3. Methods of Measurements and Data Processing

[6]  The ionosphere investigation by the Kharkov IS radar is based on the measurements of the signal correlation function (CF). Measurement and data processing methods were described by Taran [1979, 2001], Emel'yanov [1999], Lysenko [1999a, 1999b], and Pulyaev [1999]. From the measured CFs, electron Te and ion Ti temperatures, ion composition, vertical component Vz of the plasma drift velocity and other ionospheric parameters are derived. The electron density profiles Ne(h) are obtained using the power profile method from the following formula [Evans, 1969]:


where h is the height of the center of the plasma scattering volume, q is the signal-to-noise ratio, and Cr is the proportionality coefficient determined by technical parameters of the radar. For Kharkov radar absolute values of Ne(h) were determined by normalizing the profile and the adjustment of its maximum to the NmF2 value calculated from the critical frequency foF2 measured by the ionosonde.

[7]  The errors in estimation of the signal CFs and ionospheric parameters depend on the signal-to-noise ratio, noise background, parameters of the equipment, and other factors. Table 1 shows the statistical errors of ionospheric parameters for typical daytime conditions, 15-min signal integration, and the given signal-to-noise ratios.

[8]  The vertical plasma velocity Vz is found from the Doppler shift of the IS signal spectrum estimated based on the measured quadrature components of the signal CF [Emel'yanov, 1999]. To increase the accuracy of the measurements, a trapezoidal smoothing of CF over altitude is performed [Holt et al., 1992; Lysenko, 1999a]. The RMS deviation sVz of the measured velocity depends on the signal-to-noise ratio q and varies with altitude. Usually, sVzapprox 5-20 m s-1 for altitudes of the ionospheric F region at q ge 0.2 and an integration time of 15 min.

[9]  The method used to determine Ti and Te temperatures should be considered in more detail. These temperatures are calculated taking into account the ion composition at altitudes below the F2 layer peak. In this case the Te/Ti and Ti/mi ratios, where mi is ion mass, are found from the measured CFs of a scattered signal [Farley, 1969] by comparing these functions with the theoretical CFs using the least squares technique. Certain conditions are imposed in order to eliminate the ambiguity in the solution of the problem [Pavlov et al., 1999; Schlesier and Buonsanto, 1999]. The average molecular weight of ions (O2+ and NO+ ) was taken equal to 31. Gradual transition from the 100% concentration of molecular ions at 120 km altitude (where it was considered that Te approx Ti approx Tn approx 355 K) to the 100% concentration of O+ ions at an altitude of 230-300 km was assumed. This height was selected depending on specific conditions: day-night, winter-summer. A change in Ti within an altitude interval of 10 km is restricted additionally: DTi( max) = pm 0.1 Ti. It should be noted that the applied technique only approximately reflects changes in the concentration of molecular ions, which are especially significant during magnetic disturbances, and results in additional error in determining Ti, Te, and Ne. The problem of correcting measured ionospheric parameters Ti, Te, and Ne depending on the applied model of ion composition was first discussed by Waldteufel [1971]. It is known that this problem is solved in the modern models of the ionosphere [see, e.g., Mikhailov and Schlegel, 1997; Schlesier and Buonsanto, 1999]. A comparison of the data on electron density obtained by the Kharkov radar using the power profile technique and the Faraday rotation measurements (this technique is described, e.g., by Grigorenko [1979]) made it possible to estimate the error in determining Ti and Te below F2 region peak. This error was not higher than 15% under quiet conditions.


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