Impact of FGGE buoy system data on southern hemisphere meteorology

Although a definitive evaluation of the performance of the FGGE buoy system will take some time, it is already clear that the buoy data have made a major contribution to both operational and research aspects of southern hemisphere meteorology. The buoys have provided a wealth of information on both the surface drift and sea surface temperature characteristics of the Southern Ocean. Their impact on operational synoptic analysis and prediction in the southern hemisphere during the FGGE year was substantial. The buoy pressure data generally provided an adequate basis for delineation of the large scale flow over the middle and high latitude oceans in a way that has not been possible before. Preliminary evaluations of the impact of the buoy data on numerical prediction have indicated small but significant improvements in skill scores. Regional Forecasting Centres in the southern States of Australia have found that the more accurate delineation of synoptic systems over the oceans to the south has contributed significantly to forecast performance in their Regions. Use of the buoy data, and synoptic analyses based on the buoy data, to examine particular problems in southern hemisphere meteorology is now underway.     

Proposal for a reference model of the middle atmosphere of the southern hemisphere

Available rocketsonde information has been used to compile tables of monthly mean temperature, pressure, density and zonal wind for the middle atmosphere of the southern hemisphere with the purpose of revising similar tables presented to COSPAR earlier. The altitude range is 25 to 80 km in steps of 5 km. The latitude range is 0° to 70°S with a 10° step. The compatability of different sets of temperature measurements is discussed. Mean values of temperature, pressure and zonal wind obtained for the southern hemisphere are compared with northern hemisphere model values. Large differences between the hemispheres (up to 20°C in temperature, 20–30% in pressure, 30–50 m/s in wind) imply that reference atmospheres such as CIRA should be complemented by southern hemisphere climatology.     

Wavelet modelling in support of IRI

The slant total electron content (STEC) of the ionosphere is defined as the integral of the electron density along the ray-path of the signal between the transmitter and the receiver. So-called geometry free GPS measurements provide information on the electron density, which is basically a four-dimensional function depending on spatial position and time. Since ground-based measurements are not very sensitive to the vertical structure within the atmosphere, the ionosphere is often represented by a spherical layer, where all electrons are concentrated. Then the STEC is transformed into the vertical total electron content (VTEC), which is a three-dimensional function depending on longitude, latitude and time.In our approach, we decompose an ionospheric function, i.e. the electron density or the VTEC, into a reference part computed from a given model like the International Reference Ionosphere (IRI) and an unknown correction term expanded in a multi-dimensional series in terms of localizing base functions. The corresponding series coefficients are calculable from GPS measurements applying parameter estimation procedures. Since the GPS receivers are located rather unbalanced, finer structures are modelable just in regions with a sufficient number of observation sites. Due to the localizing feature of B-spline functions we apply a tensor product spline expansion to model the correction term regionally. Furthermore, the multi-resolution representation derived from wavelet analysis allows monitoring the ionosphere at different resolutions levels. We demonstrate the advantages of this procedure by representing a simulated VTEC data set over South America.     

Improved analytical representation of electron temperature in the IRI

In the last decade extensive measurements from incoherent scatter stations and from several satellite missions have considerably improved our knowledge of long- and short-term variations in the ionospheric electron temperature. Comparisons with IRI have revealed some shortcomings of this earlier model. It is obvious that in different altitude regions one has to concentrate the modelling efforts on different parameters. Here a model representation is proposed that will facilitate approaches (for the different altitude regions) in one analytic form.     

The state of the art in D-region IRI modelling

While describing the formulation of IRI-79, in so far as it is applicable to the sub E-peak electron density profile, an attempt is made to compare its predictions with the available experimental evidence. The improvements needed for D-region electron density modelling are summarized. The current D-region IRI modelling effort is illustrated with a typical example.     

Computerized ionospheric tomography with the IRI model

Computerized ionospheric tomography (CIT) is a method to estimate ionospheric electron density distribution by using the global positioning system (GPS) signals recorded by the GPS receivers. Ionospheric electron density is a function of latitude, longitude, height and time. A general approach in CIT is to represent the ionosphere as a linear combination of basis functions. In this study, the model of the ionosphere is obtained from the IRI in latitude and height only. The goal is to determine the best representing basis function from the set of Squeezed Legendre polynomials, truncated Legendre polynomials, Haar Wavelets and singular value decomposition (SVD). The reconstruction algorithms used in this study can be listed as total least squares (TLS), regularized least squares, algebraic reconstruction technique (ART) and a hybrid algorithm where the reconstruction from the TLS algorithm is used as the initial estimate for the ART. The error performance of the reconstruction algorithms are compared with respect to the electron density generated by the IRI-2001 model. In the investigated scenario, the measurements are obtained from the IRI-2001 as the line integral of the electron density profiles, imitating the total electron content estimated from GPS measurements. It has been observed that the minimum error between the reconstructed and model ionospheres depends on both the reconstruction algorithm and the basis functions where the best results have been obtained for the basis functions from the model itself through SVD.     

A plan for compilation of experimental data on drifts for IRI

Methods for constructing a dynamical part of the IRI model are described.     

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.     

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.     

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.     

New input to IRI from the worldwide digisonde network

Electron density profiles derived from Digisonde ionograms at Argentia, Millstone Hill, Wallops Island, Bermuda, Dourbes and Karachi are compared with IRI model prediction. Four months of data for 1989/90 were analyzed. For a number of station/months N(h) profiles were available every 15 or 30 minutes providing a good statistical database for the evaluation of the IRI model in terms of diurnal and seasonal variations. The data presented here are part of the VIM study (Validation of Ionospheric Models) initiated by the URSI Working Group G3 on Ionospheric Informatics.     

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.     

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.     

D-region IRI profiles in relation to radio observations

The D-region IRI profiles are compared with the direct rocket measurements as well as with ground-based radio observations by a variety of techniques. The characteristics of D-region IRI profiles and the dependence of electron density on solar zenith angle, sunspot number, latitude and season are discussed. The sensitivity of certain reflection coefficients on the height distribution of electron density below 70 km is illustrated with a typical example. For D-region modelling, the results show the importance of simultaneous measurement of reflection and conversion coefficients together with polarization phase over a wide frequency range.     

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.