Low density regions observed at high altitudes and their connection with equatorial spread F

Observations from the high resolution spherical electrostatic analyzer experiment aboard ISIS 1 have been used to study large amplitude irregularities at low latitudes in the tipside ionosphere. The irregularities appeared as plasma depletions near the magnetic equator and were observed up to satellite apogee (3500 km). The altitude local time distribution of the depletions was such that those at altitudes greater than 2000 km were found only in the post-midnight sector. This result agrees with the predictions of a model for plasma bubbles drifting under the influence of gravity-buoyancy forces. Evidence is presented that the initial steep gradients observed at low altitudes are reduced by anomalous diffusion due to drift waves.