P.F. Denisenko - Dr. Sc. (Phys.-Math.), Professor, Head of Department of Radiophysics and Space Research, Research Institute of Physics SFU (Rostov-on-Don). E-mail: denis@ip.rsu.ru
A.A. Khomyakov - Post-graduate Student, Faculty of Physics, Southern Federal University (Rostov-on-Don). E-mail: artem_rsu@mail.ru

Briefly set out the methodology of detecting wave like structures of ionosphere. The initial experimental data to identify the wave structure are obtained by calculating the profiles of electron concentration from ionograms of topside sounding. They were registered on-board satellite «ISIS 2» 22 October 1979 in the time intervals UT = 22h31m27s−22h42m15s with changes: geomagnetic longitude from −145,5° to −151,5°, geomagnetic latitude from 7,6° to 41,4°, the height of satellite from 1385 to 1417 km. To determine the disturbance of ionosphere we use the electron density (plasma frequency) at fixed altitudes as a function of geomagnetic latitude. For selection of quasi-wave structure the Singular Spectrum Analysis (SSA) is used. The basic aim of SSA is to decompose the time series into the sum of interpretable components such as trend, periodic components and noise with no a priori assumptions about the parametric form of these components. SSA-method is based on analysis of eigenvalues of trajectory matrix. Wave like structures are found by grouping of eigenvectors with similar eigenvalues. This wave like structure associated with the «frozen» travelling ionospheric disturbances (TID) is approximated in the form of single harmonic with appropriate amplitude, wavelength and phase. Altitude dependences of these characteristics are determined from 250 to 400 km in steps 5 km. This structure belong to a class of large-scale travelling ionospheric disturbances with wavelength from 2385 to 2790 km. Their amplitude decrease from 250 to 400 km; from 400 km wave structures are not selected by SSA. These results are agree with theory of TID in outer ionosphere.