Magnetospheric ducts for medium frequency (MF) radio waves are formed, for L>1.2, between about 20 and 22 hours local time. Their occurrence frequency remains constant for the remainder of the night and begins to decrease after sunrise. This decrease continues until near sunset. The seasonal and longitudinal variation of the ducts and of low-latitude spread F implies that both are related to hemispheric asymmetry of the winds in the dynamo region and hence are probably generated by field-aligned currents as suggested by K. D. Cole. These currents generate significant irregularities in the E region of the ionosphere at night when E region densities are low; these irregularities in turn produce perpendicular electric fields due to winds. The small-scale electric fields (≲ few kilometers) responsible for ducts can only be transferred to the F region and magnetosphere along magnetic field lines when the parallel conductivity is fairly large between the E and F layers. These electric fields interchange tubes of flux and hence can change tube densities. The transfer of electric fields occurs before the formation of the deep E-F density valley. The valley forms about 22 hours, which explains why ducts with L>1.2 can only form after the E region has low density (after 19 hours) and before the E-F valley forms. For L≲1.2 the percentage occurrence of MF ducts increases from about 20 to roughly 02 hours local time and decreases from roughly 02 to about 10 hours. It is thought that these ducts are associated with ionospheric 'bubbles'. The frequency of occurrence of ducts with L≲1.2 continues to increase for roughly 3 hours after the frequency of occurrence of ducts with L>1.2 stops increasing. Since the estimated rise time for bubbles from the bottom of the F layer to the topside is about 3 hours, these ducts may also have been initiated before about 22 hours.