A study of gravity waves in ionospheric electron content at L = 4

Travelling ionospheric disturbances have been observed in the electron content at L = 4 determined from satelltie radio beacon transmissions. Waves are a common feature at this latitude, present for at least 98 percent of all daylight hours. The amplitude is usually 1-4 percent of the mean electron content, and periods range between 15 and 90 minutes. Simultaneous observation of two satellite beacons, giving an effective east-west separation of 350 km, indicated apparent east-to-west velocity components of 200-700 m/s. A search was made for a likely source of the waves, using data from magnetometers and riometers, from incoherent scatter radar measurements of Joule heating, and from orbiting satellite measurements of electron influx, but no definite source could be established. It is shown that travelling disturbances are closely related to occurrences of spread-F on ionograms.