O.A. Maltseva

Obtaining Ionospheric Conditions according to Data of Navigation Satellites

Defining ionospheric conditions, the deviation of the observational value of the total electron content TEC(obs), measured by means of navigation satellites, from a median is a bench mark. According to more than 40 ionospheric stations during April 2014 it is shown that synchronism of change of deviations of TEC and critical frequency foF2 of the ionosphere is kept under quiet and moderate disturbed conditions. This fact allows to use a median of the equivalent slab thickness τ(med) as a reliable calibration factor to calculate foF2 from TEC(obs). The efficiency coefficient of joint use of τ(med) and TEC(obs) changes from 1.5 to 4 with average value 2.2 across the globe. The highest coefficient corresponds to middle latitudes, however the estimations obtained for high- and low-latitude areas indicate possibility to use τ(med) and TEC(obs) in these areas.

Impact of magnetic storms on the global TEC distribution

The study is focused on the analysis of Total Electron Content (TEC) variations during six geomagnetic storms of 10 different intensity: from Dstmin = – 46 nT to Dstmin = -223 nT. The values of TEC deviations from its 27-day median value (δTEC) were calculated during the periods of the storms along three meridians: American, Euro-African and AsianAustralian. The following results were obtained. For the majority of the storms almost simultaneous occurrence of δTEC maximums was observed along all three meridians at the beginning of the storm. The transition from weak storm to superstorm (the increase of magnetic activity) almost does not affect the intensity of δTEC maximum. The seasonal effect 15 was most pronounced at Asian-Australian meridian, less often at Euro-African meridian and was not revealed at American meridian. Sometimes the seasonal effect can penetrate to the opposite hemisphere. The character of averaged δTEC variations for the intense storms was confirmed by GOES satellite data. Though there are some common features of TEC variation revealed during each storm phase, in general no clear dependence of TEC responses on the storm phases was found: the effects were different during each storm at different locations. The behaviour of correlation coefficient (R) 20 between δTEC at three meridians was analyzed for each storm. In general, R>0.5 between δTEC averaged along each meridian. This result is new. The possible reasons for the exceptions (when R < 0.5) were provided: time-shift of δTEC maximum at different latitudes along the American meridian, the complexity of phenomena during the intense storms and discordance in local time of geomagnetic storm beginning at different meridians. Notwithstanding the complex dependence of R on the intensity of magnetic disturbance, in general R decreased with the growth of storm intensity. 25

Effectiveness of using correcting multipliers in calculations of the total electron content according to the IRI2007 model

A comparison of the total electron content (TEC) of the ionosphere calculated using different correcting multipliers in the IRI2007 model with experimental data indicates that this model, which is not empirical with respect to the TEC, does not adequately reflect the quantitative and qualitative features of the TEC behavior. The situation can be improved by using new empirical models of the critical frequency and equivalent thickness of the ionosphere and new methods for determining the TEC.

Verification of Ionospheric Models by TEC and Satellite Measurements

Despite a huge stream of the experimental data, allowing us to estimate ionospheric parameters in near real time, necessity for ionospheric modeling does not decrease. Transition to developing a model of the most changeable structure - the main ionospheric trough (MIT) was now outlined. The latest model is the Karpatchev and co-authorss model of 2016 which concerns night winter conditions (the most probable occurrence of a trough). The model is constructed according to vertical sounding and measurements of plasma frequency on satellites. As the increasing role in an estimation of ionospheric conditions is played by the total electron content (TEC), in the present paper the problem of comparison of TEC behaviour with the MIT model was put. It is shown, that TEC always shows presence of the trough however its form is more smoothed. Good enough conformity exists for longitudinal dependence of MIT, but latitudinal dependence of the TEC trough is closer to data of low-orbit satellites. Testing of the International Reference Ionosphere model is a traditional problem. In the present paper, the basic attention was given to conformity of ionospheric and plasma frequencies at height of the satellite and to use TEC. It is shown, that the usage of TEC increases accuracy of definition of critical frequencies in 1.5 times in comparison with the model in a range 5° on a latitude and 90° on a longitude from ionosondes.

One of possibilities to use TEC-GPS data for HF links

Many applications of high frequency (HF) propagation via the ionosphere (e.g., bearing-fingers, single station location, utilization of oblique ionograms and so on) demand a determination of distance from radio wave transmitters to receivers. It is shown that main errors of the distance determination are introduced by errors of angle measurements and errors of model description of propagation conditions. Since it is unable essentially to lower the first type of errors in real conditions the main attention must be paid to the increase of the model ionospheric description precision. For this purpose the correction of the International Reference Ionosphere (IRI) model by total electron content TEC-GPS data is used. Results of maximum usable frequency MUF and ground range D determination are compared with results of the IRI model adaptation by means of vertical sounding (VS) data.

Use of TEC to determine foF2: differences and similarities at high and low latitudes

The total electron content TEC finds wide application in various scientific and technological areas. One of applications is the use of TEC to obtain critical frequencies foF2 of the ionosphere. The most studied is the situation in middle latitudes. High and low latitudes are considered as problematic zones. In the present work, the situation in high and low latitudes is investigated, including at comparison with results in middle latitudes. Possibility of the TEC use to obtain foF2 is connected with their high correlation however in details this correlation was not investigated. In the present work, on an example of three stations of the American zone (Thule, Millstone Hill, Puerto Rico), latitudinal section of the northern hemisphere on a meridian 75°W it is shown, that: 1) correlation coefficients p(TEC, foF2) between foF2 and TEC lay in a range 0.7-1.0 in a seasonal course and 0.6-0.85 in a daily course with the least values for the station Thule. 2. The most clear seasonal dependence is related to that in the winter (January, December) correlation is worse, in an equinox (March, September) is better. 3. There is a tendency to higher correlation and smaller scatter of values in years of high solar activity, than in low activity. 4. Correlation coefficients ρ(δTEC, δfoF2) of deviations between δTEC and δfoF2 lay in the same range, as coefficients for magnitudes, and not strongly worsen during the disturbed periods, the least values here are at the station Puerto Rico. Estimation of a dependence ρ(δTEC, δfoF2) from indexes Kp and Dst and its approximations by a polynomial with degree n has shown, that: 1) degree of polynomial approximation and the best index of correlation depend on latitude (for high-latitude stations the greatest correlation is observed with index Kp, for middle - and low-latitude stations correlation with Dst can prevail). 2. Degree and the best index of correlation depend on a season (the greatest correlation is observed in winter months for high-latitude stations and in the summer for low-latitude stations). 3. Degree n=3 is insufficient for approximation, often degree of reliability R^2 for n=4-6 exceeds reliability for n=3 twice. 4. Reliability degree can exceed the level 0.5 testifying strong dependence and possibility of the forecast of parameters. As a whole, differences between high and low latitudes consist in a difference of coefficients of correlation, similarities - in possibility of the TEC use to obtain foF2 practically at all latitudes, i.e., details of behavior of TEC can differ, but the main result is identical: the use of TEC allows to improve conformity of the calculated values of foF2 to experimental data in 1.5-2.5 times. Residual deviations do not exceed 10 %.

Correction of Electron Density Profiles in the Low Ionosphere Based on the Data of Vertical Sounding with the IRI Model

The method of correcting the daytime vertical profiles of electron plasma frequency in the low ionosphere from International Refererence Ionosphere (IRI) model in accordance with the measured data of the virtual heights and absorption of signal radiowaves (method А1) reflected from the bottom of E-region at vertical sounding (VS) is presented. The method is based on the replacement of the IRI model profile by an approximation of analytical dependence with parameters determined according to VS data and partially by the IRI model. The method is tested by the results of four joint ground-based and rocket experiments carried out in the 1970s at midlatitudes of the European part of Russia upon the launches of high-altitude geophysical rockets of the Vertical series. It is shown that the consideration of both virtual reflection heigths and absorption makes it possible to obtain electron density distributions that show the best agreement with the rocket measurements made at most height ranges in the D- and E-regions. In additional, the obtained distributions account more adequately than the IRI model for the contributions of D- and E-regions to absorption of signals reflected above these regions.

Estimates of ionosphere state over Mexico with TEC data

The ionosphere behaviour over Mexico was studied. We analysed variations of Total Electron Content (TEC) and critical frequency of F2-layer of the ionosphere (foF2). TEC values were extracted from data of local GNSS receivers and if missed then from global ionospheric maps. foF2 values were reconstructed using TEC measured over Mexico and equivalent slab thickness of the ionosphere measured in the adjacent to Mexico regions having ionosondes. Diurnal and seasonal patterns of TEC and foF2 behaviour over Mexico were revealed. The peculiarity of TEC behaviour during disturbances were studied. The presence of strong positive enhancements is a characteristic feature for Mexico that is confirmed by measurements of electron concentration at satellites CHAMP and DSMP.

Empirical Modeling of the Total Electron Content of the Ionosphere

With the appearance of such satellite systems as GPS, GLONASS, Galileo, and others, the total electron content TEC measured by means of navigational satellites became a key parameter characterizing a state of the ionized space. In turn, functioning of navigational and telecommunication systems needs models of TEC for an estimation of accuracy of positioning, for the short-term and long-term prediction of this parame‐ ter. In this Chapter, empirical models of the total electron content are presented. The new result is their comparison. It is shown that the majority of them provide an adequate accuracy and reliability. As the basic application of TEC measurements, the problem of determination of maximum concentration NmF2 of the ionosphere with use of its equivalent slab thickness τ is considered. It is shown that existing models of τ are not global and do not provide sufficient accuracy in determining NmF2. An approach for new global model is offered.

The Use of the Total Electron Content Measured by Navigation Satellites to Estimate Ionospheric Conditions

Measurements of delays of the signals radiated by transmitters of navigational satellites allow us to obtain the total electron content (TEC). In addition, measurements of TEC allow solving problems such as development of local, regional, and global models of TEC and correction of ionospheric delay for increasing accuracy of positioning. Now, it is possible to set the task of calculation of critical frequency foF2 with the use of experimental values of TEC in a global scale. For this purpose it is necessary to know an equivalent slab thickness of the ionosphere which is a coefficient of proportionality between TEC and a maximum density of the ionosphere. The present paper is devoted to the analysis of investigation and utilization of this parameter. It is shown that (1) existing models of τ are not empirical and not always can provide an adequate accuracy of foF2 calculation, (2) experimental median (med) provides much larger accuracy of foF2 calculation than the empirical model and variations from day to day and allows filling gaps in the ionosonde data, and (3) it is possible to use a hyperbolic approximation and coefficient for development of a global model of .