Satellite studies of magnetospheric substorms on August 15, 1968: 3. Some features of magnetospheric convection

A combination of Ogo 4 and 5 data and ground data for August 13–15, 1968, has revealed new details of unsteady magnetospheric convection and the gradual erosion of the plasmasphere during a weak magnetic storm. Evidence of erosion was detected by Ogo 5 in the form of complex density structure near the duskside plasmapause and by Ogo 4 as a persistent diminution in plasmapause L value near 0500 magnetic local time (MLT). Evidence of unsteady convection includes (1) Ogo 5 observations of the irregular duskside density profile, (2) Ogo 4 and Byrd ground whistler observations of an apparent rotation of the plasmasphere bulge during quieting, and (3) Ogo 4 observations near 0500 MLT of fluctuations of plasmapause L value around the slowly decreasing average value. It is inferred that most of the variations in plasmapause L value observed near 0500 MLT were the result of rotation under the satellite of spatial patterns imposed by convection activity in the midnight and dusk sectors. A pronounced decrease in plasmapause L value from L ∼ 3.4 to L ∼ 2.5 followed within a few hours the major substorms of early August 15. From the size of this decrease, and from previous whistler research on cross-L plasma drifts, it is inferred that westward electric fields of the order of 0.5 mv/m were present in the midnight sector during the substorms of interest.