Vol49,4_5,2006 891 ANNALS OF GEOPHYSICS, VOL. 49, N. 4/5, August/October 2006 Mailing address: Dr. Miguel A. Cabrera, Laboratorio de Ionósfera, Departamento de Física, Facultad de Ciencias Exactas y Tecnolgía, Universidad Nacional de Tucumán, Av. Independencia 1800, 4000 San Miguel de Tucumán, Argentina; e-mail: mcabrera@herrera.unt.edu.ar Key words Total Electron Content – ionosphere – space weather – satellite – scale height 1. Introduction For satellite and aircraft navigation systems or satellite orbit and position determination it is necessary to use radio signals transmitted be- tween the satellite and the ground station (Har- ris et al, 2001; among others). The ionosphere produces several effects on transionospheric ra- dio waves (Hargreaves, 1992). These effects are proportional to the number of free electrons en- countered by the wave on its passage through the ionosphere (total electron content, TEC) (Rishbeth and Garriot, 1969). TEC is a key pa- rameter that describes the major impact of the ionized atmosphere on the propagation of radio waves, which is crucial for terrestrial and Earth- space communications. The ionospheric correc- tions that have to be applied to determine the position accurately are proportional TEC along the radar-satellite path (Hartmann and Leitin- ger, 1984; Lin, 2001; among others). So, for ionospheric corrections, TEC measurements are required, or TEC predictions from ionos- pheric models can be a useful tool. Different ionospheric models have been de- veloped to predict the electron density (N) distri- bution in height, which is called N-profile (Chiu, 1975; Anderson et al, 1987; among others), in- cluding the IRI model (Bilitza, 1990; Rawer and Bilitza, 1990). With this N-profile the vertical to- tal electron content can be obtained. Neverthe- less, most of the signal paths are slant paths. Slant total electron content for Sirio-Mortelliccio ray path Miguel A. Cabrera (1)(3), Rodolfo G. Ezquer (1)(2)(3) and Paolo Spalla (4) (1) Laboratorio de Ionósfera, Departamento de Física, Facultad de Ciencias Exactas y Tecnolgía, Universidad Nacional de Tucumán, San Miguel de Tucumán, Argentina (2) Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Buenos Aires, Argentina (3) Facultad Regional Tucumán, Universidad Tecnológica Nacional, San Miguel de Tucumán, Argentina (4) Istituto di Fisica dell’Atmosfera (IFA), CNR, Roma, Italy Abstract The Total Electron Content (TEC) is used to indicate the ionisation of the ionosphere. TEC is a quantity that con- cern for predicting space weather effects on telecommunications, improving the accuracy of satellite navigation, fly control vehicles and other systems that use transionospheric signals, because the ionospheric layer affects the mentioned signals. In this work the Slant Total Electron Content (STEC) was calculated with a technique that uses so-called «auxiliaries stations model», and a Chapman layer with scale height equal to atomic oxygen scale height (CHO). The validity was checked with STEC measurements obtained from geosynchronous satellite sig- nals, for SIRIO-Mortelliccio link considering solstices and equinox, in high solar activity period. In general, the deviations between predictions and measurements were lower than 30% for 16 h per day (average). The results suggest that additional studies for other links and solar activity are required in order to improve the model pre- dictions. 892 Miguel A. Cabrera, Rodolfo G. Ezquer and Paolo Spalla In general to model the ionosphere, the so- called thin layer approximation is adopted (Manucci et al, 1999; among others), the STEC is related with the vertical total electron content (VTEC) through the piercing point with the obliquity factor (Ciraolo and Spalla, 2002; Brunini et al., 2004; among others). The purpose of the present work is to calcu- late the TEC along the ground station-satellite ray path, called Slant Total Electron Content (STEC). A computation method and ionospher- ic model are used. We adopted a Chapman lay- er (Chapman, 1931) with scale height equal to atomic oxygen scale height, hereafter referred to as CHO. 2. Method To calculate the STEC the length of the slant path is divided into segments of 20 km. The ver- ticals that pass through the ends of these seg- ments intersect the Earth’s surface in different points that we call «auxiliaries stations». The co- ordinates of these stations are determined. With an ionospheric model the electron densities at the points where the slant path intersects the verticals of the auxiliaries stations are calculated and from them the slant N-profile is obtained. Then, a nu- merical integration method and this slant N-pro- file are used to calculated STEC up to 2000 km of altitude (Cabrera, 2003). 3. Model and discussion The Chapman layer offers a simple way to explain the vertical structure of plasma in upper atmosphere (Yonezawa, 1955; Yonezawa and Takahashi, 1960; Titheridge, 1993; Huang and Reisnich, 2001). The Chiu (1975) empirical model assumes that, F2 region plasma density can be expressed by two standard Chapman profile expressions, one for the bottomside of the F layer and one for the topside. The semi- empirical low-latitude ionospheric model (SLIM) (Anderson et al., 1987) assumes two modified Chapman expressions to obtain the F region N profile, one for the bottomside and one for the topside. Wright (1960) has shown that is possible to apply the following Chapman expression to the F region: (3.1) where z= ( h−hm)/H is the normalised height measured from hm in units of the scale height H. He purposed to take H as the scale height of atomic oxygen. In this paper we assume the Chapman expression given by eq. (3.1) with H equal to atomic oxygen scale height. Nm and hm correspond to peak characteristics and are obtained from CCIR global maps (CCIR, 1982). H was calculated using H = kT/mg, where k, T, m and g are Boltzman’s constant, neutral temperature, atomic oxygen mass and the acceleration due to gravity, respectively. The value of T is obtained from MSIS-86 mod- el (Hedin, 1987). The model predictions were compared with measurements obtained at Mortelliccio, a mid- dle latitude station. Figure 1 shows the calculat- ed STEC for SIRIO-Mortelliccio (42.9°N, 10.7°E) ray path. For the considered link the satellite was placed at (0°N, 345°E). Equinoxes and solstices for high solar activity period are considered. It can be seen that, in general, there is a good agreement between CHO model and measure- ments for solstices (June and December), 16 h per day (average). In December the model over- estimates STEC during 14 h. April picture shows a secondary maximum, at 19 UT, that the model does not describe. The fact that the modelled STEC values corresponding to winter are grater than those of summer shows that the model pre- dicts the occurrence of the «winter anomaly» (Rishbeth and Garriot, 1969). The worst dis- agreement is observed for August, the model overestimates the measurements since 0 to 5 UT and 20 to 23 UT, with deviations greater than 30%. In general the model represents the STEC with good agreement for hours of high ionisation, for the considered period. Table I shows the deviations (D) between modelled values and measurements. The white, light grey and dark grey boxes correspond to the cases where D ≤ 30%, 30 < D ≤ 50% and D > 50%, respectively. It can be seen that for ( ) . ( )exp expN z Nm z z0 5 1= - - -6 @" , Slant total electron content for Sirio-Mortelliccio ray path 893 Fig. 1. Calculated and measured STEC for Mortelliccio-Sirio ray path: median values. 894 Miguel A. Cabrera, Rodolfo G. Ezquer and Paolo Spalla daytime hours a great amount of white boxes are observed. Few cases show deviations greater than 50% and they correspond to night time hours. 4. Conclusions This work calculated the Slant Total Elec- tron Content (STEC). A Chapman layer with scale height equal to atomic oxygen scale height (CHO) was considered. The validity was checked with STEC meas- urements obtained from geosynchronous SIRIO satellite signals and received at Mortelliccio, con- sidering solstices and equinox, in high solar ac- tivity period. For the considered cases, the results suggest that CHO model shows an adequate perform- ance to predict the STEC. In general the devia- tions between predictions and measurements were lower than 30%, for 16 h per day. The observed disagreements between pre- dictions and measurements could arise because peak characteristics or the shape of N profile, or both are not well predicted. Additional studies for other links and condi- tions are required in order to study the perform- ance of the model to predict STEC. REFERENCES ANDERSON, D.N., M. MENDILLO and B. HERNITER (1987): A semi-empirical, low-latitude ionospheric model, Re- port AFGL-TR-85-0254 (Air Force Geophysics Labo- ratory, Hanscom AFB, Massachusetts). BILITZA, D. (1990): International reference ionosphere, NSSDC/WDC-A-R&S 90-22 (Maryland, EEUU). BRUNINI, C., A. MEZA, F. AZPILICUATA, M.A. VAN ZELE, M. GENDE and A. DÍAZ (2004): A new ionosphere monitor- ing technology based on GPS, Astrophys. Space Sci., 290, 415-429. CABRERA, M.A. (2003): Ph.D. Thesis (Universidad Na- cional de Tucumán, Argentina). CCIR (1982): Comité Consultatif International des Radio- communications (International Telecomunication Union, Place des Nations, Switzerland). CHIU, Y.T. (1975): A improved phenomenological model of ionosphere density, J. Atmos. Terr. Phys., 37, 1536. CIRAOLO, L. and P. SPALLA (2002): TEC time variability, in Proceedings of the IRI Task Force Activity 2001, IC/IR/2002/23, Trieste. HARGREAVES, J.K. (Editor) (1992): The Solar-Terrestrial Environment (Cambridge Atmospheric and Space Sci- ence Series, Cambridge University Press), pp. 420. HARRIS, I.L., A.J. MANNUCCI, B.A. IIJIMA, U.J. LINDQWIS- TER, D. MUNA, X. PI and B.D. WILSON (2001): Ionos- pheric specification algorithms for precise GPS-based aircraft navigation, Radio Sci., 36 (2), 287-298. HARTMANN, G.K. and R. LEITINGER (1984): Range error due to ionospheric and tropospheric effects for signals fre- cuencies above 100 MHz, Bull. Geodin., 58, 109-136. HEDIN, A.E. (1987): MSIS-86 thermosphere model, J. Geo- physics Res., 92, 4649. HUANG, X. and B. REINISCH (2001): Vertical electron con- tent from ionograms in real time, Radio Sci., 36 (2), 335-342. LIN, LAO-SHENG (2001): Remote sensing of ionosphere us- ing GPS measurements, in Proceedings Asian Assosia- tion on Remote Sensing, Singapore, vol. 1, 69-74. MANUCCI, A.J., B.A. IIJIMA, U.J. LINDQWISTER, X. PI, L. Table I. Percentual deviations between modelled and measured values. CHO Mortelliccio Date April-82 June-82 August-82 December-81 R12 124 117 109 138 0 UT 35.8 34.2 76.4 12.7 1 27.1 31.5 61.5 31.9 2 10.9 32.4 46.0 28.9 3 31.5 38.0 33.1 33.4 4 29.1 44.1 52.9 39.1 5 43.9 41.9 60.6 42.8 6 14.1 2.4 29.2 41.8 7 9.5 9.7 17.8 19.7 8 25.5 5.7 15.9 1.2 9 25.9 6.8 13.0 38.0 10 28.6 6.4 10.6 39.2 11 22.8 8.0 6.5 16.4 12 16.2 9.4 1.8 19.5 13 14.8 3.0 2.5 22.8 14 16.5 1.8 3.1 25.4 15 18.6 9.1 17.3 17.8 16 27.0 16.4 19.7 20.3 17 39.9 18.3 20.2 27.2 18 21.0 12.7 23.8 51.9 19 4.9 10.1 23.6 28.8 20 10.2 2.8 39.1 28.7 21 16.3 21.8 57.6 30.6 22 20.8 25.2 70.6 12.4 23 44.7 27.4 82.6 0.8 895 Slant total electron content for Sirio-Mortelliccio ray path SPARKS and B.D. WILSON (1999): GPS and Ionosphere, Revised Submission to URSI Reviews of Radio Science (Jet Propulsion Laboratory, Pasadena, CA). RAWER, K. and D. BILITZA (1990): International reference ionosphere-plasma densities: status 1988, Adv. Space res., 10 (8), 5-14. RISHBETH, H. and O.K. GARRIOT (1969): An Introduction to Ionospheric Physics (Academic Press, New York and London), pp. 331. TITHERIDGE, J.E. (1993): Atmospheric winds calculated from diurnal changes in the mid-latitude ionosphere, J. Atmos. Terr. Phys., 55, 1637-1659. WRIGHT, J.W. (1960): A model of de F region above hmaxF2, J. Geophys. Res., 65 (1), 185-191. YONEZAWA, T. (1955): On influence of the electron-ion dif- fusion on the electron density and height of the noctur- nal F2 layer, J. Radio Res. Lab., 2 (8), 125-136. YONEZAWA, T. and H. TAKAHASHI (1960): On the electron and ion density distributions from the lower uppermost part of the F region, J. Radio Res. Lab., 7 (32), p. 335. (received September 15, 2005; accepted July 27, 2006)