Modelling of ionospheric drifts in view of IRI

D/E region drift data, as worldwide obtained from 1957 to 1970 by methods D1 and D3, are statistically analyzed and a computer program describing the average variations in geomagnetic latitude and time is developed.     

Comparisons of foF2 with IRI model and equatorial vertical drifts

Measurements of the critical frequency, foF2 recorded over Ibadan: 7.4°N, 3.9°E (geographic), 6°S (dip angle) have been compared with the International Reference Ionosphere (IRI-2007) model for solar maximum geomagnetically quiet conditions, with a view to determining what modifications might bring about better predictions for the model. Our results reveal that the present version of IRI essentially reproduces diurnal trends and the general features of the experimental observations for all seasons, except for nighttime June solstice periods, which the model seriously overestimated. The model errors ranging from 50% to 125% over the four seasons considered in this study. It is also indicated that the percentage relative deviations between the observed and the modeled values vary approximately from −11% to 12% (March), −34% to 11% (June), −16% to 12% (September), and −10% to 13% (December). An unexpected feature of foF2 is obvious and remarkable reduction in values during nighttime June solstice periods compared to that in other seasons. Relationship between equatorial vertical drift and foF2 is also investigated. However, cross correlation analysis reveals strong anti-correlation between vertical drift and critical frequency during the daytime hours, but exceptionally opposite is the case for the nighttime sector. The discrepancies which are noted, particularly during June solstice season are attributed to processes most likely within the thermosphere and from meteorological influences during quiet magnetic conditions.     

A morphological study of GPS-TEC data at Agra and their comparison with the IRI model

The temporal and seasonal variations of Total Electron Content (TEC) are studied at Agra (Geographic Lat. 27.17°N, Long. 78.89°E, Dip: 41.4°), India, which is in the equatorial anomaly region, for a period of 12 months from 01 January to 31 December, 2007 using a Global Positioning System (GPS) receiver. The mean TEC values show a minimum at 0500 h LT (LT = UT + 5.5 h) and a peak value at about 1400 h LT. The lowest TEC values are observed in winter whereas largest values are observed in equinox and summer. Anomalous variations are found during the period of magnetic disturbances. These results are compared with the TEC derived from IRI-2007 using three different options of topside electron density, NeQuick, IRI01-corr, and IRI-2001. A good agreement is found between the TEC obtained at Agra and those derived from IRI models.     

Verification of IRI plasma temperature at great altitude by satellite data

Electron temperature data obtained aboard two satellites are compared with the IRI model. Our comparison may serve as an input to a future version of IRI. HINOTORI data show IRI90 to predict night time T_e quite well with the exception of a few peculiar phenomena, in particular a morning overshoot at height around 600 km and anomalous T_e observations of AKEBONO for the first time allowed to extend reliably the temperature profile up to − 8000km. This profile clearly depends on local time at all invariant latitudes. The data suggest strong influence of solar activity on electron temperature.     

Comparison of A1-absorption data with theoretically computed values based on the International Reference Ionosphere (IRI)

Total absorption of hf radio waves at vertical incidence is calculated using the IRI electron density N(h) profiles at a low latitude for low and high solar activities and the calculated values of absorption are compared with the observed values. It is found that the IRI model holds good in this respect only for equinoxial months in years of low solar acitvity; however, it yields much higher values of absorption than observed during years of high solar activity (all seasons). It is suggested that seasonal anomalous variations of gas composition and bottomside thickness of the E-layer may be given due weight in revising the IRI.     

Regional 4-D modeling of the ionospheric electron density from satellite data and IRI

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.     

Adaptation of the bottomside electron density profile of the IRI model to data of topside radiosounding

A method is proposed for reconstructing the electron density profiles N(h) of the IRI model from ionograms of topside satellite sounding of the ionosphere. An ionograms feature is the presence of traces of signal reflection from the Earth's surface. The profile reconstruction is carried out in two stages. At the first stage, the N(h) –profile is calculated from the lower boundary of the ionosphere to the satellite height (total profile) by the method presented in this paper using the ionogram. In this case, the monotonic profile of the topside ionosphere is calculated by the classical method. The profile of the inner ionosphere is represented by analytical functions, the parameters of which are calculated by optimization methods using traces of signal reflection, both from the topside ionosphere and from the Earth. At the second stage, the profile calculated from the ionogram is used to obtain the key parameters: the height of the maximum hmF2 of the F2 layer, the critical frequency foF2, the values of B0 and B1, which determine the profile shape in the F region in the IRI model. The input of key parameters, time of observation, and coordinates of sounding into the IRI model allows obtaining the IRI-profile corrected to real experimental conditions. The results of using the data of the ISIS-2 satellite show that the profiles calculated from the ionograms and the IRI profiles corrected from them are close to each other in the inner ionosphere and can differ significantly in the topside ionosphere. This indicates the possibility of obtaining a profile in the inner ionosphere close to the real distribution, which can significantly expand the information database useful for the IRTAM (IRI Realmax Assimilative Modeling) model. The calculated profiles can be used independently for local ionospheric research.     

Evaluation of the IRI model for the European region

The aim of this paper is to investigate various aspects of the International Reference Ionosphere (IRI) performance in European area and to evaluate its accuracy and efficiency for: long term prediction of the critical frequencies foF2 and the maximum usable frequencies (MUF); using storm-time correction option (ST); the total electron content (TEC) and the maximum observable frequency (MOF) updating. Data of foF2, TEC, MOF are related to 2005. It is obtained that median values of foF2 can be predicted with the mean error σ(med)∼ 0.49 MHz. For median values of MUF absolute σ was 1.39 MHz and relative σ_r was 8.8%. For instanteneous values estimates are increased to 1.58σ(med) MHz for foF2 and could reach 3.84 MHz for MUF. Using correction ST-option and TEC values provided ∼30% improvement but TEC seems to be more preferable. However, from considered parameters of the IRI updating (ST-factor, TEC, MOF) the best results were demonstrated by MOF. Using the IRI2007 to calculate TEC gives 20–50% improvement of TEC correspondence to experimental values but this improvement is not enough to treat TEC without the IRI model adaptation.     

A new model of the ion composition at 75 to 1000 km for IRI

Ion composition of the ionosphere is an important parameter of any ionospheric model. The International Reference Ionosphere-1979 includes a program for the relative ion composition computation. The program was constructed on the basis of the Danilov and Semenov /1/ empirical model, which averaged 42 rocket measurements of the ion composition at middle latitudes below 200 km, on “AEROS” satellite measurements, and on Taylor's data /2/ above that altitude.     

Improvement of the solar-cycle variation of IRI lower ionosphere models by means of radio wave propagation data

Data on day-time and night-time radio wave absorption in the frequency range 50 to 2614 kHz, obtained in long-term observational programmes in Central Europe, are compared with corresponding absorption values calculated from electron density profiles of the International Reference Ionosphere (IRI-1979) using the full-wave method.Discrepancies between calculated and observed absorption values were found for the diurnal and the solar-cycle variation, the amplitudes of the solar-cycle variation of absorption being considerably larger than the observed variation.A modification of the solar-activity dependence of the D-region electron density parameters is derived, which provides an improvement of the solar-cycle variation as well as the diurnal variation of the IRI electron density profiles.     

Analytical expression of O---H ion transition surface for use in IRI

A global surface of O⁺---H⁺ transition level is constructed, based on published data from OGO-6, Intercosmos-2, Alouette-1, ISS-b, and TAIYO satellites. This surface covers ±60° dipole latitude, all longitudes, two levels of solar activity, summer and winter solstices, and 00 and 12 hours local time. The surface is used as input data to a mathematical model which calculates transition levels in 5-dimensional space: sunspot number (R), month (M), local time (LT), dipole latitude (DL), and longitude (LONG). This model is based on a generalized multivariable polynomial, using a system of linearly independent functions. Model transition levels are compared with averaged data from AE-E and AE-C, as well as rocket measurements from Vertical-6 and Vertical-10. The obtained analytical expression can be directly used in IRI.     

Highlights of the 1975 URSI/COSPAR workshop on the IRI

Some of the presentations made at the IRI Workshop held at Louvain in 1985 have not appeared in the Proceedings. Some discussions which took place during the workshop could be of interest in future work. This article tries to highlight the IRI modelling aspects in the four principal height regions of the ionosphere.     

Global validation of IRI TEC for high and medium solar activity conditions

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.     

A simplified D-region ion chemistry scheme and its possible use for IRI lower ionosphere modelling

Ion composition of the D region is principally characterized by the existence of two distinct regions of predominant molecular ions and predominant cluster ions, separated from each other by a rather sharp ‘transition height’, which is proposed to be included in the IRI as an additional parameter, supplementing the electron density models. It is possible to predict the position of this ‘transition height’ at a given place and time with the aid of a simplified ion chemistry scheme which is shown to be satisfactorily compatible with experimental ion composition data available in the literature. Our suggested method of this prediction makes use of the (IRI or experimental) electron density profile at the location and season in question, together with an effective clustering rate coeeficient calculated from corresponding temperature and density profiles taken from a suitable reference model of the neutral atmosphere.     

A new global F2 peak electron density model for the International Reference Ionosphere (IRI)

A new neural network (NN) based global empirical model for the F2 peak electron density (NmF2) has been developed using extended temporal and spatial geophysical relevant inputs. Measured ground based ionosonde data, from 84 global stations, spanning the period 1995 to 2005 and, for a few stations from 1976 to 1986, obtained from various resources of the World Data Centre (WDC) archives (Space Physics Interactive Data Resource SPIDR, the Digital Ionogram Database, DIDBase, and IPS Radio and Space Services) have been used for training a NN. The training data set includes all periods of quiet and disturbed magnetic activity. A comprehensive comparison for all conditions (e.g., magnetic storms, levels of solar activity, season, different regions of latitudes, etc.) between foF2 value predictions using the NN based model and International Reference Ionosphere (IRI) model (including both the International Union of Radio Science (URSI) and International Radio Consultative Committee (CCIR) coefficients) with observed values was investigated. The root-mean-square (RMS) error differences for a few selected stations are presented in this paper. The results of the foF2 NN model presented in this work successfully demonstrate that this new model can be used as a replacement option for the URSI and CCIR maps within the IRI model for the purpose of F2 peak electron density predictions.     

Towards a new reference model of hmF2 for IRI

A numerical model of the peak height of the F2 layer, hmF2_top, is derived from the topside sounding database of 90,000 electron density profiles for a representative set of conditions provided by ISIS1, ISIS2, IK19 and Cosmos-1809 satellites for the period of 1969–1987. The model of regular hmF2 variations is produced in terms of local time, season, geomagnetic latitude, geodetic longitude and solar radio flux. No geomagnetic activity trends were discernible in the topside sounding data. The nighttime peak of hmF2_top evident for mid-latitudes disappears near the geomagnetic equator where a maximum of hmF2_top occurs at sunset hours when it can exceed 500 km at solar maximum. The hmF2 given by the IRI exceeds hmF2_top at the low solar activities. The hmF2_top, obtained by extrapolation of the first derivative of the topside profile to zero shows saturation similar to foF2 the greater the solar activity. The proposed model differs from hmF2 given by IRI based on M(3000)F2 to hmF2 conversion by empirical relationships in terms of foF2, foE and R12 with these quantities mapped globally by the ITU-R (former CCIR) from ground-based ionosonde data. The differences can be attributed to the different techniques of the peak height derivation, different epochs and different global distribution of the source data as well as the different mathematical functions involved in the maps and the model presentation.