Simultaneous data obtained with the Chatanika incoherent scatter radar and the Dynamics Explorer 2 (DE 2) and NOAA 6 satellites are used to relate the locations of the precipitating particles, field-aligned currents, and E and F region ionization structures in the evening-sector auroral oval. The auroral E layer observed by the radar extends about 2¿ equatorward of the electron precipitation region, and its equatorward edge coincides with the equatorward edges of the region 2 field-aligned current and intense convection region (E≂50 mV/ m). It is shown that precipitating protons are responsible for part of the E region ionization within the electron precipitation region as well as south of it. E region density profiles calculated from ion spectra measured by the DE 2 and NOAA 6 satellites are in fairly good agreement with the Chatanika data. In the F region, a channel of enhanced ionization density, elongated along the east-west direction and having a width of about 100 km, marks the poleward edge of the main trough. It is colocated with the equatorward boundary of the electron precipitation from the central plasma sheet.

Although enhanced fluxes of soft electrons are observed at this boundary, the energy input to the ionospheric electron gas, calculated from the radar data, shows that this ionization channel is not locally produced by this soft precipitation, but that it is rather a convected feature. In fact, both the trough and the ionization channel are located in a region where the plasma flows sunward at high speed, but the flux tubes associated with these two features have different convective time histories. Keeping in mind that several processes operate together in the F region, our data set is consistent with the following trough and ionization channel formation mechanisms. (1) The midlatitude trough, located equatorward of the electron precipitation region, is mainly the result of transport and enhanced recombination due to large electric fields.

Flux tubes on the low-latitude edge of the trough have most probably corotated eastward before flowing sunward at higher latitudes where magnetospheric convection predominates. The trough thus forms by recombination during the long time the flux tubes stagnate in the region where the flow reverses. (2) Flux tubes associated with the ionization channel have drifted antisunward in the polar cap before drifting sunward in the auroral zone. Its formation results from the distortion of polar cap F region ionization structures, due to the incompressibility of the flow. ¿American Geophysical Union 1987