A combination of EISCAT CP-3 (latitude scans) and satellite (DMSP) data have been used to study thestructure of the quiet-time evening-sector auroral and subauroral ionosphere, inparticular the mid-latitude trough. The main mechanism behindtrough formation in the quiet-time evening sector ionosphere isbelieved to be flow stagnation in a region whereconvection and corotation counteract each other.However, there is also the possibility that field-aligned currents(FAC) more directlymodify the ionospheric density if the current is carried by thermal ionosphericelectrons.A quantitative test of the flow-stagnation scenario and an estimate of the possible direct effects caused by field-aligned currents have been performed.We found that the electron densities observed can indeed be explained bythe flow-stagnation scenario, but the electron density altitudeprofiles in the trough sometimes differ from what should be expected fromflow stagnation. The effect of a downward field-aligned current cannotbe identified in the data, but a simple estimateshows that it can affect the ionospheric plasma density, causing decreasedionospheric densities. In the quiet-timeregion 2 current/trough region there is typically a significant ionproduction resulting from proton precipitation which may counteract this effect.Charge exchange of the precipitating protons causes a lateral spreadand a smooth associated conductance enhancement.Thus, whereas theproton number flux is insufficient to directly carry the eveningsector region 2 current, the precipitation in practice produces thenecessary charge carriers. We suggest that precipitatingprotons do play a crucial role in the electrodynamics of the dark evening sector ionosphere bycausing a small but smooth conductance enhancement and producing the charge carriersnecessary to carry the trough-associated downward field-aligned current.
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