| Summary: | Abstract The high-latitude polar ionosphere is characterized by two regions, the polar cap and the auroral oval. In the polar cap, the geomagnetic field lines are open and connect to the solar wind, whereas the field lines in the auroral oval are closed and map to the plasma sheet and the plasma sheet boundary layer in the magnetosphere. The two substantially different magnetic and plasma domains are separated by a separatrix, the polar cap boundary (PCB), which is an ionospheric projection of the open-closed field line boundary (OCB) in the magnetosphere. In this thesis, a new method to determine the location of the PCB in the nightside ionosphere based on electron temperature measurements by EISCAT incoherent scatter radars is introduced. Comparisons with other PCB proxies like poleward boundary of the auroral emissions, poleward edge of the auroral electrojets and poleward boundary of energetic particle precipitation show general agreement. By applying the method to several events together with other supporting ground-based and space-borne observations, dynamic processes and phenomena in the vicinity of the PCB and inside the auroral oval are studied. The main results include the following. During substorm expansion, the PCB moves poleward in a burstlike manner with individual bursts separated by 2–10 min, indicating impulsive reconnection in the magnetotail. In one event, a possible signature of the high-altitude counterpart of the Earthward flowing field-aligned current of the Hall current system at the magnetotail reconnection site is observed. Investigation of the relation between the auroral activity and the local reconnection rate estimated from the EISCAT measurements reveals direct association between individual auroral poleward boundary intensifications (PBIs) and intensifications in the ionospheric reconnection electric field within the same MLT sector. The result confirms earlier suggestions of positive correlation between PBIs and enhanced flux closure in the magnetotail. In another event, ...
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