (the max errors are 5TECU, 10TECU, 15TECU, respectively). In addition, the key preset parameter, SF $$\gamma $$ , which is given by the International Reference Ionosphere model (IRI2012). The experiment is designed to test the sensitivity of the new algorithm to SF variations. The results show that the IRI2012 is capable of providing initial SF values. Also in this paper, the seismic-ionosphere disturbance (SID) of the 2011 Japan earthquake is studied using the new CIT algorithm. Combined with the TEC time sequence of Sat.15, we find that the SID occurrence time and reaction area are highly related to the main shock time and epicenter. According to CIT images, there is a clear vertical electron density upward movement (from the 150-km layer to the 450-km layer) during this SID event; however, the peak value areas in the different layers were different, which means that the horizontal movement velocity is not consistent among the layers. The potential physical triggering mechanism is also discussed in this paper. Compared with the SLM, the RMS of the new CIT model is improved by 16.78%, while the CIT model could provide the three-dimensional variation in the ionosphere.

The three-dimensional ionospheric tomography (3DCIT) algorithm based on Global Navigation Satellite System (GNSS) observations have been developed into an effective tool for ionospheric monitoring in recent years. However, because the rays that come into or come out from the side of the inversion region cannot be used, the distribution of the rays in the edge and bottom part of the inversion region is scarce and the electron density cannot be effectively improved in the inversion process. We present a three-dimensional tomography algorithm with side rays (3DCIT-SR) applying the side rays to the inversion. The partial slant total electron content (STEC) of side rays in the inversion region is obtained based on the NeQuick2 model and GNSS-STEC. The simulation experiment results show that the algorithm can effectively improve the distribution of GNSS rays in the inversion region. Meanwhile, the iteration accuracy has also been significantly improved. After the same number of iterations, the iterative results of 3DCIT-SR are closer to the truth than 3DCIT, in particular, the inversion of the edge regions is improved noticeably. The GNSS data of the International GNSS Service (IGS) stations in Europe are used to perform real data experiments, and the inversion results show that the electron density profiles of 3DCIT-SR are closer to the ionosonde measurements. The accuracy improvement of 3DCIT-SR is up to 56.3% while the improvement is more obvious during the magnetic storm compared to the case of a calm ionospheric state .

The global ionospheric storm in March, 2015 was investigated using data from over 3000 GPS stations worldwide. In this study, Total Electron Content(TEC), Rate of TEC(ROT) and ROT's standard deviation ROTI, as well as the second-order difference operator TECT, were considered as main characteristic methods to distinguish ionosphereic disturbances.The results show that, (1) Based on the multiple methods above, we all observed that, for the first time, there were three equatorward traveling ionospheric disturbances (TIDs) in the main phase of this storm. In North America, the disturbance zone expanded to ~40°N; the disturbance periods and AE peak stages were roughly synchronous. We suggest that these three TIDs were induced by the propagation of AGWs to low latitudes under the action of AE. (2) The most intense positive storm occurred over South America and the South Atlantic (over 300% enhancement; 00:00–05:00 UT March 18), whereas a negative storm was observed in the corresponding region of the NH. Such inverse hemispheric asymmetry in intensity and structure can be explained by the variations of the thermospheric composition, the IMF By component and the geomagnetic intensity. (3) On March 18, a negative storm dominated globally (except at certain low latitudes), and tended to propagate equatorward and decay with time, which could be largely attributed to the storm circulation theory. And the evolution of the negative storm was further characterized by the foF2 variations of ionosondes.