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The daytime average zonal electric field is well known to be eastward in the equatorial ionosphere. Geomagnetic observations have revealed that the equatorial electrojet or zonal electric field occasionally becomes westward in the afternoon sector during quiet times. We present the first analysis of the dependence of the afternoon downward plasma drifts on the season, longitude, solar activity, and lunar phase using the ROCSAT‐1 satellite observations during 1999–2004. The results show that there are ~20% of downward plasma drifts at 13–17 LT. The occurrence rate of the downward plasma drift shows a peak around January and a secondary peak in July. The occurrence rate presents a slight decrease trend with the solar activity. Especially, the occurrence rate can attain ~40–50% around 60°E in June solstice and 145–245°E in December solstice, which is nearly comparable with the occurrence rate of the upward plasma drifts in these longitudes. The downward plasma drift occurs most frequently around the full moon in the afternoon sector. The geomagnetic declination induces the hemispheric differences in the dynamo current in the conjugate regions and controls the occurrence of the westward electric fields. A wavenumber‐4 longitudinal structure also appears to be present in the occurrence rate in June–September, and the wavenumber‐4 peak coincides with the diurnal eastward tidal wind, which drives the generation of the westward electric fields. Key Points: It is the first time to identify the occurrence rate of the upward and downward plasma drifts in the afternoon equatorial ionosphereThe westward electric field shows a comparable occurrence with the eastward electric field in the wide longitudesThe geomagnetic declination and atmospheric tides affect the occurrence of the westward electric field [ABSTRACT FROM AUTHOR]

The ionosphere is very active and complex due to photo‐ionization from the solar activity, while traditional empirical models can only give a rough description of its actual variations. Nowadays, global ionosphere maps (GIMs) derived from denser Global Navigation Satellite Systems (GNSS) world‐tracking data provide an excellent total electron content (TEC) data set for global ionospheric research and modeling. In this paper, long‐tern variations of 16‐year (2003–2018) TEC time series from GIMs are investigated by using the principal mode analysis (PCA) technique. We analyze the resulting modes in the time‐spectral domain and parameterize the main contributions in terms of solar and magnetospheric forcing, local solar time (LST), and annual variations. The results show that the TEC variability is strongly dependent on the geographical location of the Earth's magnetic field, and the Earth's diurnal rotation modulates its spatial patterns of variability. The latitudinal asymmetry in the global distribution of TEC variations is due to the effects caused by the irregular shape of the Earth's magnetic field along with its diurnal rotation. The analyses of residuals show that periodicities are correlated to the solar wind speed and magnetospheric forcing, especially those located near the southern dip pole at the night side. Furthermore, we found a TEC anomaly at about 15° from the South magnetic dip at the night side, more prominent around 52°S 155°E. Key Points: The solar and the magnetospheric forcing are the main drivers of nonperiodic ionospheric TEC variationsMain periodic contributions to TEC variations are related to the frequencies of the solar rotation, annual, and subharmonicsTEC anomaly has been found at about 15° from the South magnetic dip at the night side, more prominent around 52°S 155°E [ABSTRACT FROM AUTHOR]

Global Navigation Satellite System (GNSS) atmospheric radio occultation (RO) has been an active remote sensing method to explore the Earth's atmosphere recently. The RO products especially atmospheric bending angles have been widely used for climate study. However, the residual ionospheric error (RIE) still exists to a certain extent after the widely used linear combination in bending angle calculation. In this study, we analyzed both CHAMP and COSMIC data and did full 3‐D ray tracing to characterize RIE. It is found that the order of RIE is around ±0.1 μrad and has an obvious dependency on a variety of factors including altitude, latitude, local time, season, and solar activity level. The amplitude and complexity of RIE generally increase with altitude. It is closely related to the strength of the E layer and the asymmetry between LEO side and GNSS side. Along magnetic latitude direction, the RIE shows a pronounced feature of two peaks and three troughs. From both the simulated and observed RIE, we can identify clearly the ionospheric seasonal variation features, such as the semiannual variation, the equinox asymmetry and the annual anomaly. A spherical asymmetry factor was also defined to quantify the correlation between ionospheric spherical asymmetry degree and RIE. The study provides a comprehensive understanding of RIE, which could contribute to the correction of RIE in the future. Key Points: Both data analysis and ray tracing technique were used to characterize residual ionospheric error in GNSS RO bending angleMore than 120, 000 COSMIC RO events were simulated under ionospheric spherical asymmetry assumption for characterizing RIEThe RIE shows obvious dependency on altitude, latitude, local time, season, and solar activity and correlates well with ionospheric variability [ABSTRACT FROM AUTHOR]

Ionospheric data are valuable records of the behavior of the ionosphere, solar activity, and the entire Sun‐Earth system. The data are critical for both societally important services and scientific investigations of upper atmospheric variability. This work investigates some of the difficulties and pitfalls in maintaining long‐term records of geophysical measurements. This investigation focuses on the ionospheric parameters contained in the historical data sets within the National Oceanic and Atmospheric Administration National Geophysical Data Center and Space Physics Interactive Data Resource databases. These archives include data from approximately 100 ionosonde stations worldwide, beginning in the early 1940s. Our study focuses on the quality and consistency of ionosonde data accessible via the primary Space Physics Interactive Data Resource node located within the National Oceanic and Atmospheric Administration National Geophysical Data Center and the World Data Center for Solar‐Terrestrial Physics located in Boulder, Colorado. We find that, although the Space Physics Interactive Data Resource archives contained an impressive amount of high‐quality data, specific problems existed involving missing and noncontiguous data sets, long‐term variations or changes in methodologies and analysis procedures used, and incomplete documentation. The important lessons learned from this investigation are that the data incorporated into an archive must have clear traceability back to the primary source, including scientific validation by the contributors, and that the historical records must have associated metadata that describe relevant nuances in the observations. Although this report only focuses on historical ionosonde data in National Oceanic and Atmospheric Administration databases, we feel that these findings have general applicability to environmental scientists interested in using long‐term geophysical data sets for climate and global change research. Key Points: This work describes problems found within long‐term ionospheric records, likely caused by distributed storage systemThe problems found still persist, and there is no appearance of reparationIonospheric data are valuable records of the behavior of the ionosphere, solar activity, and the entire Sun‐Earth system [ABSTRACT FROM AUTHOR]

This brief introductory review of some recent developments in atmosphere–ionosphere science is written for the “Vertical Coupling Special Issue” that is motivated by the 5th IAGA/ICMA/SCOSTEP Workshop on Vertical Coupling in the Atmosphere–Ionosphere System. Basic processes of vertical coupling in the atmosphere–ionosphere system are discussed, focusing on the effects of internal waves, such as gravity waves and solar tides, sudden stratospheric warmings (SSWs), and of solar activity on the structure of the atmosphere. Internal waves play a crucial role in the current state and evolution of the upper atmosphere–ionosphere system. SSW effects extend into the upper atmosphere, producing changes in the thermospheric circulation and ionospheric disturbances. Sun, the dominant energy source for the atmosphere, directly impacts the upper atmosphere and modulates wave-induced coupling. The emphasis is laid on the most recent developments in the field, while giving credits to older works where necessary. Various international activities in atmospheric vertical coupling, such as SCOSTEP's ROSMIC project, and a brief contextual discussion of the papers published in the special issue are presented. [ABSTRACT FROM AUTHOR]

Multi-year (1995-2013) velocity data collected by the Super Dual Auroral Network (SuperDARN) HF radars are considered to investigate seasonal and solar cycle variations of the convection reversal boundary (CRB) location for interplanetary magnetic field (IMF) Bz < 0. By considering monthly data sets we show that the CRB is at higher latitudes in summer between 1995 and 2007. The poleward shifts are on the order of 2-5°. After 2007, the seasonal effect weakens, and the highest latitudes for the CRB start to occur during the winter time. We show that the CRB latitudes decrease with an increase of the IMF transverse component at a rate of (1-2°)/2nT. Because of this effect, on average, the CRB latitudes are lower during high solar activity periods with stronger IMFs. We also confirm the effect of the CRB dawn-dusk shifts related to the IMF changes in the IMF By sign. [ABSTRACT FROM AUTHOR]

The results of studies of longitudinal and LT variations in parameters of the ionosphere-plasmasphere system, obtained using the Global Self-Consistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP), assimilation ionospheric model IRI Real-Time Assimilation Mapping (IRTAM), and satellite and ground-based observational data are presented in the paper. The study of the main morphological features of longitudinal and LT variations in the critical frequency of the ionospheric F2 layer ( foF2) and total electron content ( TEC) depending on latitude in the winter solstice during a solar-activity minimum (December 22, 2009) is carried out. It is shown that the variations in foF2 and TEC, on the whole, are identical, and so mutually substitutable, while creating empirical models of these parameters in quiet geomagnetic conditions. The longitudinal and LT variations in both foF2 and TEC are within an order of magnitude everywhere except for the equator anomaly region, where LT variation is larger by an order of magnitude than longitudinal variation. According to the results of the study, in the American longitudinal sector at all latitudes of the Southern (summer) Hemisphere, maxima of foF2 and TEC are formed. The near-equatorial and high-latitudinal maxima are separated out from these. The estimate of the contribution into the longitudinal variation in foF2 and TEC for various local time sectors and at various latitudes has been obtained for the first time. In the Southern (summer) Hemisphere, longitudinal variation in foF2 and TEC is formed in the nighttime. [ABSTRACT FROM AUTHOR]

A correlation was found between indices of solar and geomagnetic activities for monthly foF2 median and average values of these indices on the basis of an analysis of data on the F2-layer critical frequency foF2 from Slough and Chilton stations for 1954-2013. It was found that there is no systematic difference between the index of solar activity for a median foF and the 81-day or yearly average index of this activity. Cases in which a foF2 median corresponds to high geomagnetic activity are about 3-4 times rarer than observed cases of this high activity. Therefore, the geomagnetic activity that corresponds to a foF2 median is lower than the monthly average index of this activity on average, and this difference increases as the geomagnetic activity grows. The behaviors of relations between solar and geomagnetic activity indices for a foF2 median and the average values of these indices almost coincide for periods of relatively high (1954-1995) and low (1996-2003) solar and geomagnetic activity. [ABSTRACT FROM AUTHOR]

It is shown that the use of initial data series of different lengths is one of the main causes of the difference in the values of trends in the F2-layer parameters, foF2 and hmF2, obtained in a series of publications of the authors and published by other researchers. Arguments are presented in favor of the statement that systematic changes in the F2 layer began around 1980. Searching for trends, the authors consider periods after 1980. At the same time the majority of other scientists use the entire available array of data beginning in 1957 and 1958 (and sometimes even earlier) to search for trends. In that case, the analyzed time interval automatically includes the period before 1980, when the trends were absent. This leads to a substantial underestimation of the magnitude of the obtained negative trends in foF2 and hmF2. Particular examples based on the data of both the authors and other scientists confirming the statement are presented. [ABSTRACT FROM AUTHOR]

The pattern of ionospheric activity along the meridional and latitudinal directions in the middle latitudes was analyzed based on the previously suggested IAI index. The contribution of the solar and geophysical factors that largely control the midlatitude ionosphere and the general level of the ionospheric disturbance were established. Changes in the ionospheric activity traced along a meridian from lower to higher latitudes in the interval from 35° to 50° N were quantified. Based on the obtained results, a method is suggested for calculating a correction of the IAI index, which makes it possible to estimate IAI at any desired latitude from the values of this index at a particular station. A neural network approach is developed for prompt diagnosis and forecasting of the state of the midlatitude ionosphere in terms of the IAI index from the level of the solar-geophysical activity. The optimal configuration of the neural network and the most efficient input parameters, which provide the maximum (75%) efficiency of the neural network forecasts of the IAI index, are determined. [ABSTRACT FROM AUTHOR]

The electron concentrations in the ionospheric E-layer maximum NmE, as measured by the Moscow ionosonde, are compared with the results of theoretical calculations of NmE for geomagnetically quiet conditions at low solar activity on April 1, 1986, and April 6, 1996, moderate solar activity on April 9, 1978, and April 6, 1998, and high solar activity on April 20, 1980, and April 15, 1991. On the basis of this comparison, a correction of the model flux of solar X-ray radiation is proposed. The discovered variability of the correction factors manifests the influence of solar X-ray radiation flux variations on NmE variability. The dependence of the influence of the neutral constituents ionization by photoelectrons on NmE on the solar activity level is studied. [ABSTRACT FROM AUTHOR]

The global ionospheric maps (GIMs), generated by Jet Propulsion Laboratory (JPL) and Center for Orbit Determination in Europe (CODE) during a period over 13 years, have been adopted as the primary source of data to provide global ionospheric correction for possible single frequency positioning applications. The investigation aims to assess the performance of new NeQuick model, NeQuick 2, in predicting global total electron content (TEC) through ingesting the GIMs data from the previous day(s). The results show good performance of the GIMs-driven-NeQuick model with average 86% of vertical TEC error less than 10 TECU, when the global daily effective ionization indices (Az) versus modified dip latitude (MODIP) are constructed as a second order polynomial. The performance of GIMs-driven-NeQuick model presents variability with solar activity and behaves better during low solar activity years. The accuracy of TEC prediction can be improved further through performing a four-coefficient function expression of Az versus MODIP. As more measurements from earlier days are involved in the Az optimization procedure, the accuracy may decrease. The results also reveal that more efforts are needed to improve the NeQuick 2 model capabilities to represent the ionosphere in the equatorial and high-latitude regions. [ABSTRACT FROM AUTHOR]

High-frequency variability of the ionosphere, or irregularities, constitutes the main threat for real-time precise positioning techniques based on Global Navigation Satellite Systems (GNSS) measurements. Indeed, during periods of enhanced ionospheric variability, GNSS users in the field-who cannot verify the integrity of their measurements-will experience positioning errors that can reach several decimeters, while the nominal accuracy of the technique is cm-level. In the frame of this paper, a climatological analysis of irregularities over the European mid-latitude region is presented. Based on a 10 years GPS dataset over Belgium, the work analyzes the occurrence rate (as a function of the solar cycle, season and local time) as well as the amplitude of ionospheric irregularities observed at a single GPS station. The study covers irregularities either due to space weather events (solar origin) or of terrestrial origin. If space weather irregularities are responsible for the largest effects in terms of ionospheric error, their occurrence rate highly depends on solar activity. Indeed, the occurrence rate of ionospheric irregularities is about 9 % during solar maximum, whereas it drops to about 0 % during medium or low solar activity periods. Medium-scale ionospheric disturbances (MSTIDs) occurring during daytime in autumn/winter are the most recurrent pattern of the time series, with yearly proportions slightly varying with the solar cycle and an amplitude of about 10 % of the TEC background. Another recurrent irregularity type, though less frequent than MSTIDs, is the noise-like variability in TEC observed during summer nighttime, under quiet geomagnetic conditions. These summer nighttime irregularities exhibit amplitudes ranging between 8 and 15 % of the TEC background. [ABSTRACT FROM AUTHOR]

We have investigated the response of ionosphere to major solar flare events that occurred during 1998 to 2011. The effect of enhanced radiation fluxes in the X-ray and EUV band on the GPS derived Total Electron Content (TEC) is examined. The data of X-ray flux from Geostationary Operational Environment Satellite (GOES) and EUV flux from Solar EUV Monitor (SEM) onboard SOHO spacecraft were correlated with the Total Electron Content (TEC) data of a high latitude station, Davis (68.570S, 77.960E). We found that peak intensities of X-ray and EUV flux correlate very well with the peak values of TEC. We also studied the correlation of peak enhancement of these fluxes with the peak enhancement of TEC and found that peak enhancement of these fluxes correlate highly with the peak enhancement of TEC than with the peak values themselves. It is also found that correlation is extraordinarily improved when these fluxes are multiplied by Cos(CMD) where CMD is Central Meridian Distance on the solar disc, thereby showing that the location of flares on the solar disc plays an important role while investigating the ionospheric influences of solar flares. [ABSTRACT FROM AUTHOR]

In the present study we have investigated the monthly and seasonal variability of TEC and amplitude scintillation index (S4) over two Indian polar stations Maitri (Antarctic) and Ny-Alesund (Arctic), during the low solar activity period 2008. We have used the Novatel's dual frequency GPS receiver GSV4004A to accomplish this study. From our analysis we observed that TEC achieves its highest values during the months of November and December while during the month of May and June the lowest values of TEC were recorded at Maitri station. Similarly during summer season the highest values of TEC are recorded while in winter season lowest values of TEC are observed. The scintillations that occurred during the year 2008 at Maitri as well as at Ny-Alesund were generally found to be of weak type (S4≥0.1), although few cases of moderate (S4≥0.3) and strong (S4≥0.5) scintillation were also observed. The occurrence characteristics of scintillations showed that maximum scintillations at Maitri occur during the month of July and August while least scintillations occur during the month of January and February. This type of ionospheric variability can be explained on the basis of solar irradiance at Polar Regions. [ABSTRACT FROM AUTHOR]

Results of statistical analysis of the properties of variability of F2-layer maximum parameters (critical frequency foF2 and the height hmF2) in quiet midlatitude ionosphere under low solar activity in the daytime (1000-1500 LT) and nighttime (2200-0300 LT) hours are presented on the basis of Irkutsk station data for 2007-2008. It is found that the distribution density of δ foF2 could be presented as consisting of two distinctly different normal laws of this distribution, one of which corresponds to weak (|δ foF2| < 10%) fluctuations in foF2 and the other corresponds to strong (30% > |δ foF2| > 10%) fluctuations. Weak fluctuations in foF2 to a substantial degree are related to ionospheric variability at times less of than 1-3 h and determine the δ foF2 variability in the daytime hours. Strong fluctuations in foF2 are mainly related to day-to-day variability of the ionosphere at a fixed local time, the variability increasing by approximately a factor of 3 during the transition from day to night and determining the δ foF2 variability in the nighttime hours. The distribution density of Δ hmF2 is close to the normal distribution law. An interpretation of the different character of the distribution densities of δ foF2 and Δ hmF2 is given. [ABSTRACT FROM AUTHOR]

Abstract: The variability of the critical frequency of F2 layer, foF2, over ionospheric station Grocka (44.48N, 20.31E) has been studied during the declining phase of solar cycle 23 from 2004 to 2006. The variability index was introduced to identify the daily and seasonal patterns characterizing the local mid-latitude ionosphere during quiet and disturbed geomagnetic conditions. In addition, the behaviour of the vertical total electron content values, vTEC, obtained from global positioning system (GPS) measurements in the surrounding area under these conditions is reported. The analysis shows a number of interesting features representative of the ionospheric variability relevant for ionospheric modelling as well as ionospheric propagation applications based on a single station approach. [Copyright &y& Elsevier]

Abstract: NeQuick ionospheric electron density model produces the full electron density profile in the ionosphere using the F2 layer peak values (foF2 and hmF2) as anchor points. Each part of the profile is modeled using Epstein layer formalism. Simple empirical relations are used to compute the thicknesses of each semi-Epstein layer. It has been observed that when NeQuick model is used to estimate total electron content at low latitudes the modeled values tend to underestimate the observed ones. Beside the F2 peak values, the most important profile parameter is the thickness of the F2 layer bottomside. The present study focuses on NeQuick model behavior at low latitudes comparing modeled profiles parameters with the ones extracted from experimental data mostly from African and Indian sector at different levels of solar activity and different time of the day. Possible model improvements are discussed. [Copyright &y& Elsevier]

Abstract: This paper concentrates on those major areas where our current physical understanding and recent advances can lead to positive predictions of the expected effects of ionospheric activity on the near-Earth space environment and on technological systems which operate within this environment. It briefly describes some of the key links between solar activity and the various physical processes, which govern ionospheric plasma structure that has been under scientific examination over past several decades but has lately received significant importance in relation to the space weather services. Specific examples during extremely intense solar event show how ionospheric monitoring techniques that have contributed immense data sets and related empirical and theoretical formulations have been incorporated in different ionospheric specification and prediction models for real-time operational applications. Finally, the general question of what might be expected as a result of current activities within different European cooperative projects is addressed. [Copyright &y& Elsevier]

Abstract: Responses of low-latitude ionospheric critical frequency of F2 layer to geomagnetic activities in different seasons and under different levels of solar activity are investigated by analyzing the ionospheric foF2 data from DPS-4 Digisonde in Hainan Observatory during 2002–2005. The results are as follows: (1) the response of foF2 to geomagnetic activity in Hainan shows obvious diurnal variation except for the summer in low solar activity period. Generally, geomagnetic activity will cause foF2 to increase at daytime and decrease at nighttime. The intensity of response of foF2 is stronger at nighttime than that at daytime; (2) seasonal dependence of the response of foF2 to geomagnetic activity is very obvious. The negative ionospheric storm effect is the strongest in summer and the positive ionospheric storm effect is the strongest in winter; (3) the solar cycle has important effect on the response of foF2 to geomagnetic activity in Hainan. In high solar activity period, the diurnal variation of the response of foF2 is very pronounced in each season, and the strong ionospheric response can last several days. In low solar activity period, ionospheric response has very pronounced diurnal variation in winter only; (4) the local time of geomagnetic activities occurring also has important effect on the responses of foF2 in Hainan. Generally, geomagnetic activities occurred at nighttime can cause stronger and longer responses of foF2 in Hainan. [Copyright &y& Elsevier]

Abstract: The Observatori de l’Ebre became operative in late 1904. From the beginning it was devoted to studying the influence of solar activity on Earth’s physical phenomena. Ionospheric studies should be, as they are, an important part of its research activity. This article reviews the development of ionospheric research at the observatory and the main contributions the Observatori has made to ionospheric science. [Copyright &y& Elsevier]

Abstract: The Utah State University modeling developments have led to new results in several areas of ionospheric research. This review will present the most recent of these results and their synergisms with both ground-based and satellite-based observations. Particular attention is given to new results associated with solar variability, a central theme to the TIGER program. The effect on the ionosphere of differences in solar X-EUV spectra will be demonstrated by studies using the time-dependent ionospheric model, while their effect on the coupled thermosphere–ionosphere system will be shown using a new model, the global average ionosphere thermosphere model. How the dayside solar-produced plasma contributes to sub auroral polarization streams, storm enhanced densities, tongues of ionization, plasma patches, polar wind jets, and polar cap scintillations, will be described via recent modeling results from a variety of coupled or driven thermosphere, ionosphere, and magnetosphere models. The review will indicate the need for improved solar output specification, whether via observations or models. These improvements need to be over the flare through solar cycle time scale, with particular attention to the short wavelength end of the spectrum. [Copyright &y& Elsevier]

Solar wind dynamic pressure pulse (DPP) structures, across which the dynamic pressure changes abruptly over timescales from a few seconds to several minutes, are often observed in the near-Earth space environment. The space weather effects of DPPs on the magnetosphere–ionosphere coupling system have been widely investigated in the last two decades. In this study, we perform a statistical survey on the properties of DPPs near 1 AU based on nearly 20 years of observations from the WIND spacecraft. It is found that only a tiny fraction of DPPs (around 4.2%) can be regarded as interplanetary shocks. For most DPPs, the total pressure (the sum of the thermal pressure and magnetic pressure) remains in equilibrium, but there also exists a small fraction of DPPs that are not pressure-balanced. The overwhelming majority of DPPs are associated with solar wind disturbances, including coronal mass ejection-related flows, corotating interaction regions, as well as complex ejecta. The annual variations of the averaged occurrence rate of DPPs are roughly in phase with the solar activity during solar cycle 23, and during the rising phase of solar cycle 24. [ABSTRACT FROM AUTHOR]