On medium scale travelling ionospheric disturbances as seen through total electron content power spectra

Modulation phase of 140 MHz with respect to 360 MHz signals from ATS 6 satellite recorded at Slough (51.5°N, 0.6°W geographic latitude and longitude; 54.3° geomagnetic latitude) at one-minute interval are power spectral analysed to derive dominant periodicities corresponding to Travelling Ionospheric Disturbances. From the significant peaks in the spectra, an occurrence peak at periods between 10 and 15 min and a secondary peak at 60–65 min are seen. From cross-spectral analysis of the same records from three stations separated by a few hundred kilometers for a short period, the speed and azimuth of the propagating disturbances are determined. During the day-time, most of the waves in the period range 30–100 min are seen to propagate at azimuths of 90–160°. At night-time they propagate poleward. Theoretical computations of the azimuth response of TEC to typical gravity waves, including the effect of neutral winds, show that the observed azimuths of propagation are in reasonably good agreement with theoretical predictions.