Midlatitude trough in the night ionosphere

The October–November 1963 flight of an ion trap on a polar-orbiting oriented satellite has revealed the presence of a well defined trough in the F-region ionization that persists over the nearly four and one-half days during which data were obtained. The trough is geomagnetically oriented along the intersection of the vehicle orbit and the L = 3.5 shell, located on the equator side of the polar auroral zone, and it is found in both the northern and southern hemispheres. Rather sharp gradients in the ion concentration are found to be associated with this trough, particularly on the polar side of the trough. The trough shape does not remain fixed, but changes rather dramatically throughout this flight, and its width seems to be inversely correlated with the intensity of electrons precipitated in the polar auroral zones.