Dynamic processes in the ionosphere during magnetic storms...

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 T_e and ion T_i temperatures, ion composition, vertical component V_z of the plasma drift velocity and other ionospheric parameters are derived. The electron density profiles N_e(h) are obtained using the power profile method from the following formula [Evans, 1969]:

(1)

where h is the height of the center of the plasma scattering volume, q is the signal-to-noise ratio, and C_r is the proportionality coefficient determined by technical parameters of the radar. For Kharkov radar absolute values of N_e(h) were determined by normalizing the profile and the adjustment of its maximum to the N_mF2 value calculated from the critical frequency f_oF2 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 V_z 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 s_{V_z} of the measured velocity depends on the signal-to-noise ratio q and varies with altitude. Usually, s_{V_z} approx 5-20 m s^-1 for altitudes of the ionospheric F region at q 0.2 and an integration time of 15 min.

[9] The method used to determine T_i and T_e 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 T_e/T_i and T_i/m_i ratios, where m_i 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 T_e approx T_i approx T_n 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 T_i within an altitude interval of 10 km is restricted additionally: DT_i( max) = 0.1 T_i. 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 T_i, T_e, and N_e. The problem of correcting measured ionospheric parameters T_i, T_e, and N_e 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 T_i and T_e below F2 region peak. This error was not higher than 15% under quiet conditions.