## 1. Introduction

[2] The ionosphere affects Earth-space communication systems, especially during extreme space weather events. Radio wave propagation is modified in a number of ways by the effects of the integrated electron density along the ionospheric ray path between the satellite and the receiver, the so-called total electron content (TEC). In consequence, TEC is a key parameter in the description of the impact of the ionized atmosphere on the propagation of radio signals, understanding of which is crucial for the operation of many applications, including navigation satellite systems like GPS, Global Navigation Satellite System (GLONASS), and the future Galileo system. TEC can be measured by a number of essentially standard techniques, including Faraday rotation, group delay, and dispersive carrier phase. In general, these techniques measure the electron content along a slant signal path between a satellite and the ground, from which the equivalent vertical TEC is found by simple geometric conversion. However, *Huang and Reinisch* [2001] proposed a new technique to estimate the total electron content of the ionosphere using ground-based ionosondes. While the proposed method had the advantage of giving a direct measurement of the vertical TEC, it must be noted that, in reality, the ionograms only contain direct information about the vertical electron density profile of the bottomside ionosphere up to the peak of the *F*_{2} layer, and normally, some two thirds of the electrons comprising the TEC are in the topside.

[3] In the method, the profile above the peak was approximated by an α Chapman function with constant scale height (*H*_{T}), derived from the bottomside profile shape at the *F*_{2} peak. The ionosonde total electron content (ITEC) was then calculated from the height integral over the entire profile according to the equation

where

with

The upper limit of the integration (*h*_{max}) was chosen to be 1000 km in the original study, a height retained for the estimation of the ITEC parameter provided by Digisonde software. The formulation of the Chapman function outlined above depends on only three parameters, the peak density (*N*_{m}*F*_{2}), the peak height (*h*_{m}*F*_{2}), and the scale height (*H*_{T}). The solar zenith angle dependence is contained in *N*_{m}*F*_{2}, which is estimated from the ionogram, while the peak height can be obtained from the true height inversion of the ionogram. It is assumed that the bottomside electron density profile (*N*_{B}) also gives an indication of the profile shape above *h*_{m}*F*_{2}. Once *N*_{B}(*h*) is calculated from the measured *h*′(*f*) trace in the ionogram, the shape of the profile at the *F*_{2} peak is used to derive an estimate of the topside scale height (*H*_{T}). Scale height can be introduced by describing the bottomside profile by an α Chapman layer with variable scale height *H*(*h*) [*Rishbeth and Garriot*, 1969]:

where

Here *H*_{m} is the scale height at the *F*_{2} peak, that is, *H*_{m} = *H*(*h*_{m}*F*_{2}). The ITEC method assumes that the determination of *H*_{m} from equation (3) will be a reasonable estimate for the scale height of the topside profile in equation (2), that is, *H*_{T} = *H*_{m}.

[4] Several attempts have been made to try to validate ITEC, essentially on the basis of case studies. These have compared ITEC estimates with TEC values derived from incoherent scatter radar and satellite observations, the latter involving Faraday rotation at middle latitudes, TOPEX at the equator, and use of GPS. They showed that ITEC is generally within about 10% of the satellite TEC [*Huang and Reinisch*, 2001; *Reinisch and Huang*, 2001; *Reinisch et al.*, 2001; *Belehaki and Tsagouri*, 2002]. Recently, a systematic comparison between ITEC and GPS-derived TEC values, estimated for the same geographic location using 12 consecutive months of measurements, found that the ITEC parameter can provide a qualitatively representation of the ionospheric electron content up to 1000 km [*Belehaki et al.*, 2003]. Indeed, it was suggested that the residual differences between GPSTEC and ITEC may provide information about the plasmaspheric contribution to the former, as deduced from the diurnal and seasonal behavior and the variation during geomagnetic storms.

[5] As a next stage, before ITEC can be considered as an operational parameter, the current study involves a statistical investigation of the relationship between ITEC and the vertical total electron content, both calculated from the same set of electron density profiles that had been obtained from incoherent scatter radar (ISR) observations. For this purpose, the electron density profile has been reconstructed using the bottomside profile from the ISR observations and a topside profile extrapolated according to the *Huang and Reinisch* [2001] method. A statistical comparison between the reconstructed and the observed ISR profiles should provide conclusive evidence on the validity of the ITEC parameter as an alternative method of measuring the total electron content from ionosonde observations.