EISCAT Observations of Main Ionization Troughs in the High-Latitude Ionosphere

F-region electron density depletions associated with main ionization troughs in the high-latitude ionosphere are studied using EISCAT CP3 data of meridian scanning experiments. The troughs in our observations are found to appear mainly in dusk sector, extending from late afternoon to pre-midnight, with higher occurrence rate during equinox and winter. Simultaneous ion drift velocity inF-region shows that the main trough minimum is mostly located at the equator ward edge of the plasma convection flow, rather than in the region where the largest ion flow are observed.

Accurate quantification of the algebraic,multiplicative algebraic,and simultaneous iterative reconstruction techniques in ionosphere rebuilding based on the TIEGCM assessment

The algebraic reconstruction technique(ART),multiplicative algebraic reconstruction technique(MART),and simultaneous iterative reconstruction technique(SIRT)are computational methodologies extensively utilized within the field of computerized ionospheric tomography(CIT)to facilitate three-dimensional reconstruction of the ionospheric morphology.However,reconstruction accuracy elicits recurrent disputes over its practical application,and people usually attribute this issue to incomplete and uneven coverage of the measurements.The Thermosphere Ionosphere Electrodynamics General Circulation Model(TIEGCM)offers a reasonable physics-based ionospheric background and is widely utilized in ionospheric research.We use the TIEGCM simulations as the targeted ionosphere because the current measurements are far from able to realistically reproduce the ionosphere in detail.Optimized designations of satellite measurements are conducted to investigate the limiting performance of CIT methods in ionospheric reconstruction.Similar to common practice,electron density distributions from outputs of the International Reference Ionosphere(IRI)model are used as the iterative initial value in CIT applications.The outcomes suggest that despite data coverage,iterative initial conditions also play an essential role in ionospheric reconstruction.In particular,in the longitudinal sectors where the iterative initial height of the F2-layer peak electron density(hmF2)differs substantially from the background densities,none of the three CIT methods can reproduce the exact background profile.When hmF2 is close but the ionospheric F2-layer peak density(NmF2)is different between the targeted background and initial conditions,the MART performs better than the ART and SIRT,as evidenced by the correlation coefficients of MART being above 0.97 and those of ART and SIRT being below 0.85.In summary,this investigation reveals the potential uncertainties in traditional CIT reconstruction,particularly when realistic hmF2 or NmF2 values differ substantially from the initial CIT conditions.

Model Simulation of Artificial Heating of the Daytime High-Latitude F-Region Ionosphere by Powerful High-Frequency Radio Waves

The large-scale disturbance of the spatial structure of the daytime high-latitude F-region ionosphere, caused by powerful high-frequency radio waves, pumped into the ionosphere by a groundbased ionospheric heater, is studied with the help of the numerical simulation. The mathematical model of the high-latitude ionosphere, developed earlier in the Polar Geophysical Institute, is utilized. The mathematical model takes into account the drift of the ionospheric plasma, strong magnetization of the plasma at F-layer altitudes, geomagnetic field declination, and effect of powerful high-frequency radio waves. The distributions of the ionospheric parameters were calculated on condition that an ionospheric heater, situated at the point with geographic coordinates of the HF heating facility near Tromso, Scandinavia, has been operated, with the ionospheric heater being located on the day side of the Earth. The results of the numerical simulation indicate that artificial heating of the ionosphere by powerful high-frequency waves ought to influence noticeably on the spatial structure of the daytime high-latitude F-region ionosphere in the vicinity of the ionospheric heater.

Comparison of Diurnal,Seasonal and Solar Cycle Variations of High-latitude,Mid-latitude and Low-latitude Ionosphere

Comparison of regular(diurnal,seasonal and solar cycle)variations of high-latitude,mid-latitude and low-latitude ionospheric characteristics has been provided on basis of local empirical models of the peak electron density and the peak height.The local empirical models were derived from the hand-scaled ionogram data recorded by DPS-4 digisondes located at Norilsk(69°N,88°E),Irkutsk(52°N,104°E)and Hainan(19°N,109°E)for a 6-year period from December,2002 to December,2008.The technique used to build the local empirical model is described.The primary focus is diurnal-seasonal behavior under low solar activity and its change with increasing solar activity.Both common and specific features of the high-latitude(Norilsk),mid-latitude(Irkutsk)and low-latitude(Hainan)regular variations were revealed using their local empirical models.

The high-latitude ionospheric phenomena observed by DPS-4 at Zhongshan Station, Antarctica

Ionograms, which were obtained from February 1995 to January 1999 by a Digisonde Portable Sounder-4 (DPS-4) at Zhongshan Station in Antarctica, have been scaled. The ionograms interpretation has shown various ionosphcric phcnomena occurred in the cusp region. They are particle precipitation effects in the E region. F region magnetic noon phenomena, slant Es phenomena and lacuna. F layer irregularity zone (FLIZ) phenomena, and auroral oval identification. Typical examples of ionograms and ionogram sequences observed at Zhongshan Station are displayed in this paper.

Assessment of International GNSS Service Global Ionosphere Map products over China region based on measurements from the Crustal Movement Observation Network of China

The global ionosphere maps(GIM)provided by the International GNSS Service(IGS)are extensively utilized for ionospheric morphology monitoring,scientific research,and practical application.Assessing the credibility of GIM products in data-sparse regions is of paramount importance.In this study,measurements from the Crustal Movement Observation Network of China(CMONOC)are leveraged to evaluate the suitability of IGS-GIM products over China region in 2013-2014.The indices of mean error(ME),root mean square error(RMSE),and normalized RMSE(NRMSE)are then utilized to quantify the accuracy of IGS-GIM products.Results revealed distinct local time and latitudinal dependencies in IGS-GIM errors,with substantially high errors at nighttime(NRMSE:39%)and above 40°latitude(NRMSE:49%).Seasonal differences also emerged,with larger equinoctial deviations(NRMSE:33.5%)compared with summer(20%).A preliminary analysis implied that the irregular assimilation of sparse IGS observations,compounded by China’s distinct geomagnetic topology,may manifest as error variations.These results suggest that modeling based solely on IGS-GIM observations engenders inadequate representations across China and that a thorough examination would proffer the necessary foundation for advancing regional total electron content(TEC)constructions.

The anisotropy of suprathermal electrons in the Martian ionosphere

Suprathermal electrons are an important population of the Martian ionosphere, either produced by photoionization of atmospheric neutrals or supplied from the Solar Wind (SW). This study is dedicated to an in-depth investigation of the pitch angle distribution of suprathermal electrons at two representative energies, 19−55 eV and 124−356 eV, using the extensive measurements made by the Solar Wind Electron Analyzer on board the Mars Atmosphere and Volatile Evolution. Throughout the study, we focus on the overall degree of anisotropy, defined as the standard deviation of suprathermal electron intensity among different directions which is normalized by the mean omni-directional intensity. The available data reveal the following characteristics: (1) In general, low energy electrons are more isotropic than high energy electrons, and dayside electrons are more isotropic than nightside electrons;(2) On the dayside, the anisotropy increases with increasing altitude at low energies but remains roughly constant at high energies, whereas on the nightside, the anisotropy decreases with increasing altitude at all energies;(3) Electrons tend to be more isotropic in strongly magnetized regions than in weakly magnetized regions, especially on the nightside. These observations indicate that the anisotropy is a useful diagnostic of suprathermal electron transport, for which the conversion between the parallel and perpendicular momenta as required by the conservation of the first adiabatic invariant, along with the atmospheric absorption at low altitudes, are two crucial factors modulating the observed variation of the anisotropy. Our analysis also highlights the different roles on the observed anisotropy exerted by suprathermal electrons of different origins.

The nature of electron density enhancement over a wide altitude range during ionosphere heating experiments at EISCAT

During the course of ionospheric heating experiments, researchers at the European Incoherent Scatter Scientific Association (EISCAT) observed an apparent electron density enhancement. The enhancement extended over a wide range of altitudes, above the reflection altitude of the high-frequency pump wave. However, whether this enhancement actually corresponds to a true enhancement in electron density remains an open question. When the dispersion relation of ion acoustic waves is followed, the frequency ratio of the enhanced ion line to the background ion line suggests that the profile of the effective ion mass may have remained unchanged. Furthermore, the solar radio flux and ion drift velocity indicate no significant changes in the ion species and their densities. In conclusion, the electron density enhancement observed at EISCAT should not, in fact, be considered a true enhancement.

Statistical characteristics and classification of ionospheric mid-latitude trough as revealed by the observations of DMSP-F18

Statistical characteristics and the classification of the topside ionospheric mid-latitude trough are systemically analyzed,using observations from the Defense Meteorological Satellite Program F18(DMSP-F18)satellite.The data was obtained at an altitude of around 860 km in near polar orbit,throughout 2013.Our study identified the auroral boundary based on the in-situ electron density and electron spectrum,allowing us to precisely determine the location of the mid-latitude trough.This differs from most previous works,which only use Total Electron Content(TEC)or in-situ electron density.In our study,the troughs exhibited a higher occurrence rate in local winter than in summer,and extended to lower latitudes with increasing geomagnetic activity.It was found that the ionospheric mid-latitude trough,which is associated with temperature changes or enhanced ion drift,exhibited distinct characteristics.Specifically,the ionospheric mid-latitude troughs related to electron temperature(Te)peak were located more equatorward of auroral oval boundary in winter than in summer.The ionospheric mid-latitude troughs related to Te-maximum were less frequently observed at 60−70°S magnetic latitude and 90−240°E longitude.Furthermore,the troughs related to ion temperature(Ti)maximums were observed at relatively higher latitudes,occurring more frequently in winter.In addition,the troughs related to ion velocity(Vi)maximums could be observed in all seasons.The troughs with the maximum-Ti and maximum-Vi were located closer to the equatorward boundary of the auroral oval at the nightside,and in both hemispheres.This implies that enhanced ion drift velocity contributes to increased collisional frictional heating and enhanced ion temperatures,resulting in a density depletion within the trough region.

Comparative analysis of empirical and deep learning models for ionospheric sporadic E layer prediction

Sporadic E(Es)layers in the ionosphere are characterized by intense plasma irregularities in the E region at altitudes of 90-130 km.Because they can significantly influence radio communications and navigation systems,accurate forecasting of Es layers is crucial for ensuring the precision and dependability of navigation satellite systems.In this study,we present Es predictions made by an empirical model and by a deep learning model,and analyze their differences comprehensively by comparing the model predictions to satellite RO measurements and ground-based ionosonde observations.The deep learning model exhibited significantly better performance,as indicated by its high coefficient of correlation(r=0.87)with RO observations and predictions,than did the empirical model(r=0.53).This study highlights the importance of integrating artificial intelligence technology into ionosphere modelling generally,and into predicting Es layer occurrences and characteristics,in particular.

The earthquake-related disturbances in ionosphere and project of the first China seismo-electromagnetic satellite

Based on the case studies and statistical analysis of earthquake-related ionospheric disturbances mainly from DEMETER satellite, ground-based GPS and ionosounding data, this paper summarizes the statisw tical characteristics of earthquake-related ionospheric disturbances, including electromagnetic emissions, plasma perturbations and variation of energetic particle flux. According to the main results done by Chinese scientists, fusing with the existed study from global researches, seismo-ionospheric disturbances usually occurred a few days or hours before earthquake occurrence. Paralleling to these case studies, lithosphere-atmosphere-ionosphere （LAI） coupling mechanisms are checked and optimized. A thermo-electric model was proposed to explain the seismo-electromagnetic effects before earthquakes. A propagation model was put forward to explain the electromagnetic waves into the ionosphere. According to the requirement of earthquake prediction research, China seismo-electromagnetic satellite, the first space-based platform of Chinese earthquake stereoscopic observation system, is proposed and planned to launch at about the end of 2014. It focuses on checking the LAI model and distinguishing earthquake-related ionospheric disturbance. The preliminary design for the satellite will adopt CAST-2000 platform with eight payloads onboard. It is believed that the satellite will work together with the ground monitoring network to improve the capability to capture seismo-electromagnetic information, which is beneficial for earthquake monitoring and prediction researches.

Penetration characteristics of VLF wave from atmosphere into lower ionosphere

The factors affecting the reflection and transmission coefficient of the ionosphere have been analyzed.These factors include wave frequency,incident angle,geomagnetic inclination,electron density and collision frequency in the ionosphere.The ionosphere refractive index is also analyzed.The ionosphere above 70 km is considered to be homogeneous and anisotropic,and the reflection and transmission coefficient matrix is calculated using matrix method.Simultaneously the Booker quartic equation is solved to get the refractive index in the ionosphere.The results show that when the wave frequency is higher,it is easier to penetrate into the ionosphere from its bottom boundary and the propagation attenuation in the ionosphere is smaller.TE（traverse electric） wave and TM（traverse magnetic） wave can both penetrate into the ionosphere with a small incident angle,while TE wave can hardly transmit into the ionosphere when the incident angle is large.The transmission coefficient decreases as the geomagnetic inclination increases.TE and TM wave cannot penetrate into the ionosphere at magnetic equator.When the electron collision frequency is higher,it is easier for VLF wave to penetrate into the ionosphere and the attenuation of ordinary wave is weaker,which may be caused by the energy transportation between the waves and the particles.The ordinary（O） wave experiences severer attenuation than extraordinary（X） wave,and X wave is a penetration mode whereas O wave is a non-penetration mode in the ionosphere.All the results indicate that VLF wave with higher frequency is easier to penetrate into the ionosphere and to be recorded by the satellites at high latitude.It is hard for ULF and the lower frequency VLF wave to transmit into the ionosphere directly for the severe reflection and attenuation.It may transmit into the ionosphere with a small incident angle due to the nonlinear effect,for example,the interaction between the waves and the particles or cross modulation,and then propagate along the whistle duct with small attenuation.This work may be a preliminary theoretical exploration for the future calculation on the response of ground based VLF artificial transmitter in the ionosphere and further study on the seismic ionosphere coupling model.

Novel adaptive Hatch filter to mitigate the effects of ionosphere and multipath on LAAS

It has been proven that carrier smoothing and differential global positioning system （DGPS） are effective to improve the accuracy of pseudorange by reducing the noise in it and eliminating almost all the common mode errors between the ground station and user. However, another issue coming with local area augmentation system （LAAS） is how to find an adaptive smoothing window width to minimize the error on account of ionosphere delay and multipath. Based on the errors analysis in carrier smoothing process, a novel algorithm is formulated to design adaptive Hatch filter whose smoothing window width flexibly varies with the characteristic of ionosphere delay and multipath in the differential carrier smoothing process. By conducting the simulation in LAAS and after compared with traditional Hatch filers, it reveals that not only the accuracy of differential correction, but also the accuracy and the robustness of positioning results are significantly improved by using the designed adaptive Hatch filter.

Online computation of International Reference Ionosphere Extended to Plasmasphere(IRI-Plas) model for space weather

Ionosphere is the most challenging part of Space Weather with its spatio-temporal variability and dispersive nature. Ionospheric models are very important in reducing positioning error in GNSS system.International Reference Ionosphere(IRI) is an empirical, deterministic and climatic model of ionosphere up to 2000 km in height. Recently, IRI Extended to Plasmasphere(IRI-Plas) model has been developed to extend the interest region of IRI to the GPS orbital height of 20,000 km. Both IRI and IRI-Plas provide ionospheric parameters such as electron density, electron and ion temperatures according to their height profiles. In order to update the model to current ionospheric conditions, IRI-Plas can input F2 layer critical frequency(foF2), maximum ionization height(hmF2), and also Total Electron Content(TEC).Online IRI-Plas is developed for the ionospheric community to run multiple tasks at various locations,dates and times with optional foF2, hmF2 and TEC inputs in a user-friendly manner. In this paper, we are going to present the capabilities of the Online IRI-Plas service and provide some comparisons between IRI-Plas outputs and ionosonde measurements. The comparison between online IRI-Plas foF2 outputs and ionosonde foF2 measurements indicates that the model with TEC input can significantly improve the representation of the current ionospheric state, which is very successful especially in the geomagnetically disturbed days.

Variability of the Martian ionosphere from the MAVEN Radio Occultation Science Experiment

The Martian ionosphere is produced by a number of controlling processes, including solar extreme ultraviolet radiation (EUV) and X-ray ionization, impact ionization by precipitating electrons, and day-to-night transport. This study investigates the structural variability of the Martian ionosphere with the aid of the radio occultation (RO) experiments made on board the recent Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. On the dayside, the RO electron density profiles are described by the superposition of two Chapman models, representing the contributions from both the primary layer and the low-altitude secondary layer. The inferred subsolar peak electron densities and altitudes are 1.24×10^5 cm^-3 and 127 km for the former, and 4.28×10^4 cm^-3 and 97 km for the latter, respectively, in general agreement with previous results appropriate for the low solar activity conditions. Our results strengthen the role of solar EUV and X-ray ionization as the driving source of plasma on the dayside of Mars. Beyond the terminator, a systematic decline in ionospheric total electron content is revealed by the MAVEN RO measurements made from the terminator crossing up to a solar zenith angle of 120°. Such a trend is indicative of day-to-night plasma transport as an important source for the nightside Martian ionosphere.

Temporal distribution characteristics of GNSS ionospheric occultation data and its effects in earthquake-ionosphere anomaly detection

The temporal distribution characteristics of COSMIC occultation data are analyzed in detail, and the limitations in earthquake-ionosphere anomaly detection caused by the temporal distribution characteristics of COSMIC occultation data are discussed using the example of the Wenchuan earthquake. The results demonstrate that there is no fixed temporal resolution for COSMIC occultation data when compared with other ionospheric observation techniques. Therefore, occultation data cannot currently be independently utilized in research studies but can only be used as a complement to other ionospheric observation techniques for applications with high temporal resolution demands, such as earthquake-ionosphere anomaly detection.

Manifestations of the earthquake preparations in the ionosphere total electron content variations

Recent ionospheric observations report anomalous total electron content (TEC) deviations prior strong earthquakes. We discuss common fetures of the pre-earthquake TEC disturbances on the basis of statistics covering 50 strong seismic events during 2005-2006. The F2-layer ionospheric plasma drift under action of the electric fields of seismic origin is proposed as the main reason of producing TEC anomalies. The origin of such electric fields is discussed in terms of the lithosphere-atmosphere-ionosphere coupling system. This theory is supported by numerical simulations using global Upper Atmosphere Model (UAM). UAM calculations show that the vertical electric current with the density of about 20 - 40 nA/m2 flowing between the Earth and ionosphere over an area of about 200 by 2000 km is required to produce the TEC disturbances with the amplitude of about 30% - 50% relatively to the non-disturbed conditions. Ionosphere responses on the variations of the latitudinal position, direction and configuration of the vertical electric currents have been investigated. We show that not only the vertical component of the ionospheric plasma drift but also horizontal components play an important role in producing pre-earthquake TEC disturbances.

An Introduction to Magnetosphere-Ionosphere-Thermosphere Coupling Small Satellite Constellation

A future Chinese mission is introduced to study the coupling between magnetosphere,ionosphere and thermosphere,i.e.the Magnetosphere-Ionosphere-Thermosphere Coupling Small Satellite Constellation(MIT).The scientific objective of the mission is to focus on the outflow ions from the ionosphere to the magnetosphere.The constellation is planning to be composed of four small satellites;each small satellite has its own orbit and crosses the polar region at nearly the same time but at different altitude.The payloads onboard include particle detectors,electromagnetic payloads,auroral imagers and neutral atom imagers.With these payloads,the mission will be able to investigate acceleration mechanism of the upflow ions at different altitudes.Currently the orbits have been determined and prototypes of some have also been completed.Competition for next phase selection is scheduled in late 2015.

Propagation characteristics of power line harmonic radiation in the ionosphere

Theoretical model and solutions on power line harmonic radiation （PLHR） propagating in the ground, air, and anisotropic homogeneous ionosphere are presented, The theoretical model is verified by the PLHR events observed by the DEMETER satellite. Some propagation characteristics of PLHR based on the model are obtained. This paper is bene- ficial to quantitatively interpret the formation mechanism of PLHR phenomenon.

Results of Ionospheric Heating Experiments Involving an Enhancement in Electron Density in the High Latitude Ionosphere

Observations are presented of the phenomenon of the enhancement in electron density and temperature that is caused by a powerful pump wave at a frequency near the fifth gyrofrequency. The observations show that the apparent enhancement in electron density extending over a wide altitude range and the enhancement in electron temperature around the reflection altitude occur as a function of pump frequency. Additionally, the plasma line spectra show unusual behavior as a function of pump frequency. In conclusion, the upper hybrid wave resonance excited by the pump wave plays a dominating role and leads to the enhancement in electron temperature at the upper hybrid altitude. The phenomenon of apparent enhancement in electron density does not correspond to the true enhancement in electron density, this may be due to some mechanism that preferentially involves the plasma transport process and leads to the strong backscatter of radar wave along the magnetic line, which remains to be determined.