TEC Maps Research Articles

SBAS-derived TEC maps: a new tool to forecast the spatial maps of maximum probable scintillation index over India

With the increase in the use of satellite-based navigation services, the forecasting of L band scintillation has turned out to be of paramount importance as it affects their accuracy and availability. Forecasting the time of occurrence or non-occurrence, strength and probable location of scintillation enables the service providers and users to take appropriate action to mitigate the effects and optimize the services. We use the recently developed method to retrieve TEC from the ionospheric correction data transmitted by the Indian satellite-based augmentation system (SBAS)–GAGAN. By making use of the established linear relation between the dusk time TEC and the maximum probable scintillation intensity (S4max), scintillation forecast maps have been generated as early as 1930 LT. The superposition of actual S4 measurements, obtained from the GAGAN network of receivers, on the forecasted S4max map shows that the actual measurements are less than the predicted S4max except on very few occasions. The potential of the simple technique to predict the 2 D maps of maximum probable scintillation index for the whole night has been demonstrated which with more refinements could evolve into a viable forecast or forewarning system.

Mitigation of Ionospheric Effects on GNSS Positioning at Low Latitudes

Ionospheric conditions at low latitudes are extremely harsh due to the frequent occurrence of scintillation and the presence of strong TEC gradients. For this study, the Sao Paulo state region in Brazil is chosen as a test area. This study presents a strategy to mitigate the ionospheric impact on RTK positioning with an experimental result. The proposed strategy explores two approaches that can be applied simultaneously: a) to mitigate the scintillation effect on the GNSS signals by refining the stochastic model of the corresponding observations and b) to precisely estimate the residual double difference ionospheric delay by exploiting an accurate TEC map. The strategy was tested on a long baseline kinematic processing under strong scintillation conditions (DOY21 in 2014). Significant improvements were observed when the combined use of the two mitigation approaches described above was compared with the use of conventional state-of-the-art approaches.

Attribution of interminimum changes in global and hemispheric total electron content

AbstractWe use a new data product of electron column density (total electron content, or TEC) maps to estimate and attribute the interminimum changes (the differences between annual averages centered on the 2008 and 1996 solar minima) in TEC global and hemispheric averages (dividing the globe in three different ways). We attribute the observed changes to corresponding changes in solar and geomagnetic activity. The new TEC map product was constructed with temporally consistent processing, and it resolves some of the apparent inconsistencies of earlier studies. The estimated global average TEC interminimum change is −19.3% ± 1.0% (2σ uncertainty), of which −9.1% is attributable to the interminimum change in solar extreme ultraviolet (EUV) irradiance (as represented by the F10.7 index), −2.2% is attributable to the change in geomagnetic activity (Kp index), and −9.3% remains unattributed. The hemispheric results are very similar to the global results, but the values tend to be slightly larger in the Southern Hemisphere, at low latitudes, and at night, compared to the opposing respective hemispheres. The interminimum changes and temporal variations of thermospheric mass density anomalies (i.e., the difference between the data and the empirical model) are very similar to those of the global and hemispheric TEC residuals, suggesting that they are driven by a common, globally distributed mechanism. Thermospheric composition changes and additional (unobserved) decreases in solar EUV irradiance are possible mechanisms behind the TEC and mass density unattributed changes; we estimate a plausible range of −6% to −13% for the solar EUV irradiance interminimum change.

Ionospheric tomography by gradient-enhanced kriging with STEC measurements and ionosonde characteristics

Abstract. The estimation of the ionospheric electron density by kriging is based on the optimization of a parametric measurement covariance model. First, the extension of kriging with slant total electron content (STEC) measurements based on a spatial covariance to kriging with a spatial–temporal covariance model, assimilating STEC data of a sliding window, is presented. Secondly, a novel tomography approach by gradient-enhanced kriging (GEK) is developed. Beyond the ingestion of STEC measurements, GEK assimilates ionosonde characteristics, providing peak electron density measurements as well as gradient information. Both approaches deploy the 3-D electron density model NeQuick as a priori information and estimate the covariance parameter vector within a maximum likelihood estimation for the dedicated tomography time stamp. The methods are validated in the European region for two periods covering quiet and active ionospheric conditions. The kriging with spatial and spatial–temporal covariance model is analysed regarding its capability to reproduce STEC, differential STEC and foF2. Therefore, the estimates are compared to the NeQuick model results, the 2-D TEC maps of the International GNSS Service and the DLR's Ionospheric Monitoring and Prediction Center, and in the case of foF2 to two independent ionosonde stations. Moreover, simulated STEC and ionosonde measurements are used to investigate the electron density profiles estimated by the GEK in comparison to a kriging with STEC only. The results indicate a crucial improvement in the initial guess by the developed methods and point out the potential compensation for a bias in the peak height hmF2 by means of GEK.

Remote Sensing of Atmospheric and Ionospheric Signals Prior to the Mw 8.3 Illapel Earthquake, Chile 2015

In the present study, a number of atmospheric and some ionospheric anomalies are analyzed, which were recorded prior to the Mw 8.3 Illapel earthquake of September 16, 2015. This very large earthquake occurred in Central Chile, close to the coast, as the result of thrust faulting on the interface between the Nazca Plate and South American continent. Using remotely sensed data extracted from NASA/Giovanni, NOAA/NCEP, and NOAA/NGDC, atmospheric and ionospheric anomalies were observed that co-registered 35–40 and 25–30 days prior to the main shock, respectively. With reference to long-term time series over the epicentral area, significant atmospheric anomalies were recorded for cloud cover, geopotential height, precipitation rates, surface air pressure, omega, stream function, and wind vectors—all in the time window of August 5–10, 2015, 35–40 days prior to the main shock. Anomalous TEC maps were recorded for the same time period. Satellite images indicate the formation of an unusual cyclone, presumably triggered by air turbulences and abnormal atmospheric conditions over the epicentral area, including strong vertical winds. Data from the Jicamarca radio observatory in Peru, more than 2000 km to the North, reveal anomalous ionospheric variations on August 15–20, 2015 with respect to international reference ionosphere thickness parameters and the altitude of the F layer. The observed anomalies are consistent with processes that occur at the ground-to-air interface due to the stress activation of peroxy defects in the hypocentral volume. The flow of positive hole charge carriers to the Earth surface expected to have led to massive air ionization, generating at first primarily positive airborne ions, then negative air ions plus ozone. Understanding the sequence of processes inside the Earth’s crust and at the ground-to-air interface provides information not previously available about the causal and temporal linkages between the various pre-earthquake phenomena and the future seismic event.

Evaluation of single frequency GPS precise point positioning assisted with external ionosphere sources

Single frequency code and single frequency code and phase GPS precise point positioning scenarios using various ionospheric sources are evaluated by assessing their performances with respect to dual frequency solutions. These include Canadian regional and global vertical total electron content (VTEC) maps produced by Natural Resources Canada and different International GNSS Service (IGS) coordination or analysis centres. Furthermore, two of the most commonly used single layer ionospheric mapping functions applied for conversion of VTEC to slant TEC are evaluated with each and every one of the ionospheric VTEC sources. Results show that the quality of code only solutions depends on ionospheric activity level, and the TEC map and mapping function selected. Code and phase single frequency solutions are also improved when assisted with an external ionosphere source.

On the utility of the ionosonde Doppler‐derived EXB drift during the daytime

AbstractVertical EXB drift measured using the ionosonde Doppler sounding during the daytime suffers from an underestimation of the actual EXB drift because the reflection height of the ionosonde signals is also affected by the photochemistry of the ionosphere. Systematic investigations have indicated a fair/good correlation to exist between the C/NOFS and ionosonde Doppler‐measured vertical EXB drift during the daytime over magnetic equator. A detailed analysis, however, indicated that the linear relation between the ionosonde Doppler drift and C/NOFS EXB drift varied with seasons. Thus, solar, seasonal, and also geomagnetic variables were included in the Doppler drift correction, using the artificial neural network‐based approach. The RMS error in the neural network was found to be smaller than that in the linear regression analysis. Daytime EXB drift was derived using the neural network which was also used to model the ionospheric redistribution in the SAMI2 model. SAMI2 model reproduced strong (weak) equatorial ionization anomaly (EIA) for cases when neural network corrected daytime vertical EXB drift was high (low). Similar features were also observed in GIM TEC maps. Thus, the results indicate that the neural network can be utilized to derive the vertical EXB drift from its proxies, like the ionosonde Doppler drift. These results indicate that the daytime ionosonde measured vertical EXB drift can be relied upon, provided that adequate corrections are applied to it.

Plasma bubble monitoring by TEC map and 630 nm airglow image

Equatorial ionosphere plasma bubbles over the South American continent were successfully observed by mapping the total electron content (TECMAP) using data provided by ground-based GNSS receiver networks. The TECMAP could cover almost all of the continent within ~4000km distance in longitude and latitude, monitoring TEC variability continuously with a time resolution of 10min. Simultaneous observations of OI 630nm all-sky image at Cachoeira Paulista (22.7°S, 45.0°W) and Cariri (7.4°S, 36.5°W) were used to compare the bubble structures. The spatial resolution of the TECMAP varied from 50km to 1000km, depending on the density of the observation sites. On the other hand, optical imaging has a spatial resolution better than 15km, depicting the fine structure of the bubbles but covering a limited area (~1600km diameter). TECMAP has an advantage in its spatial coverage and the continuous monitoring (day and night) form. The initial phase of plasma depletion in the post-sunset equatorial ionization anomaly (PS-EIA) trough region, followed by development of plasma bubbles in the crest region, could be monitored in a progressive way over the magnetic equator. In December 2013 to January 2014, periodically spaced bubble structures were frequently observed. The longitudinal spacing between the bubbles was around 600–800km depending on the day. The periodic form of plasma bubbles may suggest a seeding process related to the solar terminator passage in the ionosphere.

A case study of ionospheric storm effects in the Chinese sector during the October 2013 geomagnetic storm

In this study, we investigate the ionospheric storm effects in the Chinese sector during 2 October 2013 geomagnetic storm. The TEC map over China sector (1°×1°) and eight ionosondes data along the longitude of 110°E are used to show significant positive ionospheric phases (enhancements in TEC and ionospheric peak electron density NmF2) in the high-middle latitude region and the negative effects at the low latitude and equatorial region during the storm. A wave structure with periods about 1–2h and horizontal speed about 680m/s, propagating from the high latitudes to the low latitudes is observed in electron densities within the height region from 200 to 400km, which is caused by the combined effects of neutral wind and the large-scale traveling disturbances (LSTIDs). In the low latitude regions, compared with those in the quiet day, the ionospheric peak heights of the F2 layer (hmF2) in the storm day obviously increase accompanying a notably decrease in TEC and NmF2, which might be as a result of the eastward prompt penetration electric field (PPEF) evidenced by the two magnetometers and the subsequent westward disturbance dynamo electric fields (DDEF). The storm-time TEC enhancement mainly occurs in the topside ionosphere, as revealed from the topside TEC, bottomside TEC and GPS TEC.

Regional TEC dynamic modeling based on Slepian functions

In this work, the three-dimensional state of the ionosphere has been estimated by integrating the spherical Slepian harmonic function and Kalman filter. The spherical Slepian harmonic functions have been used to establish the observation equations because of their properties in local modeling. Spherical harmonics are poor choices to represent or analyze geophysical processes without perfect global coverage but the Slepian functions afford spatial and spectral selectivity. The Kalman filter has been utilized to perform the parameter estimation due to its suitable properties in processing the GPS measurements in the real-time mode. The proposed model has been applied to the real data obtained from the ground-based GPS observations across some portion of the IGS network in Europe. Results have been compared with the estimated TECs by the CODE, ESA, IGS centers and IRI-2012 model. The results indicated that the proposed model which takes advantage of the Slepian basis and Kalman filter is efficient and allows for the generation of the near-real-time regional TEC map.

Features of the Ionosphere around the Time of the 5th August, 2014 M5.5 Orkney Earthquake

In this paper, presentation of specific features of TEC behaviour as a possible precursor of Orkney earthquake of August 5th, 2014 (M5.5) was made. To achieve this, TEC data from Suth station and GIM TEC maps were used. The period understudied (prior to and after the earthquake) experienced quiet geomagnetic situation which made the analysis much easier. A characteristic anomaly was noticed from 4 days before the earthquake and it was found out that the day-time TEC increases significantly and consistently relative to the background condition up to the day before the earthquake. To reveal the spatial dimensions of seismo-ionospheric anomaly longitudinally, the maps from IONEX database were used. The ionosphere, experiencing a modification as an increase of electron concentration over a large area, was revealed.

SHPTS: towards a new method for generating precise global ionospheric TEC map based on spherical harmonic and generalized trigonometric series functions

To take maximum advantage of the increasing Global Navigation Satellite Systems (GNSS) data to improve the accuracy and resolution of global ionospheric TEC map (GIM), an approach, named Spherical Harmonic plus generalized Trigonometric Series functions (SHPTS), is proposed by integrating the spherical harmonic and the generalized trigonometric series functions on global and local scales, respectively. The SHPTS-based GIM from January 1st, 2001 to December 31st, 2011 (about one solar cycle) is validated by the ionospheric TEC from raw global GPS data, the GIM released by the current Ionospheric Associate Analysis Center (IAAC), the TOPEX/Poseidon satellite and the DORIS. The present results show that the SHPTS-based GIM over the area where no real data are available has the same accuracy level (approximately 2–6 TECu) to that released by the current IAAC. However, the ionospheric TEC in the SHPTS-based GIM over the area covered by real data is more accurate (approximately 1.5 TECu) than that of the GIM (approximately 3.0 TECu) released by the current IAAC. The external accuracy of the SHPTS-based GIM validated by the TOPEX/Poseidon and DORIS is approximately 2.5–5.5 and 1.5–4.5 TECu, respectively. In particular, the SHPTS-based GIM is the best or almost the best ranked, along with those of JPL and UPC, when they are compared with TOPEX/Poseidon measurements, and the best (in addition to UPC) when they are validated with DORIS data. With the increase in the number of GNSS satellites and contributing stations, the performance of the SHPTS-based GIM can be further improved. The SHPTS-based GIM routinely calculated using global GPS, GLONASS and BDS data will be found at the website http://www.gipp.org.cn.

Analysis of the ionosphere/plasmasphere electron content variability during strong geomagnetic storm

The ionosphere/plasmasphere electron content (PEC) variations during strong geomagnetic storms in November 2004 were estimated by combining of mid-latitude Kharkov incoherent scatter radar observations and GPS TEC data derived from global TEC maps. The comparison between two independent measurements was performed by analysis of the height-temporal distribution for specific location corresponding to the mid-latitudes of Europe. The percentage contribution of PEC to GPS TEC indicated the clear dependence from the time with maximal values (more than 70%) during night-time. During day-time the lesser values (30–45%) were observed for quiet geomagnetic conditions and rather high values of the PEC contribution to GPS TEC (up to 90%) were observed during strong negative storm. These changes can be explained by the competing effects of electric fields and winds, which tend to raise the layer to the region with lower loss rate and movement of the ionospheric plasma to the plasmasphere.

GIM-TEC adaptive ionospheric weather assessment and forecast system

The Ionospheric Weather Assessment and Forecast (IWAF) system is a computer software package designed to assess and predict the world-wide representation of 3-D electron density profiles from the Global Ionospheric Maps of Total Electron Content (GIM-TEC). The unique system products include daily-hourly numerical global maps of the F2 layer critical frequency (foF2) and the peak height (hmF2) generated with the International Reference Ionosphere extended to the plasmasphere, IRI-Plas, upgraded by importing the daily-hourly GIM-TEC as a new model driving parameter. Since GIM-TEC maps are provided with 1- or 2-days latency, the global maps forecast for 1 day and 2 days ahead are derived using an harmonic analysis applied to the temporal changes of TEC, foF2 and hmF2 at 5112 grid points of a map encapsulated in IONEX format (−87.5°:2.5°:87.5°N in latitude, −180°:5°:180°E in longitude). The system provides online the ionospheric disturbance warnings in the global W-index map establishing categories of the ionospheric weather from the quiet state (W=±1) to intense storm (W=±4) according to the thresholds set for instant TEC perturbations regarding quiet reference median for the preceding 7 days. The accuracy of IWAF system predictions of TEC, foF2 and hmF2 maps is superior to the standard persistence model with prediction equal to the most recent ‘true’ map. The paper presents outcomes of the new service expressed by the global ionospheric foF2, hmF2 and W-index maps demonstrating the process of origin and propagation of positive and negative ionosphere disturbances in space and time and their forecast under different scenarios.

Equinoctial asymmetry in solar activity variations of <I>Nm</I>F2 and TEC

Abstract. The ionosonde NmF2 data (covering several solar cycles) and the JPL TEC maps (from 1998 through 2009) were collected to investigate the equinoctial asymmetries in ionospheric electron density and its variation with solar activity. With solar activity increasing, the equinoctial asymmetry of noontime NmF2 increases at middle latitudes but decreases or changes little at low latitudes, while the equinoctial asymmetry of TEC increases at all latitudes. The latitudinal feature of the equinoctial asymmetry at high solar activity is different from that at low solar activity. The increases of NmF2 and TEC with the solar proxy P = (F10.7+F10.7A)/2 also show equinoctial asymmetries that depend on latitudes. The increase rate of NmF2 with P at March equinox (ME) is higher than that at September equinox (SE) at middle latitudes, but the latter is higher than the former at the EIA crest latitudes, and the difference between them is small at the EIA trough latitudes. The phenomenon of higher increase rate at SE than at ME does not appear in TEC. The increase rate of noontime TEC with P at ME is higher than that at SE at all latitudes, and the difference between them peaks at both sides of dip equator. It is mentionable that the equinoctial asymmetries of NmF2 and TEC increase rates present some longitudinal dependence at low latitude. The influences of equinoctial differences in the thermosphere and ionospheric dynamics processes on the equinoctial asymmetry of the electron density were briefly discussed.

Use of TEC global maps and the IRI model to study ionospheric response to geomagnetic disturbances

Period of October–November 2003 provides a wide range of geomagnetic conditions. This paper looks at three intervals: a moderate magnetic disturbance (13–16.10.2003) and two intense magnetic storms (29–31.10 and 20–21.11). One purpose of this paper is to define the behavior of the two parameters of the ionosphere, foF2 (or NmF2) and TEC, in this period. Data of foF2 of European ionosondes and TEC values of global maps for the corresponding coordinates are used. The emphasis is on the study of the synchrony of these parameters, which determine the behavior of the proportionality factor τ(obs) between the TEC and NmF2. The second objective is to compare two methods of determining foF2 during the disturbances, using: (i) the STORM-factor of the IRI model, (ii) the median equivalent slab thickness τ(med), together with the global TEC maps. It is shown that synchrony of variations of TEC and NmF2 does not always exist, which leads to unpredictable behavior of τ(obs). Acceptable option of the calculation of foF2 from TEC is to use τ(med), ensuring compliance with the experimental data better than 0.5MHz in the case of synchrony, and 1.5MHz when it does not exist. The relationship between τ(obs) and the plasmaspheric part of the TEC of the IRI-Plas model is considered. A comprehensive investigation of this relationship in cases where no synchrony may contribute to the development of model of τ(obs) during the disturbances.

Role of thermosphere‐ionosphere coupling in a global ionospheric specification

In this paper we present our recent research development in the area of ionospheric specification by means of data assimilation of ground‐based observations. NOAA's Total Electron Content specification methodology (namely, a Gauss‐Markov Kalmanfilter with an empirical model of the ionosphere as a background model) over the continental United States has lately been expanded to the multiregional domains and to the entire globe. Analyses of the global TEC maps reveal clear signatures of thermosphere‐ionosphere coupling, in both dynamical and compositional nature, even though the underlying specification methodology does not take thermospheric effects into account. This suggests that ground‐based observations of electron density contain some information about the state of the thermosphere. By using a thermosphere ionosphere general circulation model in a prototype Ensemble Kalman filter (EnKF), we examine the role of thermosphere‐ionosphere coupling in a global ionospheric specification. Observing system simulation experiments, designed for a global network of ionosondes, suggest that ionospheric data assimilation considerably benefits from self‐consistent treatment of thermosphere‐ionosphere coupling in a forecast model as well as in assimilation schemes, both of which can be achieved inherently by using the EnKF.

Energetic particle precipitation and the influence on the sub-ionosphere in the SED plume during a super geomagnetic storm

[1] In this paper, we present precipitation characteristic of energetic particles and the influence on the sub-ionosphere in the storm enhanced density (SED) plume in the event of the geomagnetic storm during March 31, 2001. With observations of the NOAA16 satellite, the peak of precipitating electron flux was located near the TEC peak in region of a SED plume recognized on two-dimensional GPS TEC maps. On the other hand, the peak of precipitating energetic proton flux was observed in the outer boundary of the SED plume. Those precipitations were associated with energetic ions/electrons injected into ring currents (RC) caused by a sequence of substorms. When the Digisonde at Millstone Hill was located in the SED plume, the minimum frequency of ionogram echoes (fmin) was observed to keep a high level. Considering the SED plume as a signature of a plasmaspheric drainage plume, the precipitation of energetic electrons is attributed to the RC-ELF hiss interaction in the plasmaspheric plume. Calculations of a simple sub-ionospheric model demonstrate that those energetic electrons can precipitate into the sub-ionosphere and cause sub-ionospheric ionization enhancements in the SED plume. As a result, the fmin observed by the Digisonde in the SED plume was kept at a high level. Therefore, the paper provides direct evidences that precipitating energetic RC electrons play a significant role in the coupling between magnetosphere and sub-ionosphere in SED plumes.

English

IRI 2001 model prediction is compared to custom orthotic design group total electron content (CODG TEC) maps prediction at Niamey during several geomagnetic (quiet, fluctuating and shock days) conditions for the period of IHY campaign. Independently of geomagnetic conditions, the qualitative comparison between CODG TEC maps and IRI 2001 TEC shows the overestimation of IRI 2001 model. The overestimation of CODG TEC maps at Niamey by IRI model is also shown by examining quantitative methods (relative deviation module means (rdmm) and percentage deviation) results. The quality of IRI prediction, that is, the amplitude of estimation depends on geomagnetic condition and month. In fact, rdmm values of March and April for quiet day periods respectively are 0.53 and 0.41. During fluctuating period, rdmm values are 0.35 in March and 0.39 in April. When act CMEs, rdmm value is 0.67; thus the work shows on one hand the necessity to improve IRI model and on the other hand to study the effect of each disturb solar event separately.   Key words: Total electron content, custom orthotic design group, international reference ionosphere, geomagnetic classes of activity, relative deviation module means, percentage deviation, concordance.

A new algorithm for single receiver DCB estimation using IGS TEC maps

A new algorithm for single receiver DCB estimation using GIM vertical TEC gridded values is proposed. It estimates receiver DCB and vertical residual ionospheric delays using the least squares approach with linear constraints. The performance of the proposed algorithm was assessed by comparing estimated receiver DCBs with those provided by the IGS. The same comparisons were done using two other algorithms for receiver DCB estimation. It is demonstrated that the proposed algorithm is capable of reproducing IGS DCB values at the level of 0.1–0.3 ns, which is better than the level of agreement observed for the other two algorithms. For our tests, we considered data from more than 100 IGS stations, daily, such that all major regions of the world were covered. Besides, both ionospherically quiet and disturbed days were considered. It provides some evidence that the aforementioned level of agreement with IGS receiver DCB values does not significantly dependent on geographical region and the state of the ionosphere. The algorithm is easy to implement and can be considered for online use.