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In this study, we validated the Jason-2 (J2) dual-frequency ionosphere delay measurements in terms of vertical total electron content (VTEC) in the Geophysical Data Record (GDR) with the coarse resolution JPL Global Ionospheric Maps (GIM), the regional ionospheric maps generated by the Crustal Motion Observation Network of China (CMONOC), and the Jason-1 (J1) interleaved tandem mission (with J2) ionosphere delay measurements. The estimates of the relative biases and their uncertainties (95% confidence, in TEC units) for various comparison cases with different data spans and regions are (1) J2–GIM: −3.07 ± 0.18 TECu (or 6.75 ± 0.40 mm in the range delay), (2) J2–CMONOC: −2.87 ± 0.38 TECu (6.31 ± 0.84 mm), (3) J1–GIM: −0.19 ± 0.18 TECu (0.42 ± 0.40 mm), and (4) J2–J1 via double-differencing, that is, (J2–GIM)–(J1–GIM): −2.88 ± 0.26 TECu (6.34 ± 0.57 mm). The scatter (RMS, about the mean differences) between J2 and GIM, CMONOC, or J1 is about 3∼8 TECu (6.6∼17.6 mm). We conclude that the results from global a...

A database of the electron temperature (Te) comprising of most of the available LEO satellite measurements is used for studying the solar activity variations of Te. The Te data are grouped for two levels of solar activity (low LSA and high HSA), five altitude ranges between 350 and 2000 km, and day and night. By fitting a theoretical expression to the Te values we obtain variation of Te along magnetic field lines and heat flux for LSA and HSA. We have found that Te increases with increase in solar activity at low and mid-latitudes during nighttime at all altitudes studied. During daytime the Te response to solar activity depends on latitude, altitude, and season. This analysis shows existence of anti-correlation between Te and solar activity at mid-latitudes below 700 km during the equinox and winter day hours. Heat fluxes show small latitudinal dependence for daytime but substantial for nighttime.

The projection of the plasmapause magnetic-field lines to low altitudes, where the light-ion chemistry is dominated by O(+), tends to occur near the minimum electron density in the main (midlatitude) electron density trough at night. With increasing attitude in the trough, where H(+) emerges as the dominant iota on the low-latitude boundary, we have found cases where the plasmapause field lines are located on the sharp low-Latitude side of the trough as expected if this topside ionosphere H(+) distribution varies in step with the plasmapause gradient in the distant plasmasphere. These conclusions are based on near-equatorial crossings of the plasmapause (corresponding to the steep gradient in the dominant species H(+) by the Explorer-45 satellite as determined from electric-field measurements by Maynard and Cauffman in the early 1970s and ISIS-2 ionospheric topside-sounder measurements. The former data have now been converted to digital form and made available at http://nssdcftp.gsfc.nasa.gov. The latter provide samples of nearly coincident observations of ionospheric main trough crossings near the same magnetic-field lines of the Explorer 45-determined equatorial plasmapause. The ISIS-2 vertical electron density profiles are used to infer where the F-region transitions from an O(+) to a H(+) dominated plasma through the main trough boundaries.

A database of electron temperature ( T e ) measurements comprising of most of the available satellite measurements in the topside ionosphere is used for studying the solar activity variations of the electron temperature T e at different latitudes, altitudes, local times and seasons. The T e data are grouped into three levels of solar activity (low, medium, high) at four altitude ranges, for day and night, and for equinox and solstices. We find that in general T e changes with solar activity are small and comparable in magnitude with seasonal changes but much smaller than the changes with altitude, latitude, and from day to night. In all cases, except at low altitude during daytime, T e increases with increasing solar activity. But this increase is not linear as assumed in most empirical T e models but requires at least a parabolic approximation. At 550 km during daytime negative as well as positive correlation is found with solar activity. Our global data base allows to quantify the latitude range and seasonal conditions for which these correlations occur. A negative correlation with solar activity is found in the invdip latitude range from 20 to 55 degrees during equinox and from 20 degrees onward during winter. In the low latitude (20 to −20 degrees invdip) F-region there is almost no change with solar activity during solstice and a positive correlation during equinox. A positive correlation is also observed during summer from 30 degrees onward.

The International Reference Ionosphere (IRI) 2007 provides two new options for the topside electron density profile: (a) a correction of the IRI-2001 model, and (b) the NeQuick topside formula. We use the large volume of Alouette 1, 2 and ISIS 1, 2 topside sounder data to evaluate these two new options with special emphasis on the uppermost topside where IRI-2001 showed the largest discrepancies. We will also study the accurate representation of profiles in the equatorial anomaly region where the profile function has to accommodate two latitudinal maxima (crests) at lower altitudes but only a single maximum (at the equator) higher up. In addition to IRI-2001 and the two new IRI-2007 options we also include the Intercosmos-based topside model of Triskova, Truhlik, and Smilauer [Triskova, L., Truhlik, V., Smilauer, J. An empirical topside electron density model for calculation of absolute ion densities in IRI. Adv. Space Res. 37 (5), 928–934, 2006] (TTS model) in our analysis. We find that overall IRI-2007-NeQ gives the best results but IRI-2007-corrected provides a more realistic representation of the altitudinal–latitudinal structure in the equatorial anomaly region. The applicability of the TTS model is limited by the fact that it is not normalized to the F2 peak density and height.

Accurate knowledge of the electron density is the key point in correcting ionospheric delays of electromagnetic measurements and in studying ionosphere physics. During the last decade Global Navigation Satellite Systems (GNSS) have become a promising tool for monitoring ionospheric parameters such as the total electron content (TEC). In this contribution we present a four-dimensional (4-D) model of the electron density consisting of a given reference part, i.e., the International Reference Ionosphere (IRI), and an unknown correction term expanded in terms of multi-dimensional base functions. The corresponding series coefficients are calculable from the satellite measurements by applying parameter estimation procedures. Since satellite data are usually sampled between GPS satellites and ground stations, finer structures of the electron density are modelable just in regions with a sufficient number of ground stations. The proposed method is applied to simulated geometry-free GPS phase measurements. The procedure can be used, for example, to study the equatorial anomaly.

The International Reference Ionosphere (IRI), a joint project of URSI and COSPAR, is the de facto international standard for the climatological specification of ionospheric parameters and as such it is currently undergoing registration as Technical Specification (TS) of the International Standardization Organization (ISO). IRI by charter and design is an empirical model based on a wide range of ground and space data. It describes monthly averages of ionospheric densities and temperatures in the altitude range 50–1500 km in the non-auroral ionosphere. Since its inception in 1969 the IRI model has been steadily improved with newer data and with better mathematical descriptions of global and temporal variation patterns. A large number of independent studies have validated the IRI model in comparisons with direct and indirect ionospheric measurements not used in the model development. A comparison with IRI is often one of the first science tasks by an ionospheric satellite or rocket team. This paper describes the latest version of the IRI model, IRI-2007, explaining the most important changes that are being introduced with this version. These include: (1) two new options for the topside electron density, (2) a new model for the topside ion composition, (3) the first-time inclusion of a model for the spread F occurrence probability, (4) a NeuralNet model for auroral E-region electron densities, (5) a model for the plasmaspheric electron temperature, and (6) the latest International Geomagnetic Reference Field (IGRF) model for the computation of magnetic coordinates including their changes due to the secular variation of the magnetic field.

The knowledge of the electron density is the key point in correcting ionospheric delays of electromagnetic measurements and in studying the ionosphere. During the last decade GNSS, in particular GPS, has become a promising tool for monitoring the total electron content (TEC), i.e., the integral of the electron density along the ray-path between the transmitting satellite and the receiver. Hence, geometry-free GNSS measurements provide informations on the electron density, which is basically a four-dimensional function depending on spatial position and time. In addition, these GNSS measurements can be combined with other available data including nadir, over-ocean TEC observations from dual-frequency radar altimetry (T/P, JASON, ENVISAT), and TECs from GPS-LEO occultation systems (e.g., FORMOSAT-3/COSMIC, CHAMP) with heterogeneous sampling and accuracy. In this paper, we present different multi-dimensional approaches for modeling spatio-temporal variations of the ionospheric electron density. To be more specific, we split the target function into a reference part, computed from the International Reference Ionosphere (IRI), and an unknown correction term. Due to the localizing feature of B-spline functions we apply tensor-product spline expansions to model the correction term in a certain multi-dimensional region either completely or partly. Furthermore, the multi-resolution representation derived from wavelet analysis allows monitoring the ionosphere at different resolutions levels. For demonstration we apply three approaches to electron density data over South America.

This paper presents an analysis of a large database of in-situ topside ionosphere electron densities ( N e ) and temperatures ( T e ) from three decades of satellite measurements with the ultimate goal of improving the International Reference Ionosphere (IRI) model. The satellite data, which range from the early Explorers to the more recent KOMPSAT and DMSP satellites, are examined to reveal the variation of the mid-latitude N e and T e at altitudes of 550, 850, and 2000 km as a function of solar activity for different local times, and seasons. Comparisons with IRI, the FLIP physical model, and the Millstone Hill incoherent scatter radar empirical model help to determine how consistent these satellite-observed variations patterns are with the current IRI model, with the theoretical expectations and with ground-based results. For N e good agreement is found between the data and models in terms of variations pattern as well as absolute values. Whereas N e always increases with solar activity, T e can increase, decrease, or stay constant depending on the specific altitude, local time and season. At 550 km the daytime T e increases with solar activity in summer, decreases in equinox and stays almost constant in winter. At this altitude range we find generally good agreement between the variations seen with the satellite data and those predicted by the FLIP and Millstone Hill models. At 850 km, however, significant discrepancies are noted. The satellite data, primarily DMSP in this altitude range, are consistently higher than the Millstone Hill model averages and include unrealistically high temperatures (4000–5000 K) at very low solar activities. The FLIP model also predicts much lower values at low solar activities but otherwise (for middle to high solar activities) agrees well with the satellite data. During nighttime the FLIP model underestimates T e at 850 and 2000 km altitude for the summer and equinox seasons. The current IRI T e model does not include variations with solar activity. The IRI values are generally in between the satellite and radar averages with a few exceptions.

Long-term databases of multiple incoherent scatter radars (ISRs) are used to create local empirical models at seven ISR sites including, from magnetic north to south and east to west, EISCAT Svalbard Radar (Norway), Sondrestrom Radar (Greenland), EISCAT TromsoRadars (Norway), Millstone Hill Radar (USA), St. Santin Radar (France), Shigaraki Middle and Upper atmosphere (MU) Radar (Japan) and Arecibo Radar (Puerto Rico). As local models, they can represent some features that may be smeared out in a global model, and are important complements to and validation tools for global models such as IRI. This paper presents comparisons between these ISR models and IRI for median solar activity and quiet magnetic activity conditions, with focus on daytime height variations for electron density, ion and electron temperatures.

In this work we compare ionospheric measurements made at several ionosonde stations with parameters inferred from airglow measurements made by the Global Ultraviolet Imager (GUVI) instrument aboard the Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED) spacecraft. The purpose of this comparison is twofold. First, our study provides a validation of the technique of using TIMED/GUVI airglow measurements to infer nighttime electron densities in F region of the ionosphere. Second, we use the global characterization of the ionosphere provided by TIMED/GUVI to identify potential biases in individual ionosonde station records. We have found for peak electron densities over the range 4:0–11:0 � 10 5 cm � 3 that the GUVI measurements are consistent with the ionosonde observations. We have also found some station to station and regional variations that warrant further study. � 2006 COSPAR. Published by Elsevier Ltd. All rights reserved.

Empirical models are an important tool for the study of the different geospace regions from Earth to Sun, providing the user with easy access to a synthesis of reliable measurements from ground and space for specific parameters and regions. This paper describes a new effort to develop a coherent model of the topside F2 layer and the plasmasphere with the goal to improve the representation of the topside electron density in the IRI model and to extend the IRI description into the plasmasphere. An α-Chapman function with a continuously varying scale height, dubbed a vary-Chap function, is used to describe the topside F2 vertical electron density profile N ( h ) that seamlessly connects the ionosphere with the plasmasphere. The Chapman scale height H ( h ) varies only slowly with height near hmF2 and increases rapidly at the O + to light–ion transition height. A hyperbolic tangent function suitably represents this variation. New plasmasphere density profile data from the IMAGE/RPI measurements and topside profiles from the ISIS topside sounders are used to construct a continuous profile from hmF2 to several R E altitude.

Non-linear Error Compensation Technique for Associative Restoration (NECTAR) is a novel approach to the task of assimilating fragmentary sensor data into a global coverage model, in which an underlying model prediction is iteratively transformed (“morphed”) into a better agreement with the available sensor data. Similarly to the Kalman filter, NECTAR gleans the system nowcast from the observed mismatch of prediction and observations; however, each NECTAR update step trying to correct the observation-model disagreements is a non-linear recursive process of manipulating a large number of internal multiscale constituents of the underlying empirical model, both spatial and temporal. The NECTAR error compensation procedure is performed by the Hopfield feed-back neural network commonly used in associative memory architectures that restore full information from its fragments. When applied to the sparse spatial data from the contributing observatories, such neural network becomes an associative multiscale interpolator of missing information. To ensure the multiscale capability of NECTAR assimilation analysis in the time domain, the spatial interpolation is performed individually for each diurnal harmonic of the differences between observation and prediction computed for each sensor location. Early results of the NECTAR model morphing applied to the assimilation of measured data from the Global Ionosphere Radio Observatory (GIRO) into the International Reference Ionosphere (IRI) model reveal its intriguing capability to predict system dynamics over no-data areas (spatial interpolation) and in time (short-term forecast).

To address challenges of assessment of modeling capabilities, the CCMC (Community Coordinated Modeling Center) initiated a series of community-wide model validation projects, such as the GEM, CEDAR and GEM-CEDAR Modeling Challenges. The CEDAR ETI (Electrodynamics Thermosphere Ionosphere) Challenge focused on the ability of ionosphere-thermosphere (IT) models to reproduce basic IT system parameters, such as electron and neutral densities, NmF2, hmF2, and TEC. Model-data time series comparisons were performed for a set of selected events with different levels of geomagnetic activity (quiet, moderate, storms). The follow-on CEDAR-GEM Challenge aims to quantify geomagnetic storm impacts on the IT system. On-going studies include quantifying the storm energy input, such as increase in auroral precipitation and Joule heating, and quantifying the storm-time variations of neutral density and TEC. The community-wide model validation activities involve international collaborations (e.g., between the CCMC and UK Met Office) to enhance the studies. In this paper, we focus on results of validation of IT models for reproducing storm impacts on TEC. In order to quantify storm impacts on TEC, we considered several parameters: TEC changes compared to quiet time (the day before storm), TEC difference between 24-hour intervals, and maximum increase/decrease during the storm. We investigated the spatial and temporal variations of the parameters during storm events (e.g., 2006 AGU storm) using ground-based GPS TEC measurements in several longitude sectors where data coverage is relatively better. The latitudinal variations were also studied. We obtained modeled TEC from various IT models. The parameters from the models were compared with each other and with the observed values. We quantified performance of the models in reproducing the TEC variations during the storm using skill scores. Model output and observational data used for the challenge will be permanently posted at the CCMC website (http://ccmc.gsfc.nasa.gov) as a resource for the space science communities to use.

Preliminary measurements by the SWARM Langmuir Probes will be compared with predictions by the International Reference Ionosphere (IRI) (D. Bilitza et al., Journal of Space Weather and Space Climate, 4, 2014, A07(1–12)) involving both the standard IRI and the Real-Time IRI. The Real-Time IRI is based on the IRI Real-Time Assimilative Mapping (IRTAM) algorithm for the F2 peak density and height that was developed by I. Galkin et al. (Radio Science, 47, 2012). IRTAM assimilates digisonde data from the Global Ionosphere Radio Observatory (GIRO) into the IRI-CCIR models for the F-peak density NmF2 and height hmF2. The assimilation for hmF2 is based on the model of Brunini et al. (Advances in Space Research, 51, 2013, pp. 2322–2332) that applies the CCIR formalism directly to hmF2.

The F2 layer of the ionosphere plays an essential role in radio communication and positioning applications as well as in space weather research. In particular, the characteristics of the F2 layer are defined by the F2 peak density NmF2NmF2, the F2 peak height hmF2hmF2 as well as the scale height HF2HF2. The International Reference Ionosphere (IRI) is the internationally recognized and recommended standard for predicting these critical parameters. However, IRI provides median monthly values of these parameters based on the International Radio Consultative Committee (CCIR) or the International Union of Radio Science (URSI) models, which were developed from data of the worldwide network of ionosondes during the years 1954 to 1958. These models provide monthly averages, and therefore, they are required to be updated with up-to-date measurements to get more accurate predictions. In this contribution, we provide a procedure to improve the parameters NmF2,hmF2 and HF2HF2 from modern space geodetic measurements, which can serve to update IRI. Specifically, we model these key parameters spatio-temporally by tensor product of B-spline expansions, and estimate the model coefficients using two types of GNSS data, namely, ground-based two-frequency GPS observations of total electron content and electron density profiles retrieved from ionospheric GPS radio occultation measurements acquired by the FORMOSAT-3/COSMIC mission. In this manner, the solution of the model parameters benefits from different sensitivities as well as from different spatio-temporal resolutions of the two observation techniques. The model is applied exemplarily over a South American region on three selected days under high solar activity (1 July 2012), moderate solar activity (16 July 2011) and low solar activity (16 July 2008), respectively. A comparison of our results with ionosonde data, a comparison of vertical total electron content (VTEC) values and a cross validation show the strengths of our approach and the potential to update the IRI model.

This presentation is given in honor of Prof. Karl Rawer’s 90th birthday. It looks back at 35 years of research and development in the framework of the International Reference Ionosphere (IRI) project. K. Rawer initiated this international modeling effort and was the first Chairman of the IRI Working Group. IRI is a joint project of the Committee on Space Research (COSPAR) and the International Union of Radio Science (URSI) whose goal it is to establish an international standard model for the ionospheric densities, temperatures and drifts. This year we are celebrating Karl Rawer’s 90th birthday and also the 35-year anniversary of the IRI effort. My talk will review the close involvement of Karl Rawer in all stages of the development and improvement of this international standard from early on and his still very active participation in this effort.

We conducted an assessment of the TOPEX dual-frequency nadir ionosphere observations in the TOPEX/Poseidon (T/P) GDR by comparing TOPEX with the Center for Orbit Determination in Europe (CODE) Global Ionosphere Map (GIM), the climatological model IRI2001, and the DORIS (onboard T/P) relative ionosphere delays. We investigated the TOPEX (TOPEX Side A and TOPEX Side B altimeters, TSA and TSB, respectively) ionosphere observations for the time period 1995–2001, covering periods of low, intermediate, and high solar activity. Here, we use absolute path delays (at Ku-band frequency of the TOPEX altimeter and with positive signs) rather than Total Electron Content (TEC). We found significant biases between GIM and TOPEX (GIM–TOPEX) nadir ionosphere path delays: −8.1 ± 0.4 {mm} formal uncertainties and equivalent to 3.7 TECu) and −9.0 ± 0.7 {mm} (4.1 TECu) for TSA and TSB, respectively, indicating that the TOPEX path delay is longer (or with higher TECu) than GIM. The estimated relative biases vary with latitude ...

An F2 region critical frequency database, from 75 ionosonde stations and 25 storms, covering a full solar cycle, was built as the basis of the STORM Time Empirical Ionospheric Correction Model. This database has been sorted by season (five intervals from summer to winter, including intermediate seasons), geomagnetic latitude (four regions, 0–20, 20–40, 40–60, and 60–80), and storm intensity (as a function of filtered a p ). For each bin of latitude-season, the standard deviation was used to quantify the scatter around the fit, i.e. the geophysical variability. The first interval, between 0 and 500 units of the filtered a p , corresponds to non-perturbed conditions; where approximately one half of the data lay. The rest of the intervals, with increments of 500 units, cover perturbed conditions. For quiet conditions, the lowest variability of the data was found to be about 0.15, in the summer hemisphere. The standard deviation of the data increases up to 0.23 for intermediate seasons, up to 0.22 for equinox, and up to 0.40 for the winter hemisphere. In general, low latitudes show higher variability for both perturbed and un-perturbed conditions, while the summer hemisphere, mid-latitudes have the lowest.

The topside segment of the International Reference Ionosphere (IRI) electron density model (and also of the Bent model) is based on the limited amount of topside data available at the time (40,OOO Alouette 1 profiles). Being established from such a small database it is therefore not surprising that the models have well-known shortcomings, for example, at high solar activities. Meanwhile a large data base of close to 200,000 topside profiles from Alouette 1,2, and ISIS I, 2 has become available online. A program of automated scaling and inversion of a large volume of digitized ionograms adds continuously to this data pool. We have used the currently available ISIs/Alouette topside profiles to evaluate the IRI topside model and to investigate ways of improving the model. The IRI model performs generally well at middle latitudes and shows discrepancies at low and high latitudes and these discrepancies are largest during high solar activity. In the upper topside IRI consistently overestimates the measurements. Based on averages of the data-model ratios we have established correction factors for the IRI model. These factors vary with altitude, modified dip latitude, and local time.

This laudation is given in honor of the 90th birthday of Prof. Karl Rawer that coincides with the 35th anniversary of the International Reference Ionosphere (IRI). The ionosphere was discovered during Karl Rawer's life, and he has dedicated his life to the exploration of this part of Earth's environment. The horrible events of world wars I and II shaped his early life, but they also launched his career as one of the eminent geophysical scientists of the twentieth century. The paper looks back at Karl's life and the 35 years of research and development in the framework of the IRI project. K. Rawer initiated this international modeling effort and was the first chairman of the IRI Working Group. IRI is a joint project of the Committee on Space Research (COSPAR) and the International Union of Radio science (URSI) that has the goal to establish an international standard model of the ionospheric densities temperatures, and drifts. © 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.

We have used data from two equatorial stations in the African longitude sector to study the monthly variability of foF2. Diurnal, seasonal, and solar activity effects were investigated. The data established by this study are proposed as equatorial input values for the development of a variability model for the International Reference Ionosphere.

This paper describes the changes that were implemented in the new version of the COSPAR/URSI International Reference Ionosphere (IRI-2000). These changes are: (1) two new options for the electron density in the D-region, (2) a better functional description of the electron density in the E-F merging region, (3) inclusion of the F1 layer occurrence probability as a new parameter, (4) a new model for the bottomside parameters B0 and B1 that greatly improves the representation at low and equatorial latitudes during high solar activities, (5) inclusion of a model for foF2 storm-time updating, (6) a new option for the electron temperature in the topside ionosphere, and (7) inclusion of a model for the equatorial F region ion drift. The main purpose of this paper is to provide the IRI users with examples of the effects of these changes.

The IRI standard model of relative ion composition assumes a constant n(H+n(He+) ratio of 9. We study the light ion ratio with Atmosphere Explorer C (AE-C) and ACTIVE satellite data for low and high solar activity. Based on the AE-C data a model is presented that described the light ion ratio in terms of latitude for day and night conditions.

Ionosonde data recorded at Korhogo, Ce Longitude - 5.4, Dip -0.67) during a year of low solar activity (1995) were used to investigate ways of improving the representation of equatorial F2 peak height (hmF2) in the International Reference Ionosphere (IRI). For this purpose we have studied the correlation between hmF2 and the equatorial F region vertical drift as given by the model of Scherliess and Fejer (1999). The positive correlation found during nighttime could be helpful in representing the post-sunset peak of hmF2 that is currently not represented by IRI. We have also investigated the reliability of the CCIR model for the propagation factor M(3OOO)F2 model since the IRI hmF2 model is based on the strong anti-correlation between hmF2 and M(3OOO)F2. Overall the CCIR model represents the diurnal variation of M(3OOO)F2 quite well but does not represent small-scale features. With the M(3OOO)F2 values deduced from the Korhogo ionograms as input, the IRI hmF2 model provides an excellent representation of the observed diurnal structure including the post-sunset peak.

We will discuss various ways in which the International Reference Ionosphere (IRI) model and ionospheric data deduced from GPS measurements can be combined to improve ionospheric determinations. A number of research groups are analyzing GPS data products and providing global maps of vertical Total Electron Content (TEC) on a regular basis. IRI predictions can guide the interpolation of regional TEC estimations, computed from GPS data, to obtain global TEC maps. GPS measurements, on the other hand, can be used to update the IRI monthly averages to actual conditions. This can be done by using the GPS-derived TEC maps or by using the actual GPS measurements of the electron content along the signal path from satellite to ground receiver. We will discuss the updating results using the actual GPS measurements.

Using the TOPEX dual-frequency (Ku- and C-band) radar altimeter measured nadir total electron contents (TECs) from an orbital altitude of 1354 km, the fidelity of the IRI95 ionosphere model and GPS-derived Global Ionosphere Map (GIM) have been evaluated. In particular, the models have been evaluated over the ocean for the time period 1993–1999, representing low, medium and high solar activity periods. In addition, the evaluation is also focused on the model performance for TEC variations due to local solar time, geographical locations, and long-term trend, or drift. The major spatial and temporal features of ionosphere depicted by IRI95 model agrees reasonably well when comparing with TOPEX ground-truth, except during high solar activity period in 1999. The error in IRI95 would translate into an apparent sea level change of 0.3–0.6 mm/yr for the majority (70%) of the global ocean. During high solar activities in 1999, the error reaches 1 mm/yr. Results indicate that GIM provides more accurate ionosphere delays during high solar activities. For the use of single-frequency altimeters (e.g., GFO) for sea level studies during high solar activity period, ionosphere models need further improvement.

En: Modeling the Ionosphere–Thermosphere System (Geophysical Monograph Series),1st Edition by J. D. Huba (Editor), Robert W. Schunk (Editor), G. V. Khazanov (Editor).

The International Reference Ionosphere (IRI) is the international standard for the specification of ionospheric densities and temperatures. It was developed and is being improved-updated by a joint working group of the International Union of Radio Science (URSI) and the Committee on Space Research (COSPAR). A new version of IRI is scheduled for release in the year 2000. This paper describes the most important changes compared to the current version of IRI: (1) an improved representation of the electron density in the region from the F peak down to the E peak including a better description of the F1 layer occurrence statistics and a more realistic description of the low-latitude bottomside thickness, (2) inclusion of a model for storm-time conditions, (3) inclusion of an ion drift model, (4) two new options for the electron density in the D region, and (5) an improved model for the topside electron temperatures. The outcome of the most recent IRI Workshops (Kuhlungsborn, 1997, and Nagoya, 1998) will be reviewed, and the status of several ongoing task force activities (e.g., efforts to improve the representation of electron and ion densities in the topside ionosphere and the inclusion of a plasmaspheric extension) will be discussed. A few typical IRI applications will be highlighted in section 6.

NASA's National Space Science Data Center (NSSDC) provides internet access to a large number of space physics data sets and models. We will review and explain the different products and services that might be of interest to the IRI community. Data can be obtained directly through anonymous ftp or through the SPyCAT WWW interface to a large volume of space physics data on juke-box type mass storage devices. A newly developed WWW system, the ATMOWeb, provides browse and sub-setting capabilities for selected atmospheric and thermospheric data. NSSDC maintains an archive of space physics models that includes a subset of ionospheric models. The model software can be retrieved via anonymous ftp. A selection of the most frequently requested models can be run on-line through special WWW interfaces. Currently supported models include the International Reference Ionosphere (IRI), the Mass Spectrometer and Incoherent Scatter (MSIS) atmospheric model, the International Geomagnetic Reference Field (IGRF) and the AE-8/AP-8 radiation belt models. In this article special emphasis will be given to the IRI interface and its various input/output options. Several new options and a Java-based plotting capability were recently added to the Web interface.

The International Reference Ionosphere (IRI) is a widely used model recommended for international use by URSI and COSPAR. An interface to Windows has been developed and the stand-alone IRI program is executable on PC systems using Windows 95/98 or Windows NT as the platform. This IRI-Windows version allows computation and plotting of any of the IRI parameters as function of any one or two variables: height, latitude, longitude, UT or LT time, and solar activity. The user can conveniently specify the variables and various options of the model. This paper gives a brief description of the program features and the operating procedure.

The International Reference Ionosphere (IRI), a standard model for ionospheric densities and temperatures developed by a joint working group of the Committee on Space Research (COSPAR) and the International Union of Radio Science (URSI), is widely used for specifications of the ionospheric environment. This paper will discuss the importance of the solar and/or ionospheric indices for driving the IRI model. The different existing indices will be reviewed and future needs will be outlined.

Detailed in situ ion composition measurements in the E- and D- region ionosphere are available only from eccentric orbiting satellites with very low perigees and from sounding rockets. The lower limit for existing satellite data is about 130 km. Below this altitude mostly sounding rockets have sampled the ion composition. These sounding rocket data sets have been analyzed in the literature individually in the past on a case-by-case basis. In order to provide the best empirical picture possible of the lower ionosphere molecular ion distributions all available published sounding rocket observations of O 2 + and NO + ion concentrations were scanned and digitized to provide an empirical data base of the lower ionosphere. There were 43 such flights selected, which were scattered in latitude and local time. There were few low latitude observations, most of the measurements were made from middle and high latitude launch sites. The distributions are characterized at all latitudes by large drops in the concentrations from day to night with the molecular ion O 2 + having the greatest range of variation above ∼ 100 km and NO + below 90 km. Features of the sounding rocket data distributions at low latitudes, middle latitudes and at high latitudes and its limitations are discussed and the measurements compared with the International Reference Ionosphere (IRI) to investigate the accuracy of the IRI ion composition model in the lower ionosphere. The IRI ion composition model is based predominantly on dayside midlatitude observations for equinoctial and summer conditions. Our results show that winter nighttime measurements need to be expanded in order to improve the model.

The electron density profile in the F region bottomside is described in the International Reference Ionosphere (IRI) by two parameters: a thickness parameter B 0 and a shape parameter B 1. The models used for B 0 and B 1 in IRI are based on ionosonde data from magnetic mid-latitude stations. Comparisons with ionosonde data from several stations close to the magnetic equator show large discrepancies between the model and the data. We propose new models for B 0 and B 1 based on data from several ionosondes including low and mid latitude stations. Close to the magnetic dip equator the new B 0 model provides an improvement over the current IRI model by a factor of up to 1.5.

The current IRI topside model is based on the data compilation and grouping that was used for the Bent model (Bent et al., 1972). The principal data source for Bent were Alouette 1 sounder data from a period of low to medium solar activity. Through a systematic search of the NASA archives, we have established a large data base of topside profiles recorded by the Alouette 1, 2, and ISIS 1, 2 topside sounder instruments. Comparison of IRI with these data reveals shortcomings of the model especially in the upper topside. The largest discrepancies are found at low latitudes and at high solar activities. We will discuss the cause of these model limitations and ways to improve the current model with the help of our data base.

The existence of a permanent network of GPS ground receivers, the International GPS Service (IGS), with available data since 1991, allows to perform an exhaustive comparison of the global electron content derived from these data using tomographic models with the International Reference Ionosphere (IRI) predictions. This work will allow the IRI upgrading for the electron content forecast, using this data set at global scale and along more than half solar cycle. In this contribution we present a comparison involving more than 1300 stations times day worldwide distributed between 1991 (∼-10 stations) and 1997 (∼-200 stations).

In just a few years the Internet has become an essential element in national and international science endeavors. Transfer of data, models, and related software across the net has skyrocketed. Interactive WorldWideWeb modules let the users do everything from simply specifying time periods for data retrieval to plotting complex data sets, to computing model parameters and to finding magnetic conjunctions between points in space. This paper reviews Internet sources for solar-terrestrial data and models, with special emphasis on systems that are related to, or a result of, the Solar Terrestrial Energy Program (STEP). We also discuss on-line systems that provide specific interactive services to the research community, like data finding, viewing, and browsing capabilities, comparison with model parameters, etc. This html document provides links to a large number of WWW sites that might be of interest for STEP research. It does not attempt to be a comprehensive guide to such sites but rather to provide access to selected typical sites for the specific capabilities discussed. Throughout the paper Universal Resource Locator (URL) addresses are highlighted in italics; the links to all the services mentioned are available on the WWW at http://nssdc.gsfc.nasa.gov/space/STEP-links.html .

We are investigating potential improvements of the IRI topside model using a large data base of topside electron density profiles (more than 100 000 profiles) obtained with the Alouette and ISIS spacecraft from 1965 to 1980. Our modeling approach is based on elements of the older BENT model and of the current IRI topside model. We divide the topside profile into 4 segments reaching up to 3500 km compared to the 3 segments and the upper limit of 1000 km used in the BENT model. Compared to the fixed segment boundaries used in BENT and IRI we use dynamic segment boundaries. In each segment we assume a linearly varying density gradient whereas the BENT model takes a constant gradient. The global and temporal variations of the segment parameters are described in terms of geomagnetic latitude, solar sunspot number and foF2 in a manner similar to the current IRI method. Comparisons with TOPEX total electron data document the dramatic improvements obtained with this new model approach.

We have analyzed the trapped electron data (0.19–3.2 MeV) taken by the Japanese OHZORA satellite operated at 350–850 km altitude in polar orbit during 1984–1987 near solar minimum. The electron observations reveal all the global attributes of the quiet-time electron radiation belts, such as the South Atlantic Anomaly, the electron “slot”, and the outer radiation belt regions. The electron data are in general agreement with the NASA AE-8 electron model, but there are differences, particularly with respect to distinctive local-time variations in the slot region. In this paper, we present results from analyses of variations of the electron pitch angle distributions with local time, L -shell and altitude.

The International Reference Ionosphere provides a representation of the electron density and the ion composition in the E- and D-region (from 60 km upward) based on measurements from the ground (ionosonde, incoherent scatter) and from rockets. We will review the current status of the IRI E- and D-region models and discuss recent improvements and plans for future updates. Shortcomings and limits of the present description will be pointed out and the need for more measurements will be stressed. We will also discuss other venues of model improvement, e.g., results from theoretical models and input of measured characteristic parameters (E peak, D ledge).

This paper describes the 1995/1996 status of IRI activities listing the improvements/additions that are currently being implemented. It also reports about a special IRI Task Force effort at the International Center for Theoretical Physics focusing on a better description of the “intermediate region“ (bottomside and F1). IRI modelling efforts are also continuing in the topside ionosphere. Better representations of the region above the F peak are of critical importance for many investigations/applications that require Total Electron Content (TEC) predictions. We will briefly report about efforts at GSFC to make Alouette/ISIS topside sounder data available for the modelling community. Reliability and accuracy of model predictions can be considerably increased through updating with real-time measurements. IRI has for some time had the facility of updates with co-located simultaneous F peak parameters (foF2, NmF2, hmF2, M(3000)F2). To expand this capability, updating algorithms are now under development that will allow updates with available peak parameters and/or Total Electron Content (TEC) measurements.

Relative performances of available ionospheric models for computing path delay corrections to radar altimeter range data have been assessed using altimeter crossover measurements computed for the single-frequency altimeter measurements collected by Geosat and ERS-1, and for the dual-frequency altimeter measurements collected by TOPEX/POSEIDON. The models studied include IRI-90, IRI-95, and JPL GIM. The models are further compared with the ionospheric delay computed from TOPEX/POSEIDON dual-frequency altimeter data. Crossover analyses indicate that IRI-95 is an improved model over IRI-90 for computing ionospheric delays for all three altimetric satellites during low to medium solar activity periods. It is found that smoothing of the TOPEX/POSEIDON dual-frequency ionospheric corection significantly improved the precision of the TOPEX/POSEIDON data.

Single-frequency altimeter measurements require an estimate of the signal retardation effect in the ionosphere. Ionospheric models like the International Reference Ionosphere (IRI) are being used to obtain these correction factors. Incorporation of worldwide ionosonde measurements is discussed as a way towards more accurate predictions. Using measurements of the F peak critical frequency (foF2) from more than 70 ionosondes we have derived (hour and month dependent) regional and global ionospheric-effective solar indices. Replacing the solar index in IRI with these new indices we could improve the IRI estimates for the GEOSAT time period (1986–1989) by a few percent.

Ionospheric models are mostly unable to correctly predict the effects of space weather events on the ionosphere. This is especially true for the International Reference Ionosphere (IRI) which by design is a monthly median (climatologic) model (Bilitza et al., J. Geodesy, 85, 909-920, 2011). The IRI electron density profile is critically dependent on the correct values of the F2 layer peak height and density, hmF2 and NmF2 (or foF2). For the ionospheric characteristics IRI uses predictions based on CCIR/URSI coefficients (W. B. Jones and R. M. Gallet, ITU Telecomm. J. 29(5), 129-149, 1962) that were derived from the monthly median values of hourly ionosonde measurements. The diurnal variation of the foF2 characteristic, for example, is presented by the Fourier series foF2(T,φ,λ,χ)=a0(φ,λ,χ) + Σn=16 (an(φ,λ,χ)cos nT + bn(φ,λ,χ)sin nT), where T is Universal Time in hours, and φ, λ, χ are the geographic latitude, longitude, and modified dip latitude, respectively. The coefficients an and bn are in turn expanded as functions φm λ, χ resulting in a set of 24 global maps of 988 coefficients each, one for each month of the year and for two levels of solar activity, R12=10 and 100, where R12 is the 12-month running-mean of the monthly sunspot number Rm (2*12*988 = 23,712 coefficients in all) (ITU-R, Information Document on Ionospheric Mapping, Oct. 2011). Real time data from the Digisonde GIRO network (B. W. Reinisch and I. A. Galkin, Earth, Planets and Space, 63(4), 377-381, 2011) can be used to adjust the coefficients to produce more accurate foF2 maps. Adjusting 988 coefficients with merely 42 measured foF2 values is a completely underdetermined problem requiring special techniques (e.g., Galkin et al., Radio Sci., 47, RS0L07, 10 PP, 2012). We have applied the mathematical tool of linear optimization to determine new sets of 988 coefficients that reduce the global deviation of the model prediction from the measured values by a factor of 2.

The ionospheric low transition height obtained from a theoretical model is compared to the IRI model using the standard ion composition option and the Danilov and Yaichinikov model option, and to Oliver's model. It is found that 1) the three models agree rather well by day except in summer for low solar activity, while large differences exist by night; 2) the day-night difference of the transition height by the IRI standard option is much smaller in summer and in winter, but is larger in equinox seasons in low solar activity, while the Danilov and Yaichnikov model always yields a rather small difference; 3) increasing solar activity leads to higher theoretical levels particularly by night, while IRI shows an opposite trend. 4) due to transport by wind the simulation gives a transition height depending on local time, while both IRI options take the solar zenith angle for variable. As for the ion composition above 150 km, the distributions of the relative percentage of O+ as well as NO+ at noon in summer and winter under low solar activity predicted by the theoretical model are approximately consistent with those of the Danilov and Yaichnikov model.

Measurements by single-frequency satellite altimeter (Geosat, ERS-1) require a ionospheric correction to account for the signal time delay in the ionosphere. We propose using the International Reference Ionosphere (IRI) for the determination of this time delay. To investigate the effectiveness of an IRI correction, we have compared the IRI values with ionospheric corrections deduce from measurements by the dual-frequency Topex altimeter. By measuring at two frequencies, the Topex instrument can record (and thus eliminate) the ionospheric influence. We find that IRI agrees with the Topex data much better than the model that is currently used in Geosat data analysis. In particular the earlier model does not represent the equatorial double-peak (equator anomaly) clearly seen in the Topex data. Overall, the use of IRI results in a 30% improvement (over the older model) in the accuracy of ionospheric corrections computed for the first year of the Topex mission.

At the last International Reference Ionosphere (IRI) Workshop it was decided that future editions of the model should include a representationof the aurroral oval boundaries. In this paper we review the different existing parameterizations of the auroral oval discussing their data base, boundary criteria, matematical formation, and overall usefulness for IRI. As a first candidate for incorporation into IRI we recomment the parameterization of the Feldstein ovals by Holzworth and Meng. Ways of implementing this model into IRI are discussed. We will also address adjustability with user-provided boundaries or boundary-related parameters, to better support strom-related studies.