The Observed Variations of Ionospheric Electron Content

A brief review is given of the behaviour of the electron content of the ionosphere and its variations that has been obtained from beacon satellite studies. This includes the daily variation, the day-to-day and hour-to-hour variability, the 27-day variation, the annual(seasonal), semi-annual and solar cycle variations and variations associated with geomagnetic disturbance and storms. It is remarkable that we are still far from a complete understanding of the causes of most of these phenomena.