Data from the University of Alberta meridian line of magnetometers are utilized to define the poleward and equatorward borders of the eastward electrojet in the evening sector. Soft particle spectrometer data from the Isis 2 polar-orbiting satellite are organized in the framework of the eastward electrojet for cases where the satellite orbital path took it close to the meridian line of ground-based magnetometer stations. It is shown that in the late evening hours the equatorward border of the eastward electrojet coincides with the equatorward edge of the central plasma sheet (cps) as marked by electrons of E>1 keV. The boundary plasma sheet (bps) spans the poleward portion of the eastward electrojet and a region up to a few degrees poleward of the eastward electrojet where the westward electrojet is known to penetrate on average. A level shift in the Y' component of the ground magnetometer data exists across the portion of the electrojet region marked by bps; this level shift is interpreted as net upward field-aligned ccurrent flow. In the hours between noon and dusk the correlation between the equatorward border of the eastward electrojet and the equatorward edge of the cps deteriorates completely, and the magnetic latitude profiles exhibit anomalous behavior. This problem will be discussed in paper 2 of this series. On the basis of this study we conclude that in the evening sector the cps is threaded by field lines carrying current flowing into the ionosphere, while the bps is threaded by field lines carrying current flowing out of the ionosphere. The electric field transition from the auroral ovval to the polar cap must occur in the heart of the bps under average conditions.