Electron content statistics offers important information for planning and operation of various application systems that make use of the transionospheric propagation of radio signals. Electron content statistics meet with an important difficulty: the majority of data stem from observations on the radio signals of orbiting satellites. The database for vertical electron content derived from observations of radio signals emitted from geostationary satellites is only very small. Therefore it is important to make use of statistics for peak density from ionosonde measurements. Quantile statistics do not contain information about ionization extremes. Therefore it is necessary to complement the statistics with case studies which reveal unusual and extreme electron content structures in space and in time.

An important part of the tasks of Working Group 2 of the COST Action 271 «Assessment of space plasma effect for satellites applications» is the assessment of novel data sources for information about the state of ionisation of the ionosphere. This report deals with those aspects which are not represented adequately in the scientific papers in this issue. Here emphasis is given to the product aspect (data and model collections, descriptions of methods and algorithms, availability of products, expected future developments) and the links between the past COST Actions 238 and 251 with the present Action 271 and with possible future cooperations. Working Group 2 was leading in the transionospheric propagation aspects of possible products for the International Telecommunication Union?s Radiocommunication (ITU-R) Study Group 3. This report gives a short overview emphasizing future developments.

Various modern applications of empirical electron density models need realistic structures of the electron density distribution with smaller scales than the model background. Travelling Ionospheric Disturbances (TIDs) produce three dimensional and time dependent disturbances of the background ionization. We present a TID model suitable to «modulate» large scale electron density distributions by multiplication. A model TID takes into account the forward tilt of the disturbance wave front, a distinct vertical structure, a fan type horizontal radiation characteristic, geometric dilution and attenuation. More complicated radiation patterns can be constructed by means of superposition. The model TIDs originate from source regions which can be chosen arbitrarily. We show examples for TID modulations of the background model family developed at Trieste and Graz (NeQuick, COSTprof and NeUoG-plas).

In winter and equinoctial season nights, the main trough of the F region is an important stable structure of the ionosphere at the border between mid and high latitudes. Therefore it has to be taken into account in modelling and mapping approaches. Werner and Prölss (1995) derived a model for the position of the trough minimum which has found wide acceptance. The model is based on in-situ electron density data measured aboard the low orbiting Dynamic Explorer satellite DE 2 (1981-1986). We present results for other properties of the main trough derived from the same data set. These results are sufficiently good for modelling purposes which need reliable information on the depth, the equatorward and poleward width and the steepness of the walls of the trough. Because of the eccentric orbit of DE 2 (orbit height between about 300-1000 km) it was necessary to «project» observed electron densities to the peak of the F 2 layer. This was done by means of the electron density model COSTprof. The database was restricted to those cases for which the height of DE 2 was below 700 km. Examples are shown for «typical» troughs observed under various conditions.

The ionospheric electron density model NeQuick is a «profiler» which uses the peaks of the E-layer, the F1-layer and the F2-layer as anchor points. In the version prepared for and submitted to the International Telecommunication Union (ITU) the model uses the ITU-R (CCIR) maps for foF2 and M(3000)F2 and adapted maps similar to the ITU-R ones for foE and foF1. Since users found problematic behaviour of NeQuick under conditions of strong differences of foE and foF2 map structures, the profiling was adapted by changing the properties of the Epstein layers used for this purpose. The new formulation avoids both strange horizontal structures of the geographic distribution of electron density in fixed heights and unrealistic peculiarities of the height profile which occasionally occurred with the old version of the model. Since the Epstein layer approach allows for 8 parameters only (3 layer amplitudes and 5 semi-thicknesses) the adaptation was no minor task but needed careful planning of suitable strategies.

The NeQuick electron density model of the ionosphere is designed for trans-ionospheric propagation applications. The model topside has been revised on the basis of ISIS 2 topside sounder profiles, producing a new formulation of its empirical shape parameter. Comparisons between experimental slant TEC data and values modelled using both versions of NeQuick topside showed that in general we have obtained a distinct improvement. However, during some months of the year and at low latitudes, the new topside formulation does not produce improvements on the slant TEC estimates. We discuss the likely reasons for this behaviour including assessment of merits and shortcomings of the ISIS 2 data in low latitudes. The topside sounder on Intercosmos 19 satellite extensively sounded the equatorial region during a period of high solar activity, which was less covered by ISIS 2. This paper presents comparisons of NeQuick and topside sounders profiles at low latitudes using Intercosmos 19 satellite data.

The IRI model offers a choice of options for the computation of the electron density profile and electron content (TEC). Recently new options for the topside electron density profile have been developed, which have a strong impact on TEC. Therefore it is important to test massively the IRI and the new options with experimental data. A large number of permanent stations record dual frequency GPS data from which it is possible to obtain TEC values. Thirty-one worldwide distributed stations have been selected to investigate the capabilities of the IRI to reproduce experimental TEC. Data for years 2000 (high solar activity) and 2004 (medium solar activity) have been analyzed computing modeled values with the IRI-2001 and the IRI-2007-NeQuick topside options. It is found that IRI-2007-NeQuick option generally improves the estimate of the slant TEC, especially in the case of high latitudes stations during high solar activity.

The topside ionosphere modeling suffers from the relative scarcity of experimental electron density data. Different modeling approaches have been applied in empirical models, like IRI, and profilers, like NeQuick. Comparing ISIS-2 topside electron density profiles with IRI and NeQuick models weak points in their topside formulation are identified and analysed. The IRI topside consists in two constant gradient sections, with a transition height kept at a fixed height in its domain. Tests of transition height and gradient modifications are presented, based on experimental evidences and model efforts. NeQuick model has a topside formulation based on a semi-Epstein layer, governed by an empirical shape parameter k. An alternative formulation is presented based on k values derived from experimental profiles.

In the presence of electron density gradients the thin shell approximation for the ionosphere, used together with a simple mapping function to convert slant total electron content (TEC) to vertical TEC, could lead to TEC conversion errors. These “mapping function errors” can therefore be used to detect the electron density gradients in the ionosphere. In the present work GPS derived slant TEC data have been used to investigate the effects of the electron density gradients in the middle and low latitude ionosphere under geomagnetic quiet and disturbed conditions. In particular the data corresponding to the geographic area of the American Sector for the days 5–7 April 2000 have been used to perform a complete analysis of mapping function errors based on the “coinciding pierce point technique”. The results clearly illustrate the electron density gradient effects according to the locations considered and to the actual levels of disturbance of the ionosphere. In addition, the possibility to assess an ionospheric shell height able to minimize the mapping function errors has been verified.

NeQuick is a three‐dimensional and time‐dependent quick run electron density model specifically designed for transionospheric propagation applications. It allows calculation of electron concentration values at any location in the ionosphere and the total electron content (TEC) along any ground station–to–satellite ray path. After specific adaptations, the model has been used to develop a near‐real‐time nontomographic electron density retrieval technique able to provide the electron density of the ionosphere above the geographic area of interest. The technique relies on the knowledge of the model driving parameter Az (ionization level) for the location considered. In the present study, the necessary Az values have been obtained through direct ingestion of Global Positioning System (GPS)–derived slant TEC data in two different ways: using data from a single GPS receiver and using data from multiple ground stations. Statistical comparisons between experimental and reconstructed slant TEC values and between experimental and retrieved maximum electron concentration values are shown.

Using the Austrian network of 10 two‐frequency GPS geodetic reference stations, whose location is known with high precision, a study of mesoscale anomalies in the ionosphere over central Europe has been made (and is still in progress for the following years). From the large number of individual observations during the period from 2000 to 2004, maps of small‐scale ionospheric behavior (with a pixel size of 0.5° in latitude and longitude) have been constructed from which local anomalies (at an equivalent height of 250 km) in terms of departure from the model total electron content (TEC) can be recognized. Not all magnetic storms which were most common during the 2000–2002 period of solar maximum showed local effects in the regional ionosphere, but mesoscale ionospheric anomalies which are not (necessarily) associated with space weather were also observed up to 20% of the days of observation, with their highest frequency of occurrence during the winter months (November–February). These anomalies have a typical duration of several hours and seem to be associated with traveling ionospheric disturbances (TIDs) whose origin could lie in gravity waves initiated in the lower atmosphere. Thus TIDs not associated with space weather may play a role in the regional ionosphere but can be detected only by virtue of the dense spatial coverage obtained by a network of GPS stations for precise positioning. From 2000 to 2004, the solar activity and the background ionization decreased considerably. Since TIDs depend on both the properties of atmospheric gravity waves and on the level of ionization, we observed smaller absolute TID amplitudes in 2003–2004 compared with 2000–2001. There is indication that the relative amplitudes have increased slightly, which is consistent with the influence of ion drag on thermosphere dynamics.

Usually regional and global electron density models provide large scale spatial structures only and smooth out the smaller scale features of the electron density distribution. We present a method to modulate existing electron density models by multiplication: M(h, φ, λ, t) = L(h, φ, λ, t) × S1(h, φ, λ, t) × S2(h, φ, λ, t) × ... Sn(h, φ, λ, t) M: resulting electron density distribution, L: large scale model, S1...Sn: modulating models for n the smaller scale structures; h: height; φ, λ: geographic coordinates, t: Universal Time. There are no restrictions to the nature of the large scale model provided it takes height and horizontal coordinates as input. Examples are models of the "profiler" type which use large scale "maps" for profile anchor points (e.g., E, F1, F2 peak properties) like the International Reference Ionosphere (IRI). Typical examples for smaller scale structures are ridges, troughs and wavelike disturbances. The advantage of modulation by multiplication is that there is no danger to get zero or negative values of electron density as long as the background and modulations are >0 everywhere. For each modulation model, unity means "undisturbed".

Remote sensing systems need verification by means of in-situ data or by means of model data. In the case of ionospheric occultation inversion, ionosphere tomography and other imaging methods on the basis of satellite-to-ground or satellite-to-satellite electron content, the availability of in-situ data with adequate spatial and temporal co-location is a very rare case, indeed. Therefore the method of choice for verification is to produce artificial electron content data with realistic properties, subject these data to the inversion/retrieval method, compare the results with model data and apply a suitable type of “goodness of fit” classification. Inter-comparison of inversion/retrieval methods should be done with sets of artificial electron contents in a “blind” (or even “double blind”) way. The set up of a relevant database for the COST 271 Action is described. One part of the database will be made available to everyone interested in testing of inversion/retrieval methods. The artificial electron content data are calculated by means of large-scale models that are “modulated” in a realistic way to include smaller scale and dynamic structures, like troughs and traveling ionospheric disturbances.

The paper reports on a series of studies carried out within the COST 271 Action relating to the measurement and use of Total Electron Content (TEC) of the ionosphere over North West Europe. Total electron content is a very important parameter for the correction of propagation effects on applied radio systems so that it is vital to have confidence in the experimental measurements and the resultant products derived as aids for the practical user. Comparative investigations have been carried out using TEC values from several different sources. It was found that in general there was broad statistical agreement between the data sets within the known limitations of the techniques, though discrepancies were identified linked to steep ionospheric gradients at the onset of geomagnetic storm disturbance and in the vicinity of the main trough. The paper also reviews recent progress in the development of tomographic inversion techniques that use total electron content measurements to image the ionosphere as an aid to various radio systems applications.

The Total Electron Content (TEC) of the ionosphere is a key parameter for describing the ionospheric state. This paper deals with the large scale behaviour of TEC under low and high solar activity conditions. Large scale structures of the plasma density are formed by fundamental ionospheric processes mainly driven by solar radiation input, neutral winds and electric fields. The monitoring of large scale structures contributes to a comprehensive understanding of these coupling mechanisms which are rather complex particularly under perturbed geomagnetic conditions. The paper addresses techniques to monitor TEC with sufficient accuracy of a few TEC units (10(16)m(-2)) to measure large scale structures over Europe and over the polar areas. The availability of GPS data from global GPS receiver networks as e.g., those from the International GPS Service (IGS) is dense enough to generate TEC maps on a continuous base. A model assisted technique is briefly described for mapping TEC over the European and polar areas. A statistical estimation of horizontal TEC gradients reveals large scale gradients of up to about 6 TECU/1000 km under high solar activity conditions at an occurrence probability level of about 1%. Occasionally, during severe ionospheric storms this value may increase by a factor of 10 or even more. A close correlation of large scale gradients and the geomagnetic activity has been found giving the chance to forecast TEC gradient amplitudes by using predicted geomagnetic indices. Since TEC is proportional to first-order range errors in Global Satellite Navigation Systems (GNSS) such as the US GPS and the Russian GLONASS the study of the behaviour of this parameter has a practical meaning in GNSS based navigation and positioning. The paper addresses the close relationship between TEC and ranging errors in GNSS. Having in view Galileo, the planned Europe's own global satellite navigation system, some aspects related to the mitigation of ionospheric propagation errors within the European Geostationary Navigation Overlay System (EGNOS) are discussed. Since EGNOS will augment the two above mentioned satellite navigation systems and make them suitable for safety critical applications such as flying aircraft or navigating ships through narrow channels the ionospheric propagation errors have to be mitigated as much as possible.

The present work analyzes results coming from global maps of ionospheric total electron content (TEC) obtained from observations and from different empirical models like the International Reference Ionosphere (IRI), the model family developed at Trieste and Graz (NeQuick, COSTprof, and NeUoG‐plas), and the GPS operational model (formulation by J. A. Klobuchar). Since they still appear in the context of assessment studies we have also included “old” models like the Bent model. The attention is focused on situations which occurred in the present period of high solar activity, pointing out features like the equatorial anomaly and polar regions, which are crucial test regions for ionospheric TEC models. Experimental estimates of slant TEC from International GPS Service (IGS) stations have been compared particularly with the predictions of the NeQuick and GPS models. A very simple picture of the ionosphere like the one given by the GPS operational model appears to be insufficient to reproduce the global complex behavior of the ionosphere, as it is needed for assessment studies or for modern operational real‐time corrections of transionospheric propagation errors. The IRI estimates of TEC still present serious problems, essentially owing to the topside under high solar activity conditions, and the model cannot be integrated to heights above 2000 km. With processing resources suitable for real‐time operation, it seems that the NeQuick model can give a more widely reliable picture of the TEC estimated from GPS measurements. Computing times for this model are considerably smaller than for more complex models like NeUoG‐plas.