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Ionospheric spread-F (SF) is a commonly observed phenomenon of electron density perturbation in the F-layer. The ionospheric irregularities structure has an adverse effect on the propagation of electromagnetic waves in the ionosphere. The automatic identification of ionospheric spread-F and statistical study of the formation of spread-F are of great significance to the study of the physical mechanism of ionospheric inhomogeneity and for prediction of ionospheric irregularities. In this paper, we describe and implement three automatic identification methods of spread-F based on machine learning: decision tree, random forest, and convolutional neural network (CNN). The performance of these automatic identification methods was verified using a large set of test data. Results show that the accuracy of all three methods on identifying ionograms with spread-F exceeded 90%. After comparing the results of the three methods, we found that the decision tree method was the simplest and with the structure easiest to be understood, and it required the shortest interpretation time. In terms of the identification results, the random forest method provided better results than the decision tree method, and the CNN method was the best at accurately identifying ionograms with spread-F.

Ionograms recorded at Puer station (PUR, 22.7°N, 101.05°E, Dip Latitude 12.9°N) in the Southwest of China from January 2015 to December 2016 were used to study characteristics of the F2 layer stratification at the northern equatorial ionization anomaly. Ionosonde observations show that the development of the F2 layer stratification is different under different conditions. Both the upward and downward movement of the F2 layer stratification could be observed. The F2 layer stratification could occur both at daytime and nighttime. The new cusp could originate from different positions on ionograms. Moreover, statistical results indicate that the F2 layer stratification occurred later in the winter than in other seasons at daytime, it occurred frequently in the local spring, and most of ionograms with the F2 layer stratification at post-midnight occurred in March and April. Our results also show that the F2 layer stratification has a correlation with solar activity.

Spread F is a commonly observed phenomenon on ionograms caused by plasma irregularities or wave-like structures in the ionosphere. In general, Spread F could induce fluctuations of amplitude and phase of radio waves which travel through the ionosphere. Therefore, investigation of Spread F could be used to not only reveal ionospheric electrodynamics process, but also have a significant engineering application. Due to a large amount of ionograms recorded by ionosondes, it is a challenge work to manually identify ionograms with Spread F to study characteristics of Spread F. Thus, much work has been devoted to automatic identification of Spread F. In the present study, a machine learning method related to decision tree was adopted to automatically identify Spread F from ionograms. First, ionograms were processed by image method and projection techniques to provide input parameters for decision tree. The output of the proposed decision tree is whether Spread F is present or not on ionograms. Then, a set of ionograms was used to construct a decision tree. At last, a set of ionograms was adopted to validate the performance of this decision tree. In this study, ionograms recorded at Puer station (Geographic latitude and longitude: 22.7°N, 101.5°E; Geomagnetic latitude: 12.8°N) in the Yunnan province were used. Results indicate that the decision tree performed well in automatic identification of Spread F on ionograms. The accuracy of automatic identification in a set of ionograms with Spread F was reached up to 89%. It inspires us to continually improve the performance of automatic identification of Spread F in the future work.

Recently, Jiang et al., (2016) reported daytime spread F at Puer station (22.7oN, 101.05oE, Dip Lat 12.9oN). As a continual work, daytime spread F at middle latitudes was investigated in this study through ionograms recorded by an ionosonde installed at Zhangye (ZHY, 39.4oN, 100.0oE, Dip Lat 29.6oN) in the Northwest of China. The ionosonde at ZHY station recorded daytime spread F three times on 6 January, 2017. The first case occurred at approximately 06:45 LT and disappeared at about 08:35 LT. The second and third cases lasted from 09:50 LT to 10:30 LT and from 11:10 LT to 11:30 LT, respectively. In addition, Swarm satellite observations were used to reveal the possible mechanism in this study. Results show that daytime spread F observed in this study might be attributed to Traveling Ionospheric Disturbances (TIDs)/atmospheric gravity waves induced by a geomagnetic storm. However, the physical processes might be different for these cases. Ionospheric instabilities produced by local F region electric field might be contributed to the first case. The second and third cases might be attributed to off-vertical reflections from wave-like ionospheric structure generated by TIDs/atmospheric gravity waves.

By using the 2002–2016 dataset of COSMIC, CHAMP and GRACE radio occultation (RO) and TIMED/TIDI, we investigate the morphology of global sporadic E (ES) layers and its relation with wind shear. The occurrence rate of ES layers depends on temporal and spatial variation. The occurrence rate of ES layers has a maximum at around 103 km in middle latitude regions in summer hemisphere. The results show that the global distribution of ES layers is relatively consistent with the wind shear distribution. The height-local time distribution of ES layers occurrences are primary modulated by diurnal and semidiurnal atmospheric tides. However, the distribution of ES layers occurrence rate cannot be explained completely according to the wind shear theory, especially at equatorial and auroral regions. More observations are required to further investigate the formation of ES layers and its relation with wind shear.

HF sky-wave backscatter sounding system is capable of measuring the large-scale, two-dimensional (2-D) distributions of ionospheric electron density. The leading edge (LE) of a backscatter ionogram (BSI) is widely used for ionospheric inversion since it is hardly affected by any factors other than ionospheric electron density. Traditional BSI inversion methods have failed to distinguish LEs associated with different ionospheric layers, and simply utilize the minimum group path of each operating frequency, which generally corresponds to the LE associated with the F2 layer. Consequently, while the inversion results can provide accurate profiles of the F region below the F2 peak, the diagnostics may not be so effective for other ionospheric layers. In order to resolve this issue, we present a new BSI inversion method using LEs associated with different layers, which can further improve the accuracy of electron density distribution, especially the profile of the ionospheric layers below the F2 region. The efficiency of the algorithm is evaluated by computing the mean and the standard deviation of the differences between inverted parameter values and true values obtained from both vertical and oblique incidence sounding. Test results clearly manifest that the method we have developed outputs more accurate electron density profiles due to improvements to acquire the profiles of the layers below the F2 region. Our study can further improve the current BSI inversion methods on the reconstruction of 2-D electron density distribution in a vertical plane aligned with the direction of sounding.

We investigate the electric field penetration of the lithosphere–atmosphere–ionosphere coupling (LAIC) problem to study abnormal seismo-ionospheric disturbance. By directly solving the LAIC electric field penetration model at the high-latitude region, we find that the additional current induced at the ground surface flows into the ionosphere completely and further generates an abnormal ionospheric electric field. Therefore, we reasonably suggest that the electric field penetration of LAIC at middle- and low-latitude regions can be solved from the perspective of the ionospheric electric field model. The current from the downward atmosphere is treated as the source term. The simulation results demonstrate the following principal findings: (a) for the high-latitude region, the horizontal electric field in the ionosphere does not change with height and the vertical electric field can be neglected; (b) for the middle- and low-latitude regions, the intensity of the total horizontal electric field increases with the latitude and the vertical electric field is more obvious at low latitudes; and (c) the penetration height of the LAIC electric field in the ionosphere is lower at low latitudes than at high latitudes. We also find that according to the diurnal change of the ionospheric conductivity, the most efficient time for electric field penetration is between 00:00 and 04:00 local time.

This study mainly investigates equatorial and low-latitude ionospheric response to a great geomagnetic storm occurred on the 17 March 2015. We found that there were some interesting ionospheric phenomena, e.g. short-term ionospheric positive effect, daytime spread F, and morning Equatorial Ionization Anomaly (EIA) in the topside ionosphere, emerged at equatorial and low-latitude region along the longitude of about 100oE. Ground-based ionosondes and in situ satellite (Swarm) were utilized to study the possible mechanisms for these ionospheric phenomena. We found that vertical downward transport of plasma or neutral induced by traveling ionospheric disturbances (TIDs) or traveling atmospheric disturbances (TADs) might make a contribution to the short-term ionospheric positive effect at the main stage of this great storm. Additionally, results suggested that the occurrence of daytime spread F at low latitudes might be due to the diffusion of equatorial ionospheric irregularities in the topside ionosphere along geomagnetic field lines. Moreover, observational evidence shows that TIDs also might be the main driver for morning EIA-like feature recorded by Swarm B satellite in the topside ionosphere. These ionospheric phenomena mentioned above could make us to gain a better understanding of ionospheric storm effects at equatorial and low-latitude region.

Electromagnetic extremely low frequency (ELF) waves play an important role in modulating the Earth’s radiation belt electron dynamics. High-frequency (HF) modulated heating of the ionosphere acts as a viable means to generate artificial ELF waves. The artificial ELF waves can reside in two different plasma regions in geo-space by propagating in the ionosphere and penetrating into the magnetosphere. As a consequence, the entire trajectory of ELF wave propagation should be considered to carefully analyze the wave radiation properties resulting from modulated ionospheric heating. We adopt a model of full wave solution to evaluate the Poynting vector of the ELF radiation field in the ionosphere, which can reflect the propagation characteristics of the radiated ELF waves along the background magnetic field and provide the initial condition of waves for ray tracing in the magnetosphere. The results indicate that the induced ELF wave energy forms a collimated beam and the center of the ELF radiation shifts obviously with respect to the ambient magnetic field with the radiation power inversely proportional to the wave frequency. The intensity of ELF wave radiation also shows a weak correlation with the size of the radiation source or its geographical location. Furthermore, the combination of ELF propagation in the ionosphere and magnetosphere is proposed on basis of the characteristics of the ELF radiation field from the upper ionospheric boundary and ray tracing simulations are implemented to reasonably calculate magnetospheric ray paths of ELF waves induced by modulated ionospheric heating.

A statistical analysis of sporadic E layer recorded from September 2011 to September 2015 at four Chinese ionospheric sounding stations of Mohe (122.37°E, 53.50°N), Beijing (116.25°E, 40.25°N), Wuhan (114.61°E, 30.53°N), and Hainan (109.13°E, 19.52°N) are presented to investigate the characteristics of sporadic E layer at the middle latitudes over China. The occurrence of the sporadic E layer in mid-latitude China region shows strong dependence on local time and season, consistent with previous studies. The occurrence of the mid-latitude sporadic E layer is prominent at local daytime in summer season. The results also reveal that the post-sunset sporadic E layer is statistically pronounced in mid-latitude China region, possibly related to the nighttime mid-latitude E region irregularities. The mid-latitude sporadic E layer is modulated by atmospheric tidal waves and planetary waves at different latitudes. The occurrence of the mid-latitude sporadic E layer also tends to increase with the level of geomagnetic activity on basis of both the statistical analysis and case study.

A software tool named ionoScaler (ionogram Scaler) was developed to carry out manual and automatic scaling of vertical incidence ionograms. Additionally, ionosonde Total Electron Content (ITEC) can be estimated by this tool through reconstructing the bottomside and topside electron density profiles of ionograms. Ionograms recorded at Puer (22.7°N, 101.05°E) and Wuhan (30.5°N, 114.3°E) were adopted to test features of the ionoScaler. Results show that the software tool performed well for scaling of ionograms and estimation of ITEC. However, bad quality ionograms would reduce the accuracy of automatic scaled (autoscaled) values. The ionoScaler presented in this paper will greatly expedite the interpretation of ionograms, and it is also an alternative method for estimation of TEC values. Moreover, this tool will be useful for research in the ionosphere and space weather.

We report observations of ionospheric disturbances in response to a North Korean underground nuclear explosion (UNE) on 3 September 2017. By using data from IGS (International GNSS Service) stations and Swarm satellites, geomagnetic conjugate ionospheric disturbances were observed. The observational evidence showed that UNE-generated ionospheric disturbances propagated radially from the UNE epicenter with a velocity of ∼280 m s−1. We propose that the ionospheric disturbances are results of electrodynamic process caused by LAIC (lithosphere–atmosphere–ionosphere coupling) electric field penetration. The LAIC electric field can also be mapped to the conjugate hemispheres along the magnetic field line and consequently cause ionospheric disturbances in conjugate regions. The UNE-generated LAIC electric field penetration plays an important role in the ionospheric disturbances in the region of the nuclear test site nearby and the corresponding geomagnetic conjugate points.

Motivated by the observations and linear theory analysis of ionospheric irregularities at Mars, we performed numerical simulations of the nonlinear evolution of the electromagnetic gradient drift instability in the Martian ionospheric dynamo region. The seeding source of ionospheric irregularities is perturbation zonal neutral wind. We found that the perturbation electric fields induced by the gradient drift instability can convect lower density plasma into higher density plasma at higher altitudes. Then, the associated perturbation magnetic field and electric field can cause the velocity shear of the plasma, which induced the Kelvin–Helmholtz instability at higher altitude. The Kelvin–Helmholtz instabilities furthermore lead to smaller-scale irregularities in plasma density, magnetic field, and electric field in the Martial ionosphere. Key points: (1) Nonlinear simulation of small-scale ionospheric irregularities at Mars was present. (2) Ionospheric irregularities at Mars can be seeded by the perturbation neutral winds. (3) Model results are comparable to linear theory analysis and satellite observations.

A local maximum value of ionospheric total electron content (TEC) in midlatitude regions, named midlatitude ionospheric peak in this study, has been frequently observed during both the daytime and nighttime. We have carried out a modeling study of midlatitude ionospheric peaks in TEC using the Thermosphere Ionosphere Electrodynamic General Circulation Model (TIEGCM) to investigate possible mechanisms driving these peaks. Comparisons between model results and Global Position System (GPS) TEC observations in the solar minimum year of 2009 show that TIEGCM can reproduce the nighttime and daytime midlatitude ionospheric peaks. Model results indicate that midlatitude ionospheric peaks in the daytime and nighttime are likely caused by different mechanisms. Nighttime midlatitude ionospheric peaks are attributed to the downwards-moving plasma ambipolar diffusive flux from the plasmasphere. However, the daytime upwards-moving plasma ambipolar diffusive flux and poleward meridional winds, which can cause ionospheric depletion, might play dominated roles in producing midlatitude ionospheric peaks in the early morning in the winter season.

Radiation belt electron flux dropouts are a kind of drastic variation in the Earth's magnetosphere, understanding of which is of both scientific and societal importance. Using electron flux data from a group of 14 satellites, we report multi-satellite simultaneous observations of magnetopause and atmospheric losses of radiation belt electrons during an event of intense solar wind dynamic pressure pulse. When the pulse occurred, magnetopause and atmospheric loss could take effect concurrently contributing to the electron flux dropout. Losses through the magnetopause were observed to be efficient and significant at L ≳ 5, owing to the magnetopause intrusion into L ∼ 6 and outward radial diffusion associated with sharp negative gradient in electron phase space density. Losses to the atmosphere were directly identified from the precipitating electron flux observations, for which pitch angle scattering by plasma waves could be mainly responsible. While the convection and substorm injections strongly enhanced the energetic electron fluxes up to hundreds of keV, they could delay other than avoid the occurrence of electron flux dropout at these energies. It is demonstrated that the pulse-time radiation belt electron flux dropout depends strongly on the specific interplanetary and magnetospheric conditions and that losses through the magnetopause and to the atmosphere and enhancements of substorm injection play an essential role in combination, which should be incorporated as a whole into future simulations for comprehending the nature of radiation belt electron flux dropouts.

This paper presents a new method to reconstruct the vertical electron density profile based on vertical Total Electron Content (TEC) using the simulated annealing algorithm. The present technique used the Quasi-parabolic segments (QPS) to model the bottomside ionosphere. The initial parameters of the ionosphere model were determined from both International Reference Ionosphere (IRI) (Bilitza et al., 2014) and vertical TEC (vTEC). Then, the simulated annealing algorithm was used to search the best-fit parameters of the ionosphere model by comparing with the GPS-TEC. The performance and robust of this technique were verified by ionosonde data. The critical frequency (foF2) and peak height (hmF2) of the F2 layer obtained from ionograms recorded at different locations and on different days were compared with those calculated by the proposed method. The analysis of results shows that the present method is inspiring for obtaining foF2 from vTEC. However, the accuracy of hmF2 needs to be improved in the future work.

A new digital low-frequency receiver system has been developed at Wuhan University for sensitive reception of low-latitude broadband Extremely Low Frequency (ELF) and Very Low Frequency (VLF) radio waves originating from either natural or artificial sources. These low-frequency radio waves are useful for ionospheric remote sensing, geospace environment monitoring, and submarine communications. This paper presents the principle and architecture of the system framework, including magnetic loop antenna design, low-noise analog front-end and digital receiver with data sampling and transmission. A new structure is adopted in the analog front end to provide high common-mode rejection and to reduce interference. On basis of field programmable gate array (FPGA) device and Universal Serial Bus (USB) architecture, the digital receiver is developed along with time keeping and synchronization module. The validity and feasibility of the self-developed ground-based ELF/VLF receiver system is evaluated by first results of experimental data that show the temporal variation of broadband ELF/VLF wave spectral intensity in Wuhan (30.54 °N, 114.37 °E). In addition to the acquisition of VLF transmitter signals at various frequencies, tweek atmospherics are also clearly captured to occur at multiple modes up to n = 6.

This is a companion study to Liang et al. (2014) which reported a “reversed” energy-latitude dispersion pattern of ion precipitation in that the lower energy ion precipitation extends to lower latitudes than the higher-energy ion precipitation. Electromagnetic ion cyclotron (EMIC) waves in the central plasma sheet (CPS) have been suggested to account for this reversed-type ion precipitation. To further investigate the association, we perform a comprehensive study of pitch angle diffusion rates induced by EMIC wave and the resultant proton loss timescales at L = 8–12 around the midnight. Comparing the proton scattering rates in the Earth's dipole field and a more realistic quiet time geomagnetic field constructed from the Tsyganenko 2001 (T01) model, we find that use of a realistic, nondipolar magnetic field model not only decreases the minimum resonant energies of CPS protons but also considerably decreases the limit of strong diffusion and changes the proton pitch angle diffusion rates. Adoption of the T01 model increases EMIC wave diffusion rates at > ~ 60° equatorial pitch angles but decreases them at small equatorial pitch angles. Pitch angle scattering coefficients of 1–10 keV protons due to H+ band EMIC waves can exceed the strong diffusion rate for both geomagnetic field models. While He+ and O+ band EMIC waves can only scatter tens of keV protons efficiently to cause a fully filled loss cone at L > 10, in the T01 magnetic field they can also cause efficient scattering of ~ keV protons in the strong diffusion limit at L > 10. The resultant proton loss timescales by EMIC waves with a nominal amplitude of 0.2 nT vary from a few hours to several days, depending on the wave band and L shell. Overall, the results demonstrate that H+ band EMIC waves, once present, can act as a major contributor to the scattering loss of a few keV protons at lower L shells in the CPS, accounting for the reversed energy-latitude dispersion pattern of proton precipitation at low energies (~ keV) on the nightside. The pitch angle coverage for H+ band EMIC wave resonant scattering strongly depends on proton energy, L shell, and field model. He+ and O+ band EMIC waves tend to cause efficient scattering loss of protons at higher energies, thereby importantly contributing to the isotropic distribution of higher energy (> ~ 10 keV) protons at higher L shells on the nightside where the geomagnetic field line is highly stretched. Our results also suggest that scattering by H+ band EMIC waves may significantly contribute to the formation of the reversed-type CPS proton precipitation on the dawnside where both the wave activity and occurrence probability is statistically high.

Powerful high-frequency (HF) radio waves can be used to efficiently modify the upper-ionospheric plasmas of the F region. The pressure gradient induced by modulated electron heating at ultralow-frequency (ULF) drives a local oscillating diamagnetic ring current source perpendicular to the ambient magnetic field, which can act as an antenna radiating ULF waves. In this paper, utilizing the HF heating model and the model of ULF wave generation and propagation, we investigate the effects of both the background ionospheric profiles at different latitudes in the daytime and nighttime ionosphere and the modulation frequency on the process of the HF modulated heating and the subsequent generation and propagation of artificial ULF waves. Firstly, based on a relation among the radiation efficiency of the ring current source, the size of the spatial distribution of the modulated electron temperature and the wavelength of ULF waves, we discuss the possibility of the effects of the background ionospheric parameters and the modulation frequency. Then the numerical simulations with both models are performed to demonstrate the prediction. Six different background parameters are used in the simulation, and they are from the International Reference Ionosphere (IRI-2012) model and the neutral atmosphere model (NRLMSISE-00), including the High Frequency Active Auroral Research Program (HAARP; 62.39° N, 145.15° W), Wuhan (30.52° N, 114.32° E) and Jicamarca (11.95° S, 76.87° W) at 02:00 and 14:00 LT. A modulation frequency sweep is also used in the simulation. Finally, by analyzing the numerical results, we come to the following conclusions: in the nighttime ionosphere, the size of the spatial distribution of the modulated electron temperature and the ground magnitude of the magnetic field of ULF wave are larger, while the propagation loss due to Joule heating is smaller compared to the daytime ionosphere; the amplitude of the electron temperature oscillation decreases with latitude in the daytime ionosphere, while it increases with latitude in the nighttime ionosphere; both the electron temperature oscillation amplitude and the ground ULF wave magnitude decreases as the modulation frequency increases; when the electron temperature oscillation is fixed as input, the radiation efficiency of the ring current source is higher in the nighttime ionosphere than in the daytime ionosphere.

The excitation of magnetospheric whistler-mode chorus in response to interplanetary (IP) shocks is investigated using wave data from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft. As an example, we show a typical chorus wave excitation following an IP shock event that was observed by THEMIS in the postnoon sector near the magnetopause on 3 August 2010. We then analyze characteristic changes during this event and perform a survey of similar events during the period 2008–2014 using the THEMIS and OMNI data set. Our statistical analysis demonstrates that the chorus wave excitation/intensification in response to IP shocks occurs only at high L shells (L > 8) on the dayside. We analyzed the variations of magnetic curvature following the arrival of the IP shock and found that IP shocks lead to more homogeneous background magnetic field configurations in the near-equatorial dayside magnetosphere; and therefore, the threshold of nonlinear chorus wave growth is likely to be reduced, favoring chorus wave generation. Our results provide the observational evidence to support the concept that the geomagnetic field line configuration plays a key role in the excitation of dayside chorus.

The ionogram autoscaling technique is very important for facilitating the statistical investigation of the ionosphere. Jiang et al. (2013) proposed an autoscaling technique for extracting ionospheric characteristics from vertical incidence ionograms. However, extensive efforts are invested in continuously improving the performance of that. The simulated annealing (SA) is used to improve the autoscaling technique in this paper. To be capable of automatic scaling of ionograms recorded at different locations, the SA is applied instead of Empirical Orthogonal Functions (EOFs) to search the best-fit parameters in the autoscaling technique. In order to validate the improvement of this autoscaling technique, ionograms recorded at Wuhan (30.5°N, 114.3°E), Puer (22.7°N, 101.05°E) and Leshan (29.6°N, 103.75°E) are investigated by comparing the autoscaled results with the values scaled by an operator. Results show that the presented work is efficient for scaling of ionograms recorded at different geographic positions. Moreover, the additional procedure can improve the accuracy of the autoscaling technique compared to results presented by Jiang et al. (2013) .

To improve our understanding of the role of electromagnetic ion cyclotron (EMIC) waves in radiation belt electron dynamics, we perform a comprehensive analysis of EMIC wave-induced resonant scattering of outer zone relativistic (>0.5 MeV) electrons and resultant electron loss time scales with respect to EMIC wave band, L shell, and wave normal angle model. The results demonstrate that while H+-band EMIC waves dominate the scattering losses of ~1–4 MeV outer zone relativistic electrons, it is He+-band and O+-band waves that prevail over the pitch angle diffusion of ultrarelativistic electrons at higher energies. Given the wave amplitude, EMIC waves at higher L shells tend to resonantly interact with a larger population of outer zone relativistic electrons and drive their pitch angle scattering more efficiently. Obliquity of EMIC waves can reduce the efficiency of wave-induced relativistic electron pitch angle scattering. Compared to the frequently adopted parallel or quasi-parallel model, use of the latitudinally varying wave normal angle model produces the largest decrease in H+-band EMIC wave scattering rates at pitch angles ~5 MeV. At a representative nominal amplitude of 1 nT, EMIC wave scattering produces the equilibrium state (i.e., the lowest normal mode under which electrons at the same energy but different pitch angles decay exponentially on the same time scale) of outer belt relativistic electrons within several to tens of minutes and the following exponential decay extending to higher pitch angles on time scales from

This paper presents a method for automatically scaling the sporadic E (Es) layer and removing its multiple reflection and backscatter echoes for vertical incidence (VI) ionograms. First, the method scales the Es layer automatically. Then, it removes the traces of multiple reflection and backscatter of the Es layer to clean VI ionograms using the parameters of the Es layer. Ionograms recorded at Wuhan (30.5°N, 114.37°E) are used to verify the performance of the proposed method compared with manually scaled values. The results indicate that the proposed method can automatically scale the Es layer and effectively improve the performance of the technique developed by Jiang et al. ( 2013 ).

The F2 layer critical frequency of the ionosphere (foF2) is one of the most significant parameters for studying the ionosphere. To investigate the large-scale characteristics of the ionosphere over particular regions, modeling foF2 is an effective method. In this paper, we use both the Kriging (KG) and neural network (NN) methods to reconstruct foF2 maps over North China. The neural network is trained by the genetic algorithm (GA) to avoid the ‘local minimum’ phenomenon in most NN applications. We then carry out a comparison between foF2 provided by both the KG and NN methods with vertical model operation of ionosonde data including Beijing, Qingdao, Suzhou, and Changchun. All of the foF2 data used in the comparison are obtained from the oblique and vertical mode operation of ionosonde from the China Ground-based Seismo-ionospheric Monitoring Network. To allow for a possible seasonal and diurnal variation, data obtained from summer, winter, and equinox months are applied in the present comparison. In addition, we make a comparison during a magnetic storm period. The results of our comparisons demonstrate that both the KG and NN methods are appropriate tools for modeling foF2 maps. However, when the data set is spare, the performance of the NN method is better than the KG method. On the other hand, the KG method is more robust than the NN method during a magnetic storm.

Fifteen months of pitch angle resolved Van Allen Probes Relativistic Electron-Proton Telescope (REPT) measurements of differential electron flux are analyzed to investigate the characteristic variability of the pitch angle distribution of radiation belt ultrarelativistic (>2MeV) electrons during storm conditions and during the long-term poststorm decay. By modeling the ultrarelativistic electron pitch angle distribution as sin(n)alpha, where alpha is the equatorial pitch angle, we examine the spatiotemporal variations of the n value. The results show that, in general, n values increase with the level of geomagnetic activity. In principle, ultrarelativistic electrons respond to geomagnetic storms by becoming more peaked at 90 degrees pitch angle with n values of 2-3 as a supportive signature of chorus acceleration outside the plasmasphere. High n values also exist inside the plasmasphere, being localized adjacent to the plasmapause and exhibiting energy dependence, which suggests a significant contribution from electromagnetic ion cyclotron (EMIC) wave scattering. During quiet periods, n values generally evolve to become small, i.e., 0-1. The slow and long-term decays of the ultrarelativistic electrons after geomagnetic storms, while prominent, produce energy and L-shell-dependent decay time scales in association with the solar and geomagnetic activity and wave-particle interaction processes. At lower L shells inside the plasmasphere, the decay time scales tau(d) for electrons at REPT energies are generally larger, varying from tens of days to hundreds of days, which can be mainly attributed to the combined effect of hiss-induced pitch angle scattering and inward radial diffusion. As L shell increases to L similar to 3.5, a narrow region exists (with a width of similar to 0.5L), where the observed ultrarelativistic electrons decay fastest, possibly resulting from efficient EMIC wave scattering. As L shell continues to increase, tau(d) generally becomes larger again, indicating an overall slower loss process by waves at high L shells. Our investigation based upon the sin(n)alpha function fitting and the estimate of decay time scale offers a convenient and useful means to evaluate the underlying physical processes that play a role in driving the acceleration and loss of ultrarelativistic electrons and to assess their relative contributions.

The solar eclipse effects on the ionosphere are very complex. Except for the ionization decay due to the decrease of the photochemical process, the couplings of matter and energy between the ionosphere and the regions above and below will introduce much more disturbances. Five ionosondes in the Northeast Asia were used to record the midlatitude ionospheric responses to the solar eclipse of 20 May 2012. The latitude dependence of the eclipse lag was studied first. The foF2 response to the eclipse became slower with increased latitude. The response of the ionosphere at the different latitudes with the same eclipse obscuration differed from each other greatly. The plasma flux from the protonsphere was possibly produced by the rapid temperature drop in the lunar shadow to make up the ionization loss. The greater downward plasma flux was generated at higher latitude with larger dip angle and delayed the ionospheric response later. The waves in the foEs and the plasma frequency at the fixed height in the F layer are studied by the time period analytic method. The gravity waves of 43–51 min center period during and after the solar eclipse were found over Jeju and I-Cheon. The northward group velocity component of the gravity waves was estimated as ~108.7 m/s. The vertical group velocities between 100 and 150 km height over the two stations were calculated as ~5 and ~4.3 m/s upward respectively, indicating that the eclipse-induced gravity waves propagated from below the ionosphere.

Many observational results have shown electromagnetic abnormality in the ionosphere before large earthquakes. The theoretical simulation can help us to understand the internal mechanism of these anomalous electromagnetic signals resulted from seismic regions. In this paper, the horizontal and vertical components of electric and magnetic field at the topside ionosphere are simulated by using the full wave method that is based on an improved transfer matrix method in the lossy anisotropic horizontally stratified ionosphere. Taken account into two earthquakes with electric field perturbations recorded by the DEMETER satellite, the numerical results reveal that the propagation and penetration of ULF (ultra-low-frequency) electromagnetic waves into the ionosphere is related to the spatial distribution of electron and ion densities at different time and locations, in which the ion density has less effect than electron density on the field intensity. Compared with different frequency signals, the minimum values of electric and magnetic field excited by earthquakes can be detected by satellite in current detection capability have also been calculated, and the lower frequency wave can be detected easier.

Wuhan atmosphere radio exploration (WARE) radar is the first mesosphere-stratosphere-troposphere radar to have become operative in the mainland of China and is dedicated to real-time atmospheric observations. Based on the WARE radar data collected for the period from September 2011 to February 2013, 2666 downward and 1735 upward inertia gravity waves (IGWs) are identified from three-dimensional (3-D) wind fields observed in the troposphere and subsequently analyzed in a statistical manner. Wave characteristics including intrinsic frequencies, vertical wavelengths, horizontal wavelengths, vertical wave number spectra, energy density spectra, and wave sources are investigated using a combination of the Lomb-Scargle spectral analysis, the quasi-monochromatic gravity waves model, and the hodograph method. Our results demonstrate that the characteristic parameters of upward and downward tropospheric IGWs are not significantly different. These results indicate that the tropospheric IGWs parameters are not directly correlated with propagation directions. Combining with the information of statistical 3-D wind field and some climatic characteristics of Hubei Province, atmospheric moist convection will contribute most in summer, whereas jet/front systems will contribute most in winter. One may expect seasonal variations to be tied to the varying importance of these sources.

Vertical incidence ionograms indicate ionospheric characteristics over the ionosonde station, from which electron density profiles can be derived. This paper describes a method for the automatic calculation of electron density profiles from vertical incidence ionograms. First, the method calculates the initial parameters of the quasi-parabolic segments (QPS) model by using the International Reference Ionosphere (IRI) model, the Nequick2 model, image processing techniques, and the Empirical Orthogonal Function (EOF). Once the initial parameters have been calculated the method then adjusts those to obtain the electron density profiles and synthesized traces that match the recorded ionograms. The algorithm then selects the best-fit synthesized trace and corresponding parameters as output from the candidate ones. Furthermore, the corresponding electron density profile of the recorded ionogram is calculated by using the best-fit parameters of the QPS model. To further test the feasibility of the proposed method, we apply it to some ionograms that were recorded at Wuhan during different seasons. As a result, our results demonstrate that the proposed method is feasible for the automatic calculation of electron density profiles.

[1] The solar eclipse on 15 January 2010 traversed Asia and completed its travel on the Shandong Peninsula in China at sunset. Two vertical incidence ionosondes at Wuhan and Beijing and the oblique incidence ionosonde network in North China were implemented to record the ionospheric response to the solar eclipse. Following the initial electron density decrease caused by the eclipse, the ionosphere was characterized by a strong premidnight enhancement, and a subsequent ionospheric decay, and a ~10 h later postmidnight enhancement. Neither geomagnetic disturbance occurred during the eclipse day nor did obvious nighttime peak appear for the 10 day mean of the F2-layer critical frequency (foF2). The electron density profilogram of the Beijing ionosonde indicates that the two enhancements were the result of the plasma flux downward from the top ionosphere, possibly due to the steep decrease of the ionospheric electron density and plasma temperature during the solar eclipse. The two-dimensional differential foF2 maps present the regional variations of the nighttime electron density peaks and decay. Both the pre- and postmidnight enhancements initially appeared in a belt almost in parallel with the eclipse track and then drifted southward. The different magnitudes of greatest eclipse in the umbra and outside tend to account for the different occurrence times of the plasma flux. The ionospheric decay following the premidnight enhancement is also considered as a consequence of the eclipse shade.

[1] A real-time mapping model of foF2 in northern China was established using neural networks (NNs). To avoid the local minimum problem associated with traditional NNs, a newly improved genetic algorithm-based NN (GA-NN) was developed using the input parameters of solar activities, geomagnetic activities, neutral winds, seasonal information, and geographical coordinates. The foF2 data were extracted by inversing the oblique ionograms obtained from the oblique ionosondes of the China Ground-based Seismo-ionospheric Monitoring Network every 30 min for the period from August 2009 to December 2011. The data associated with five transmitter stations (Beijing, Changchun, Qingdao, Xinxiang, and Suzhou) and one receiver station in Binzhou were considered the input parameters for the real-time foF2 mapping model, and the data from the Dalian and Jinyang transmitter stations were used to verify the results. The Jining transmitter station data were used to test the capability of the model. The root-mean-square error and percent deviation were calculated to estimate the performance of the model. The correlation coefficient was used to evaluate the correlation of observed and predicted values. In addition, observations of foF2 from the vertical ionosondes at Beijing, Changchun, Qingdao, and Suzhou stations are compared with the model prediction of foF2. The results indicate that the developed real-time foF2 mapping model based upon genetic algorithm-based NN is very promising for ionospheric studies.

A regional model for the ionospheric critical frequency of the F2 layer (foF2) over China is developed by neural networks (NNs) trained by the genetic algorithm (GA). In order to avoid the ‘local minimum’ phenomena in most NN applications, GA is utilized here to optimize the initial weights of NNs. The input parameters used in this GA-NN based foF2 prediction model include Beijing time (BJT, GMT+8), day number (day of the year), seasonal information, solar cycle information, magnetic activity, magnetic declination, magnetic dip angle, angle of meridian relative to sub-solar point, solar zenith angle, and geographic coordinates. The foF2 datasets employed in this model are obtained from nine ionosonde stations located in China for the time span of 1990–2004 that covers one entire sunspot cycle. The datasets for 1996 and 2000 are selected for validation instead of for training use. Prediction results of GA-NN model, unimproved NN model, and International Reference Ionosphere 2007 (IRI2007) model (from International Radio Consultative Committee (CCIR) coefficient) are compared with the observation data for the year of 1996 and 2000 respectively. The results indicate that GA-NN model is superior to the unimproved NN model and the IRI2007 model for foF2 prediction. According to the statistical analysis of average RMSE, the GA-NN method offers an improvement of 4.89% over NN method and an improvement of 27.79% over IRI2007 model. The improvement of accuracy for one single station forecasting is validated with the data from the Wuhan ionosonde station both at 00:00 (BJT) and 12:00 (BJT) in 1996 (solar minimum) and 2000 (solar maximum).

[1] As an essential concept of resonant wave-particle interactions, the strong diffusion limit DSD is an important variable to explore the efficiency of wave-induced pitch angle scattering for particle precipitation loss to the atmosphere. Determined by the size of equatorial loss cone on a given field line and the bounce period at a given energy, the value of DSD sets a lower limit to the precipitation timescale for loss cone filling, regardless of the strength of wave-particle interactions. However, no efforts have ever been made to evaluate DSD in the realistic magnetosphere considering the impact of various geomagnetic activities. To perform a systematic exploration of the dependence of DSD on geomagnetic condition, spatial location, and global magnetic field model, we have numerically computed DSD using the dipolar and non-dipolar Tsyganenko magnetic field models under three representative (quiet, moderate, and active) geomagnetic conditions. Use of more realistic Tsyganenko magnetic field models introduces non-negligible or considerable differences in DSD magnitude from that obtained using a dipolar field. The difference can be over an order of magnitude at the field lines with equatorial crossings ≥6 Re during geomagnetically disturbed times. We also report that in the realistic magnetosphere both DSD magnitude and its variations have a strong dependence on the spatial location. Computed DSD shows the maximum tending to occur on the dayside (MLT = 12 and 16) and the minimum DSD more likely to occur at MLT = 00. Compared to the dipolar results, largest deviation in DSD occurs for MLT = 00, 04, and 20, while DSD variations on the dayside are relatively small. Our results demonstrate that accurate evaluation of DSD besides scattering rates in the realistic magnetosphere, especially at high spatial locations and under geomagnetically disturbed conditions for which a dipolar approximation fails, can make an important contribution to quantifying the wave effect on particle resonant diffusion, which should be incorporated into future modeling efforts for comprehending the role of resonant wave-particle interactions and the dynamics of magnetospheric electrons under a variety of geomagnetic conditions.

In this paper, we present results of parametric instability induced by X-mode wave heating observed by EISCAT (European Incoherent Scatter Scientific Association) radar at Tromso, Norway. Three typical X-mode ionospheric heating experiments on October 22, 2013, October 19, 2012 and February 21, 2013 are investigated in details. Both parametric decay instability (PDI) and oscillating two-stream instability (OTSI) are observed during the X-mode heating period. We suggest that the full dispersion relationship of the Langmuir wave can be employed to analyze the X-mode parametric instability excitation. Parallel electric field component of X-mode heating wave can also exceed the parametric instability excitation threshold under certain conditions.

Using the electron flux measurements obtained from five satellites (GOES-15 and POES 15, 16, 18, and 19), we investigate the flux variations of radiation belt electrons during forty solar wind dynamic pressure pulses identified between September 2012 and December 2014. By utilizing the mean duration of the pressure pulses as the epoch timeline and stretching or compressing the time phases of individual events to normalize the duration by means of linear interpolation, we have performed normalized superposed epoch analysis to evaluate the dynamic responses of radiation belt energetic electrons corresponding to various groups of solar wind and magnetospheric conditions in association with solar wind dynamic pressure pulses. Our results indicate that by adopting the timeline normalization we can reproduce the typical response of the electron radiation belts to pressure pulses. Radiation belt electron fluxes exhibit large depletions right after the Pdyn peak during the periods of northward IMF Bz and are more likely to occur during the Pdyn pulse under southward IMF Bz conditions. For the pulse events with large negative values of (Dst)min, radiation belt electrons respond in a manner similar to those with southward IMF Bz, and the corresponding post-pulse recovery can extend to L ~ 3 and exceed the pre-pulse flux levels. Triggered by the solar wind pressure enhancements, deeper earthward magnetopause erosion provides favorable conditions for the prompt electron flux dropouts that extend down to L ~ 5, and the pressure pulses with longer duration tend to produce quicker and stronger electron flux decay. In addition, the events with high electron fluxes before the Pdyn pulse tend to experience more severe electron flux dropouts during the course of the pulse, while the largest rate of electron flux increase before and after the pulse occurs under the pre-conditioned low electron fluxes. These new results help us understand how electron fluxes respond to solar wind dynamic pressure pulses and how these responses depend on the solar wind and geomagnetic conditions and on the preconditions in the electron radiation belts.

The high frequency management system with backscatter radar supplies the real time ionosphere channel conditions to high frequency users, which leads to the demand for the ground range between the radar location and the scatters on the distant ground. The ionosphere electron density profile is usually inversed to obtain the ground range. An inversion algorithm, with which the ground range on the leading edge of the backscatter ionograms can be obtained without electron density, is presented in this paper. The ray path geometry of the backscatter sounding and the change in the group path on the leading edge with operating frequency are used to derive the ground range. Synthesized backscatter ionogram and experimental backscatter ionograms are processed to validate the algorithm. The results indicate that the algorithm is usable for high frequency management system.

Spread F on ionograms has been considered to be a phenomenon mainly occurred at nighttime. This study presented a case study of daytime spread F observed by the ionosonde installed at Puer (PUR; 22.7°N, 101.05°E; dip latitude 12.9°N), where daytime spread F that lasted for more than 2 h (about 08:30 LT~10:45 LT) was observed on 14 November 2015. To investigate the possible mechanism, ionograms recorded at PUR and Chiang Mai (18.76°N, 98.93°E; dip latitude 9.04°N) were used in this study. We found that traveling ionospheric disturbances were observed before the occurrence of daytime spread F. Meanwhile, the movement of the peak height of the ionosphere was downward. We suggested that downward vertical neutral winds excited by traveling atmospheric disturbances/atmospheric gravity waves might play a significant role in forming daytime spread F over PUR during geomagnetic storms.

An exceptionally long total solar eclipse occurred over the Yangtze River Basin in the mid-latitudes of China on 22 July 2009. The moon’s umbral shadow crossed through the ionospheric equatorial anomaly region. During the solar eclipse, new ionospheric behaviors were observed using a multi-station sounding approach. These new phenomena include: (1) visible Doppler spreading of F layer echoes at multiple group distances during the solar eclipse period, (2) strong ionospheric response near the peak of the northern equatorial anomaly crest and (3) synchronous oscillations in the E s and F layer during the recovery phase of the solar eclipse.