Spectral behaviour of TIDs detected from rapid sequence ionograms

The main purpose of this paper is to illustrate the usefulness of ionosonde observations in the study of the spectral evolution of travelling ionospheric disturbances (TIDs). An inversion method is introduced to yield the spatial and temporal variation of the ionospheric velocity from data of modern ionosondes or rapid-run ionosondes. Some inversion results of rapid sequence ionograms are spectrally analysed to obtain the height- and time-dependent power spectra of TIDs, and the evolution of these spectra is discussed in detail. It is found that spectral peaks shift regularly with time. As time increases, the peaks of the lower frequency components or those at the lower heights tend to shift towards lower frequency, while those of higher frequency or at higher altitudes lend to higher frequency. This property is explained by the current dispersion theory of atmospheric gravity waves. It is concluded that the spectral behaviour of TIDs can be well studied by using ground-based HF techniques, such as ionosonde observation.