The’ scintillations’ observed on signals received in the equatorial region from GPS satellites are due to plasma instabilities in the F region of the ionosphere, also detected as spread F These instabilities give rise to depletions of ionisation or ‘bubbles’. The occurrence of these events and their relation to the equatorial electrojet are reviewed. Possibilities of short-term forecasting are examined with particular attention to problems encountered in modelling the equatorial electrojet.