Ion convection and the formation of the mid‐latitude F region ionization trough

Measurements of total ion concentration and ion drift velocity from Atmosphere Explorer C have been used to examine the convective flow of plasma in the vicinity of the mid‐latitude F region ionization trough. It is found that the low‐latitude portion of the premidnight trough is a region of slow eastward plasma drift while the poleward portion is characterized by generally westward flow. The reversal from eastward to westward flow typically occurs on the low‐latitude edge of the trough where N i is observed to vary smoothly. We propose that the plasma flux tubes within the premidnight trough have corotated eastward into the evening local time sector from dusk before stagnating and flowing westward at higher latitudes in the nighttime sector. In the absence of ionization sources, rather deep troughs can be formed by F region recombination during the long time required for the flow to stagnate and reverse. The proposed drift paths can result quite naturally from the combined effects of the corotation and magnetospheric electric fields. Large plasma convection velocities that act to increase the F region decay rate are not required to produce a trough but may cause the plasma composition changes that are occasionally observed within the trough.